linux.git/fs/fuse/file.c, branch v2.6.26

fuse: add flag to turn on big writes

2008-05-13T15:02:26+00:00

Prior to 2.6.26 fuse only supported single page write requests.  In theory all
fuse filesystem should be able support bigger than 4k writes, as there's
nothing in the API to prevent it.  Unfortunately there's a known case in
NTFS-3G where big writes cause filesystem corruption.  There could also be
other filesystems, where the lack of testing with big write requests would
result in bugs.

To prevent such problems on a kernel upgrade, disable big writes by default,
but let filesystems set a flag to turn it on.

Signed-off-by: Miklos Szeredi 
Cc: Szabolcs Szakacsits 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: use clamp() rather than nested min/max

2008-05-01T15:04:02+00:00

clamp() exists for this use.

Signed-off-by: Harvey Harrison 
Cc: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: fix race in llseek

2008-04-30T15:29:51+00:00

Fuse doesn't use i_mutex to protect setting i_size, and so
generic_file_llseek() can be racy: it doesn't use i_size_read().

So do a fuse specific llseek method, which does use i_size_read().

[akpm@linux-foundation.org: make `retval' loff_t]
Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: fix max i/o size calculation

2008-04-30T15:29:51+00:00

Fix a bug that Werner Baumann reported: fuse can send a bigger write request
than the maximum specified.  This only affected direct_io operation.

In addition set a sane minimum for the max_read and max_write tunables, so I/O
always makes some progress.

Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: update file size on short read

2008-04-30T15:29:50+00:00

If the READ request returned a short count, then either

  - cached size is incorrect
  - filesystem is buggy, as short reads are only allowed on EOF

So assume that the size is wrong and refresh it, so that cached read() doesn't
zero fill the missing chunk.

Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: implement perform_write

2008-04-30T15:29:50+00:00

Introduce fuse_perform_write.  With fusexmp (a passthrough filesystem), large
(1MB) writes into a backing tmpfs filesystem are sped up by almost 4 times
(256MB/s vs 71MB/s).

[mszeredi@suse.cz]:

 - split into smaller functions
 - testing
 - duplicate generic_file_aio_write(), so that there's no need to add a
   new ->perform_write() a_op.  Comment from hch.

Signed-off-by: Nick Piggin 
Signed-off-by: Miklos Szeredi 
Cc: Christoph Hellwig 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: clean up setting i_size in write

2008-04-30T15:29:50+00:00

Extract common code for setting i_size in write functions into a common
helper.

Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: support writable mmap

2008-04-30T15:29:50+00:00

Quoting Linus (3 years ago, FUSE inclusion discussions):

  "User-space filesystems are hard to get right. I'd claim that they
   are almost impossible, unless you limit them somehow (shared
   writable mappings are the nastiest part - if you don't have those,
   you can reasonably limit your problems by limiting the number of
   dirty pages you accept through normal "write()" calls)."

Instead of attempting the impossible, I've just waited for the dirty page
accounting infrastructure to materialize (thanks to Peter Zijlstra and
others).  This nicely solved the biggest problem: limiting the number of pages
used for write caching.

Some small details remained, however, which this largish patch attempts to
address.  It provides a page writeback implementation for fuse, which is
completely safe against VM related deadlocks.  Performance may not be very
good for certain usage patterns, but generally it should be acceptable.

It has been tested extensively with fsx-linux and bash-shared-mapping.

Fuse page writeback design
--------------------------

fuse_writepage() allocates a new temporary page with GFP_NOFS|__GFP_HIGHMEM.
It copies the contents of the original page, and queues a WRITE request to the
userspace filesystem using this temp page.

The writeback is finished instantly from the MM's point of view: the page is
removed from the radix trees, and the PageDirty and PageWriteback flags are
cleared.

For the duration of the actual write, the NR_WRITEBACK_TEMP counter is
incremented.  The per-bdi writeback count is not decremented until the actual
write completes.

On dirtying the page, fuse waits for a previous write to finish before
proceeding.  This makes sure, there can only be one temporary page used at a
time for one cached page.

This approach is wasteful in both memory and CPU bandwidth, so why is this
complication needed?

The basic problem is that there can be no guarantee about the time in which
the userspace filesystem will complete a write.  It may be buggy or even
malicious, and fail to complete WRITE requests.  We don't want unrelated parts
of the system to grind to a halt in such cases.

Also a filesystem may need additional resources (particularly memory) to
complete a WRITE request.  There's a great danger of a deadlock if that
allocation may wait for the writepage to finish.

Currently there are several cases where the kernel can block on page
writeback:

  - allocation order is larger than PAGE_ALLOC_COSTLY_ORDER
  - page migration
  - throttle_vm_writeout (through NR_WRITEBACK)
  - sync(2)

Of course in some cases (fsync, msync) we explicitly want to allow blocking.
So for these cases new code has to be added to fuse, since the VM is not
tracking writeback pages for us any more.

As an extra safetly measure, the maximum dirty ratio allocated to a single
fuse filesystem is set to 1% by default.  This way one (or several) buggy or
malicious fuse filesystems cannot slow down the rest of the system by hogging
dirty memory.

With appropriate privileges, this limit can be raised through
'/sys/class/bdi//max_ratio'.

Signed-off-by: Miklos Szeredi 
Cc: Peter Zijlstra 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: save space in struct fuse_req

2008-02-06T18:41:13+00:00

Move the fields 'dentry' and 'vfsmount' into the request specific union, since
these are only used for the RELEASE request.

Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

fuse: pass open flags to read and write

2007-11-29T17:24:54+00:00

Some open flags (O_APPEND, O_DIRECT) can be changed with fcntl(F_SETFL, ...)
after open, but fuse currently only sends the flags to userspace in open.

To make it possible to correcly handle changing flags, send the
current value to userspace in each read and write.

Signed-off-by: Miklos Szeredi 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds