<feed xmlns='http://www.w3.org/2005/Atom'>
<title>linux.git/fs/btrfs/compression.c, branch v4.0</title>
<subtitle>Linux kernel source tree</subtitle>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/'/>
<entry>
<title>Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs</title>
<updated>2014-12-12T19:15:23+00:00</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2014-12-12T19:15:23+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=bdeb03cada1c305346505c48e5b1dab37e9acc4e'/>
<id>bdeb03cada1c305346505c48e5b1dab37e9acc4e</id>
<content type='text'>
Pull btrfs update from Chris Mason:
 "From a feature point of view, most of the code here comes from Miao
  Xie and others at Fujitsu to implement scrubbing and replacing devices
  on raid56.  This has been in development for a while, and it's a big
  improvement.

  Filipe and Josef have a great assortment of fixes, many of which solve
  problems corruptions either after a crash or in error conditions.  I
  still have a round two from Filipe for next week that solves
  corruptions with discard and block group removal"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs: (62 commits)
  Btrfs: make get_caching_control unconditionally return the ctl
  Btrfs: fix unprotected deletion from pending_chunks list
  Btrfs: fix fs mapping extent map leak
  Btrfs: fix memory leak after block remove + trimming
  Btrfs: make btrfs_abort_transaction consider existence of new block groups
  Btrfs: fix race between writing free space cache and trimming
  Btrfs: fix race between fs trimming and block group remove/allocation
  Btrfs, replace: enable dev-replace for raid56
  Btrfs: fix freeing used extents after removing empty block group
  Btrfs: fix crash caused by block group removal
  Btrfs: fix invalid block group rbtree access after bg is removed
  Btrfs, raid56: fix use-after-free problem in the final device replace procedure on raid56
  Btrfs, replace: write raid56 parity into the replace target device
  Btrfs, replace: write dirty pages into the replace target device
  Btrfs, raid56: support parity scrub on raid56
  Btrfs, raid56: use a variant to record the operation type
  Btrfs, scrub: repair the common data on RAID5/6 if it is corrupted
  Btrfs, raid56: don't change bbio and raid_map
  Btrfs: remove unnecessary code of stripe_index assignment in __btrfs_map_block
  Btrfs: remove noused bbio_ret in __btrfs_map_block in condition
  ...
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
Pull btrfs update from Chris Mason:
 "From a feature point of view, most of the code here comes from Miao
  Xie and others at Fujitsu to implement scrubbing and replacing devices
  on raid56.  This has been in development for a while, and it's a big
  improvement.

  Filipe and Josef have a great assortment of fixes, many of which solve
  problems corruptions either after a crash or in error conditions.  I
  still have a round two from Filipe for next week that solves
  corruptions with discard and block group removal"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs: (62 commits)
  Btrfs: make get_caching_control unconditionally return the ctl
  Btrfs: fix unprotected deletion from pending_chunks list
  Btrfs: fix fs mapping extent map leak
  Btrfs: fix memory leak after block remove + trimming
  Btrfs: make btrfs_abort_transaction consider existence of new block groups
  Btrfs: fix race between writing free space cache and trimming
  Btrfs: fix race between fs trimming and block group remove/allocation
  Btrfs, replace: enable dev-replace for raid56
  Btrfs: fix freeing used extents after removing empty block group
  Btrfs: fix crash caused by block group removal
  Btrfs: fix invalid block group rbtree access after bg is removed
  Btrfs, raid56: fix use-after-free problem in the final device replace procedure on raid56
  Btrfs, replace: write raid56 parity into the replace target device
  Btrfs, replace: write dirty pages into the replace target device
  Btrfs, raid56: support parity scrub on raid56
  Btrfs, raid56: use a variant to record the operation type
  Btrfs, scrub: repair the common data on RAID5/6 if it is corrupted
  Btrfs, raid56: don't change bbio and raid_map
  Btrfs: remove unnecessary code of stripe_index assignment in __btrfs_map_block
  Btrfs: remove noused bbio_ret in __btrfs_map_block in condition
  ...
</pre>
</div>
</content>
</entry>
<entry>
<title>btrfs: zero out left over bytes after processing compression streams</title>
<updated>2014-11-30T17:33:51+00:00</updated>
<author>
<name>Chris Mason</name>
<email>clm@fb.com</email>
</author>
<published>2014-11-30T13:56:33+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=2f19cad94cee3c9bd52d0c9ca584ef506302fb7c'/>
<id>2f19cad94cee3c9bd52d0c9ca584ef506302fb7c</id>
<content type='text'>
Don Bailey noticed that our page zeroing for compression at end-io time
isn't complete.  This reworks a patch from Linus to push the zeroing
into the zlib and lzo specific functions instead of trying to handle the
corners inside btrfs_decompress_buf2page

Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
Reviewed-by: Josef Bacik &lt;jbacik@fb.com&gt;
Reported-by: Don A. Bailey &lt;donb@securitymouse.com&gt;
cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
Don Bailey noticed that our page zeroing for compression at end-io time
isn't complete.  This reworks a patch from Linus to push the zeroing
into the zlib and lzo specific functions instead of trying to handle the
corners inside btrfs_decompress_buf2page

Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
Reviewed-by: Josef Bacik &lt;jbacik@fb.com&gt;
Reported-by: Don A. Bailey &lt;donb@securitymouse.com&gt;
cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>Btrfs: don't ignore compressed bio write errors</title>
<updated>2014-11-21T01:14:26+00:00</updated>
<author>
<name>Filipe Manana</name>
<email>fdmanana@suse.com</email>
</author>
<published>2014-10-07T00:48:26+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=7bdcefc103849386ef7f3029dd94ecfd4a822a67'/>
<id>7bdcefc103849386ef7f3029dd94ecfd4a822a67</id>
<content type='text'>
Our compressed bio write end callback was essentially ignoring the error
parameter. When a write error happens, it must pass a value of 0 to the
inode's write_page_end_io_hook callback, SetPageError on the respective
pages and set AS_EIO in the inode's mapping flags, so that a call to
filemap_fdatawait_range() / filemap_fdatawait() can find out that errors
happened (we surely don't want silent failures on fsync for example).

Signed-off-by: Filipe Manana &lt;fdmanana@suse.com&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
Our compressed bio write end callback was essentially ignoring the error
parameter. When a write error happens, it must pass a value of 0 to the
inode's write_page_end_io_hook callback, SetPageError on the respective
pages and set AS_EIO in the inode's mapping flags, so that a call to
filemap_fdatawait_range() / filemap_fdatawait() can find out that errors
happened (we surely don't want silent failures on fsync for example).

Signed-off-by: Filipe Manana &lt;fdmanana@suse.com&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>btrfs: use enum for wq endio metadata type</title>
<updated>2014-10-02T15:30:28+00:00</updated>
<author>
<name>David Sterba</name>
<email>dsterba@suse.cz</email>
</author>
<published>2014-07-29T22:25:45+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=bfebd8b5441755f228ad02273682d675d3335123'/>
<id>bfebd8b5441755f228ad02273682d675d3335123</id>
<content type='text'>
The enum exists but is not consistently used.

Signed-off-by: David Sterba &lt;dsterba@suse.cz&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
The enum exists but is not consistently used.

Signed-off-by: David Sterba &lt;dsterba@suse.cz&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>btrfs: use DIV_ROUND_UP instead of open-coded variants</title>
<updated>2014-09-17T20:37:17+00:00</updated>
<author>
<name>David Sterba</name>
<email>dsterba@suse.cz</email>
</author>
<published>2014-06-04T23:59:57+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=ed6078f70335f158ca79790a0d0708ce558a6e9a'/>
<id>ed6078f70335f158ca79790a0d0708ce558a6e9a</id>
<content type='text'>
The form

  (value + PAGE_CACHE_SIZE - 1) &gt;&gt; PAGE_CACHE_SHIFT

is equivalent to

  (value + PAGE_CACHE_SIZE - 1) / PAGE_CACHE_SIZE

The rest is a simple subsitution, no difference in the generated
assembly code.

Signed-off-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
The form

  (value + PAGE_CACHE_SIZE - 1) &gt;&gt; PAGE_CACHE_SHIFT

is equivalent to

  (value + PAGE_CACHE_SIZE - 1) / PAGE_CACHE_SIZE

The rest is a simple subsitution, no difference in the generated
assembly code.

Signed-off-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>btrfs compression: reuse recently used workspace</title>
<updated>2014-06-28T20:48:46+00:00</updated>
<author>
<name>Sergey Senozhatsky</name>
<email>sergey.senozhatsky@gmail.com</email>
</author>
<published>2014-06-24T15:00:44+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=c39aa7056f50fa7b49265ea35c0f3eddb7c7be68'/>
<id>c39aa7056f50fa7b49265ea35c0f3eddb7c7be68</id>
<content type='text'>
Add compression `workspace' in free_workspace() to
`idle_workspace' list head, instead of tail. So we have
better chances to reuse most recently used `workspace'.

Signed-off-by: Sergey Senozhatsky &lt;sergey.senozhatsky@gmail.com&gt;
Reviewed-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
Add compression `workspace' in free_workspace() to
`idle_workspace' list head, instead of tail. So we have
better chances to reuse most recently used `workspace'.

Signed-off-by: Sergey Senozhatsky &lt;sergey.senozhatsky@gmail.com&gt;
Reviewed-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>btrfs: return ptr error from compression workspace</title>
<updated>2014-06-10T00:20:22+00:00</updated>
<author>
<name>Zach Brown</name>
<email>zab@redhat.com</email>
</author>
<published>2014-05-09T21:15:09+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=774bcb35f0d9ccb9adfd3a391328cc25523659fc'/>
<id>774bcb35f0d9ccb9adfd3a391328cc25523659fc</id>
<content type='text'>
The btrfs compression wrappers translated errors from workspace
allocation to either -ENOMEM or -1.  The compression type workspace
allocators are already returning a ERR_PTR(-ENOMEM).  Just return that
and get rid of the magical -1.

This helps a future patch return errors from the compression wrappers.

Signed-off-by: Zach Brown &lt;zab@redhat.com&gt;
Reviewed-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
The btrfs compression wrappers translated errors from workspace
allocation to either -ENOMEM or -1.  The compression type workspace
allocators are already returning a ERR_PTR(-ENOMEM).  Just return that
and get rid of the magical -1.

This helps a future patch return errors from the compression wrappers.

Signed-off-by: Zach Brown &lt;zab@redhat.com&gt;
Reviewed-by: David Sterba &lt;dsterba@suse.cz&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>mm + fs: prepare for non-page entries in page cache radix trees</title>
<updated>2014-04-03T23:21:00+00:00</updated>
<author>
<name>Johannes Weiner</name>
<email>hannes@cmpxchg.org</email>
</author>
<published>2014-04-03T21:47:46+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=0cd6144aadd2afd19d1aca880153530c52957604'/>
<id>0cd6144aadd2afd19d1aca880153530c52957604</id>
<content type='text'>
shmem mappings already contain exceptional entries where swap slot
information is remembered.

To be able to store eviction information for regular page cache, prepare
every site dealing with the radix trees directly to handle entries other
than pages.

The common lookup functions will filter out non-page entries and return
NULL for page cache holes, just as before.  But provide a raw version of
the API which returns non-page entries as well, and switch shmem over to
use it.

Signed-off-by: Johannes Weiner &lt;hannes@cmpxchg.org&gt;
Reviewed-by: Rik van Riel &lt;riel@redhat.com&gt;
Reviewed-by: Minchan Kim &lt;minchan@kernel.org&gt;
Cc: Andrea Arcangeli &lt;aarcange@redhat.com&gt;
Cc: Bob Liu &lt;bob.liu@oracle.com&gt;
Cc: Christoph Hellwig &lt;hch@infradead.org&gt;
Cc: Dave Chinner &lt;david@fromorbit.com&gt;
Cc: Greg Thelen &lt;gthelen@google.com&gt;
Cc: Hugh Dickins &lt;hughd@google.com&gt;
Cc: Jan Kara &lt;jack@suse.cz&gt;
Cc: KOSAKI Motohiro &lt;kosaki.motohiro@jp.fujitsu.com&gt;
Cc: Luigi Semenzato &lt;semenzato@google.com&gt;
Cc: Mel Gorman &lt;mgorman@suse.de&gt;
Cc: Metin Doslu &lt;metin@citusdata.com&gt;
Cc: Michel Lespinasse &lt;walken@google.com&gt;
Cc: Ozgun Erdogan &lt;ozgun@citusdata.com&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Roman Gushchin &lt;klamm@yandex-team.ru&gt;
Cc: Ryan Mallon &lt;rmallon@gmail.com&gt;
Cc: Tejun Heo &lt;tj@kernel.org&gt;
Cc: Vlastimil Babka &lt;vbabka@suse.cz&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
shmem mappings already contain exceptional entries where swap slot
information is remembered.

To be able to store eviction information for regular page cache, prepare
every site dealing with the radix trees directly to handle entries other
than pages.

The common lookup functions will filter out non-page entries and return
NULL for page cache holes, just as before.  But provide a raw version of
the API which returns non-page entries as well, and switch shmem over to
use it.

Signed-off-by: Johannes Weiner &lt;hannes@cmpxchg.org&gt;
Reviewed-by: Rik van Riel &lt;riel@redhat.com&gt;
Reviewed-by: Minchan Kim &lt;minchan@kernel.org&gt;
Cc: Andrea Arcangeli &lt;aarcange@redhat.com&gt;
Cc: Bob Liu &lt;bob.liu@oracle.com&gt;
Cc: Christoph Hellwig &lt;hch@infradead.org&gt;
Cc: Dave Chinner &lt;david@fromorbit.com&gt;
Cc: Greg Thelen &lt;gthelen@google.com&gt;
Cc: Hugh Dickins &lt;hughd@google.com&gt;
Cc: Jan Kara &lt;jack@suse.cz&gt;
Cc: KOSAKI Motohiro &lt;kosaki.motohiro@jp.fujitsu.com&gt;
Cc: Luigi Semenzato &lt;semenzato@google.com&gt;
Cc: Mel Gorman &lt;mgorman@suse.de&gt;
Cc: Metin Doslu &lt;metin@citusdata.com&gt;
Cc: Michel Lespinasse &lt;walken@google.com&gt;
Cc: Ozgun Erdogan &lt;ozgun@citusdata.com&gt;
Cc: Peter Zijlstra &lt;peterz@infradead.org&gt;
Cc: Roman Gushchin &lt;klamm@yandex-team.ru&gt;
Cc: Ryan Mallon &lt;rmallon@gmail.com&gt;
Cc: Tejun Heo &lt;tj@kernel.org&gt;
Cc: Vlastimil Babka &lt;vbabka@suse.cz&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
</pre>
</div>
</content>
</entry>
<entry>
<title>Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs</title>
<updated>2014-02-09T19:12:26+00:00</updated>
<author>
<name>Linus Torvalds</name>
<email>torvalds@linux-foundation.org</email>
</author>
<published>2014-02-09T19:12:26+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=9c1db7798141e2658e4b5bb170128dfdc3270ff4'/>
<id>9c1db7798141e2658e4b5bb170128dfdc3270ff4</id>
<content type='text'>
Pull btrfs fixes from Chris Mason:
 "This is a small collection of fixes"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs:
  Btrfs: fix data corruption when reading/updating compressed extents
  Btrfs: don't loop forever if we can't run because of the tree mod log
  btrfs: reserve no transaction units in btrfs_ioctl_set_features
  btrfs: commit transaction after setting label and features
  Btrfs: fix assert screwup for the pending move stuff
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
Pull btrfs fixes from Chris Mason:
 "This is a small collection of fixes"

* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mason/linux-btrfs:
  Btrfs: fix data corruption when reading/updating compressed extents
  Btrfs: don't loop forever if we can't run because of the tree mod log
  btrfs: reserve no transaction units in btrfs_ioctl_set_features
  btrfs: commit transaction after setting label and features
  Btrfs: fix assert screwup for the pending move stuff
</pre>
</div>
</content>
</entry>
<entry>
<title>Btrfs: fix data corruption when reading/updating compressed extents</title>
<updated>2014-02-09T01:57:15+00:00</updated>
<author>
<name>Filipe David Borba Manana</name>
<email>fdmanana@gmail.com</email>
</author>
<published>2014-02-08T15:47:46+00:00</published>
<link rel='alternate' type='text/html' href='https://git.tavy.me/linux.git/commit/?id=a2aa75e18a21b21952dc6daa9bac7c9f4426f81f'/>
<id>a2aa75e18a21b21952dc6daa9bac7c9f4426f81f</id>
<content type='text'>
When using a mix of compressed file extents and prealloc extents, it
is possible to fill a page of a file with random, garbage data from
some unrelated previous use of the page, instead of a sequence of zeroes.

A simple sequence of steps to get into such case, taken from the test
case I made for xfstests, is:

   _scratch_mkfs
   _scratch_mount "-o compress-force=lzo"
   $XFS_IO_PROG -f -c "pwrite -S 0x06 -b 18670 266978 18670" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "falloc 26450 665194" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "truncate 542872" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foobar

This results in the following file items in the fs tree:

   item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
       inode generation 6 transid 6 size 542872 block group 0 mode 100600
   item 5 key (257 INODE_REF 256) itemoff 15863 itemsize 16
       inode ref index 2 namelen 6 name: foobar
   item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
       extent data disk byte 0 nr 0 gen 6
       extent data offset 0 nr 24576 ram 266240
       extent compression 0
   item 7 key (257 EXTENT_DATA 24576) itemoff 15757 itemsize 53
       prealloc data disk byte 12849152 nr 241664 gen 6
       prealloc data offset 0 nr 241664
   item 8 key (257 EXTENT_DATA 266240) itemoff 15704 itemsize 53
       extent data disk byte 12845056 nr 4096 gen 6
       extent data offset 0 nr 20480 ram 20480
       extent compression 2
   item 9 key (257 EXTENT_DATA 286720) itemoff 15651 itemsize 53
       prealloc data disk byte 13090816 nr 405504 gen 6
       prealloc data offset 0 nr 258048

The on disk extent at offset 266240 (which corresponds to 1 single disk block),
contains 5 compressed chunks of file data. Each of the first 4 compress 4096
bytes of file data, while the last one only compresses 3024 bytes of file data.
Therefore a read into the file region [285648 ; 286720[ (length = 4096 - 3024 =
1072 bytes) should always return zeroes (our next extent is a prealloc one).

The solution here is the compression code path to zero the remaining (untouched)
bytes of the last page it uncompressed data into, as the information about how
much space the file data consumes in the last page is not known in the upper layer
fs/btrfs/extent_io.c:__do_readpage(). In __do_readpage we were correctly zeroing
the remainder of the page but only if it corresponds to the last page of the inode
and if the inode's size is not a multiple of the page size.

This would cause not only returning random data on reads, but also permanently
storing random data when updating parts of the region that should be zeroed.
For the example above, it means updating a single byte in the region [285648 ; 286720[
would store that byte correctly but also store random data on disk.

A test case for xfstests follows soon.

Signed-off-by: Filipe David Borba Manana &lt;fdmanana@gmail.com&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</content>
<content type='xhtml'>
<div xmlns='http://www.w3.org/1999/xhtml'>
<pre>
When using a mix of compressed file extents and prealloc extents, it
is possible to fill a page of a file with random, garbage data from
some unrelated previous use of the page, instead of a sequence of zeroes.

A simple sequence of steps to get into such case, taken from the test
case I made for xfstests, is:

   _scratch_mkfs
   _scratch_mount "-o compress-force=lzo"
   $XFS_IO_PROG -f -c "pwrite -S 0x06 -b 18670 266978 18670" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "falloc 26450 665194" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "truncate 542872" $SCRATCH_MNT/foobar
   $XFS_IO_PROG -c "fsync" $SCRATCH_MNT/foobar

This results in the following file items in the fs tree:

   item 4 key (257 INODE_ITEM 0) itemoff 15879 itemsize 160
       inode generation 6 transid 6 size 542872 block group 0 mode 100600
   item 5 key (257 INODE_REF 256) itemoff 15863 itemsize 16
       inode ref index 2 namelen 6 name: foobar
   item 6 key (257 EXTENT_DATA 0) itemoff 15810 itemsize 53
       extent data disk byte 0 nr 0 gen 6
       extent data offset 0 nr 24576 ram 266240
       extent compression 0
   item 7 key (257 EXTENT_DATA 24576) itemoff 15757 itemsize 53
       prealloc data disk byte 12849152 nr 241664 gen 6
       prealloc data offset 0 nr 241664
   item 8 key (257 EXTENT_DATA 266240) itemoff 15704 itemsize 53
       extent data disk byte 12845056 nr 4096 gen 6
       extent data offset 0 nr 20480 ram 20480
       extent compression 2
   item 9 key (257 EXTENT_DATA 286720) itemoff 15651 itemsize 53
       prealloc data disk byte 13090816 nr 405504 gen 6
       prealloc data offset 0 nr 258048

The on disk extent at offset 266240 (which corresponds to 1 single disk block),
contains 5 compressed chunks of file data. Each of the first 4 compress 4096
bytes of file data, while the last one only compresses 3024 bytes of file data.
Therefore a read into the file region [285648 ; 286720[ (length = 4096 - 3024 =
1072 bytes) should always return zeroes (our next extent is a prealloc one).

The solution here is the compression code path to zero the remaining (untouched)
bytes of the last page it uncompressed data into, as the information about how
much space the file data consumes in the last page is not known in the upper layer
fs/btrfs/extent_io.c:__do_readpage(). In __do_readpage we were correctly zeroing
the remainder of the page but only if it corresponds to the last page of the inode
and if the inode's size is not a multiple of the page size.

This would cause not only returning random data on reads, but also permanently
storing random data when updating parts of the region that should be zeroed.
For the example above, it means updating a single byte in the region [285648 ; 286720[
would store that byte correctly but also store random data on disk.

A test case for xfstests follows soon.

Signed-off-by: Filipe David Borba Manana &lt;fdmanana@gmail.com&gt;
Signed-off-by: Chris Mason &lt;clm@fb.com&gt;
</pre>
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