linux.git/fs/netfs/write_issue.c, branch v6.16

netfs: Fix wait/wake to be consistent about the waitqueue used

2025-05-21T12:35:21+00:00

Fix further inconsistencies in the use of waitqueues
(clear_and_wake_up_bit() vs private waitqueue).

Move some of this stuff from the read and write sides into common code so
that it can be done in fewer places.

To make this work, async I/O needs to set NETFS_RREQ_OFFLOAD_COLLECTION to
indicate that a workqueue will do the collecting and places that call the
wait function need to deal with it returning the amount transferred.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Signed-off-by: David Howells 
Link: https://lore.kernel.org/20250519090707.2848510-5-dhowells@redhat.com
cc: Marc Dionne 
cc: Steve French 
cc: Ihor Solodrai 
cc: Eric Van Hensbergen 
cc: Latchesar Ionkov 
cc: Dominique Martinet 
cc: Christian Schoenebeck 
cc: Paulo Alcantara 
cc: Jeff Layton 
cc: v9fs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Fix the request's work item to not require a ref

2025-05-21T12:35:20+00:00

When the netfs_io_request struct's work item is queued, it must be supplied
with a ref to the work item struct to prevent it being deallocated whilst
on the queue or whilst it is being processed.  This is tricky to manage as
we have to get a ref before we try and queue it and then we may find it's
already queued and is thus already holding a ref - in which case we have to
try and get rid of the ref again.

The problem comes if we're in BH or IRQ context and need to drop the ref:
if netfs_put_request() reduces the count to 0, we have to do the cleanup -
but the cleanup may need to wait.

Fix this by adding a new work item to the request, ->cleanup_work, and
dispatching that when the refcount hits zero.  That can then synchronously
cancel any outstanding work on the main work item before doing the cleanup.

Adding a new work item also deals with another problem upstream where it's
sometimes changing the work func in the put function and requeuing it -
which has occasionally in the past caused the cleanup to happen
incorrectly.

As a bonus, this allows us to get rid of the 'was_async' parameter from a
bunch of functions.  This indicated whether the put function might not be
permitted to sleep.

Fixes: 3d3c95046742 ("netfs: Provide readahead and readpage netfs helpers")
Signed-off-by: David Howells 
Link: https://lore.kernel.org/20250519090707.2848510-4-dhowells@redhat.com
cc: Paulo Alcantara 
cc: Marc Dionne 
cc: Steve French 
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Add retry stat counters

2025-02-13T15:00:48+00:00

Add stat counters to count the number of request and subrequest retries and
display them in /proc/fs/netfs/stats.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20250212222402.3618494-3-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Change the read result collector to only use one work item

2024-12-20T21:34:08+00:00

Change the way netfslib collects read results to do all the collection for
a particular read request using a single work item that walks along the
subrequest queue as subrequests make progress or complete, unlocking folios
progressively rather than doing the unlock in parallel as parallel requests
come in.

The code is remodelled to be more like the write-side code, though only
using a single stream.  This makes it more directly comparable and thus
easier to duplicate fixes between the two sides.

This has a number of advantages:

 (1) It's simpler.  There doesn't need to be a complex donation mechanism
     to handle mismatches between the size and alignment of subrequests and
     folios.  The collector unlocks folios as the subrequests covering each
     complete.

 (2) It should cause less scheduler overhead as there's a single work item
     in play unlocking pages in parallel when a read gets split up into a
     lot of subrequests instead of one per subrequest.

     Whilst the parallellism is nice in theory, in practice, the vast
     majority of loads are sequential reads of the whole file, so
     committing a bunch of threads to unlocking folios out of order doesn't
     help in those cases.

 (3) It should make it easier to implement content decryption.  A folio
     cannot be decrypted until all the requests that contribute to it have
     completed - and, again, most loads are sequential and so, most of the
     time, we want to begin decryption sequentially (though it's great if
     the decryption can happen in parallel).

There is a disadvantage in that we're losing the ability to decrypt and
unlock things on an as-things-arrive basis which may affect some
applications.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-28-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Add support for caching single monolithic objects such as AFS dirs

2024-12-20T21:34:06+00:00

Add support for caching the content of a file that contains a single
monolithic object that must be read/written with a single I/O operation,
such as an AFS directory.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-20-dhowells@redhat.com
cc: Jeff Layton 
cc: Marc Dionne 
cc: netfs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Don't use bh spinlock

2024-12-20T21:34:04+00:00

All the accessing of the subrequest lists is now done in process context,
possibly in a workqueue, but not now in a BH context, so we don't need the
lock against BH interference when taking the netfs_io_request::lock
spinlock.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-11-dhowells@redhat.com
cc: Jeff Layton 
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Christian Brauner

netfs: Make netfs_advance_write() return size_t

2024-12-20T21:34:02+00:00

netfs_advance_write() calculates the amount of data it's attaching to a
stream with size_t, but then returns this as an int.  Switch the return
value to size_t for consistency.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-7-dhowells@redhat.com
cc: Jeff Layton 
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org
cc: linux-mm@kvack.org
Signed-off-by: Christian Brauner

netfs: Abstract out a rolling folio buffer implementation

2024-12-20T21:34:02+00:00

A rolling buffer is a series of folios held in a list of folio_queues.  New
folios and folio_queue structs may be inserted at the head simultaneously
with spent ones being removed from the tail without the need for locking.

The rolling buffer includes an iov_iter and it has to be careful managing
this as the list of folio_queues is extended such that an oops doesn't
incurred because the iterator was pointing to the end of a folio_queue
segment that got appended to and then removed.

We need to use the mechanism twice, once for read and once for write, and,
in future patches, we will use a second rolling buffer to handle bounce
buffering for content encryption.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-6-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Add a tracepoint to log the lifespan of folio_queue structs

2024-12-20T21:34:02+00:00

Add a tracepoint to log the lifespan of folio_queue structs.  For tracing
illustrative purposes, folio_queues are tagged with the debug ID of
whatever they're related to (typically a netfs_io_request) and a debug ID
of their own.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-5-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Work around recursion by abandoning retry if nothing read

2024-12-20T21:07:57+00:00

syzkaller reported recursion with a loop of three calls (netfs_rreq_assess,
netfs_retry_reads and netfs_rreq_terminated) hitting the limit of the stack
during an unbuffered or direct I/O read.

There are a number of issues:

 (1) There is no limit on the number of retries.

 (2) A subrequest is supposed to be abandoned if it does not transfer
     anything (NETFS_SREQ_NO_PROGRESS), but that isn't checked under all
     circumstances.

 (3) The actual root cause, which is this:

	if (atomic_dec_and_test(&rreq->nr_outstanding))
		netfs_rreq_terminated(rreq, ...);

     When we do a retry, we bump the rreq->nr_outstanding counter to
     prevent the final cleanup phase running before we've finished
     dispatching the retries.  The problem is if we hit 0, we have to do
     the cleanup phase - but we're in the cleanup phase and end up
     repeating the retry cycle, hence the recursion.

Work around the problem by limiting the number of retries.  This is based
on Lizhi Xu's patch[1], and makes the following changes:

 (1) Replace NETFS_SREQ_NO_PROGRESS with NETFS_SREQ_MADE_PROGRESS and make
     the filesystem set it if it managed to read or write at least one byte
     of data.  Clear this bit before issuing a subrequest.

 (2) Add a ->retry_count member to the subrequest and increment it any time
     we do a retry.

 (3) Remove the NETFS_SREQ_RETRYING flag as it is superfluous with
     ->retry_count.  If the latter is non-zero, we're doing a retry.

 (4) Abandon a subrequest if retry_count is non-zero and we made no
     progress.

 (5) Use ->retry_count in both the write-side and the read-size.

[?] Question: Should I set a hard limit on retry_count in both read and
    write?  Say it hits 50, we always abandon it.  The problem is that
    these changes only mitigate the issue.  As long as it made at least one
    byte of progress, the recursion is still an issue.  This patch
    mitigates the problem, but does not fix the underlying cause.  I have
    patches that will do that, but it's an intrusive fix that's currently
    pending for the next merge window.

The oops generated by KASAN looks something like:

   BUG: TASK stack guard page was hit at ffffc9000482ff48 (stack is ffffc90004830000..ffffc90004838000)
   Oops: stack guard page: 0000 [#1] PREEMPT SMP KASAN NOPTI
   ...
   RIP: 0010:mark_lock+0x25/0xc60 kernel/locking/lockdep.c:4686
    ...
    mark_usage kernel/locking/lockdep.c:4646 [inline]
    __lock_acquire+0x906/0x3ce0 kernel/locking/lockdep.c:5156
    lock_acquire.part.0+0x11b/0x380 kernel/locking/lockdep.c:5825
    local_lock_acquire include/linux/local_lock_internal.h:29 [inline]
    ___slab_alloc+0x123/0x1880 mm/slub.c:3695
    __slab_alloc.constprop.0+0x56/0xb0 mm/slub.c:3908
    __slab_alloc_node mm/slub.c:3961 [inline]
    slab_alloc_node mm/slub.c:4122 [inline]
    kmem_cache_alloc_noprof+0x2a7/0x2f0 mm/slub.c:4141
    radix_tree_node_alloc.constprop.0+0x1e8/0x350 lib/radix-tree.c:253
    idr_get_free+0x528/0xa40 lib/radix-tree.c:1506
    idr_alloc_u32+0x191/0x2f0 lib/idr.c:46
    idr_alloc+0xc1/0x130 lib/idr.c:87
    p9_tag_alloc+0x394/0x870 net/9p/client.c:321
    p9_client_prepare_req+0x19f/0x4d0 net/9p/client.c:644
    p9_client_zc_rpc.constprop.0+0x105/0x880 net/9p/client.c:793
    p9_client_read_once+0x443/0x820 net/9p/client.c:1570
    p9_client_read+0x13f/0x1b0 net/9p/client.c:1534
    v9fs_issue_read+0x115/0x310 fs/9p/vfs_addr.c:74
    netfs_retry_read_subrequests fs/netfs/read_retry.c:60 [inline]
    netfs_retry_reads+0x153a/0x1d00 fs/netfs/read_retry.c:232
    netfs_rreq_assess+0x5d3/0x870 fs/netfs/read_collect.c:371
    netfs_rreq_terminated+0xe5/0x110 fs/netfs/read_collect.c:407
    netfs_retry_reads+0x155e/0x1d00 fs/netfs/read_retry.c:235
    netfs_rreq_assess+0x5d3/0x870 fs/netfs/read_collect.c:371
    netfs_rreq_terminated+0xe5/0x110 fs/netfs/read_collect.c:407
    netfs_retry_reads+0x155e/0x1d00 fs/netfs/read_retry.c:235
    netfs_rreq_assess+0x5d3/0x870 fs/netfs/read_collect.c:371
    ...
    netfs_rreq_terminated+0xe5/0x110 fs/netfs/read_collect.c:407
    netfs_retry_reads+0x155e/0x1d00 fs/netfs/read_retry.c:235
    netfs_rreq_assess+0x5d3/0x870 fs/netfs/read_collect.c:371
    netfs_rreq_terminated+0xe5/0x110 fs/netfs/read_collect.c:407
    netfs_retry_reads+0x155e/0x1d00 fs/netfs/read_retry.c:235
    netfs_rreq_assess+0x5d3/0x870 fs/netfs/read_collect.c:371
    netfs_rreq_terminated+0xe5/0x110 fs/netfs/read_collect.c:407
    netfs_dispatch_unbuffered_reads fs/netfs/direct_read.c:103 [inline]
    netfs_unbuffered_read fs/netfs/direct_read.c:127 [inline]
    netfs_unbuffered_read_iter_locked+0x12f6/0x19b0 fs/netfs/direct_read.c:221
    netfs_unbuffered_read_iter+0xc5/0x100 fs/netfs/direct_read.c:256
    v9fs_file_read_iter+0xbf/0x100 fs/9p/vfs_file.c:361
    do_iter_readv_writev+0x614/0x7f0 fs/read_write.c:832
    vfs_readv+0x4cf/0x890 fs/read_write.c:1025
    do_preadv fs/read_write.c:1142 [inline]
    __do_sys_preadv fs/read_write.c:1192 [inline]
    __se_sys_preadv fs/read_write.c:1187 [inline]
    __x64_sys_preadv+0x22d/0x310 fs/read_write.c:1187
    do_syscall_x64 arch/x86/entry/common.c:52 [inline]
    do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Closes: https://syzkaller.appspot.com/bug?extid=1fc6f64c40a9d143cfb6
Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241108034020.3695718-1-lizhi.xu@windriver.com/ [1]
Link: https://lore.kernel.org/r/20241213135013.2964079-9-dhowells@redhat.com
Tested-by: syzbot+885c03ad650731743489@syzkaller.appspotmail.com
Suggested-by: Lizhi Xu 
cc: Dominique Martinet 
cc: Jeff Layton 
cc: v9fs@lists.linux.dev
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Reported-by: syzbot+885c03ad650731743489@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner