linux-stable.git/include/linux/netfs.h, branch v6.15.2

netfs: Fix a number of read-retry hangs

2025-02-13T15:00:38+00:00

Fix a number of hangs in the netfslib read-retry code, including:

 (1) netfs_reissue_read() doubles up the getting of references on
     subrequests, thereby leaking the subrequest and causing inode eviction
     to wait indefinitely.  This can lead to the kernel reporting a hang in
     the filesystem's evict_inode().

     Fix this by removing the get from netfs_reissue_read() and adding one
     to netfs_retry_read_subrequests() to deal with the one place that
     didn't double up.

 (2) The loop in netfs_retry_read_subrequests() that retries a sequence of
     failed subrequests doesn't record whether or not it retried the one
     that the "subreq" pointer points to when it leaves the loop.  It may
     not if renegotiation/repreparation of the subrequests means that fewer
     subrequests are needed to span the cumulative range of the sequence.

     Because it doesn't record this, the piece of code that discards
     now-superfluous subrequests doesn't know whether it should discard the
     one "subreq" points to - and so it doesn't.

     Fix this by noting whether the last subreq it examines is superfluous
     and if it is, then getting rid of it and all subsequent subrequests.

     If that one one wasn't superfluous, then we would have tried to go
     round the previous loop again and so there can be no further unretried
     subrequests in the sequence.

 (3) netfs_retry_read_subrequests() gets yet an extra ref on any additional
     subrequests it has to get because it ran out of ones it could reuse to
     to renegotiation/repreparation shrinking the subrequests.

     Fix this by removing that extra ref.

 (4) In netfs_retry_reads(), it was using wait_on_bit() to wait for
     NETFS_SREQ_IN_PROGRESS to be cleared on all subrequests in the
     sequence - but netfs_read_subreq_terminated() is now using a wait
     queue on the request instead and so this wait will never finish.

     Fix this by waiting on the wait queue instead.  To make this work, a
     new flag, NETFS_RREQ_RETRYING, is now set around the wait loop to tell
     the wake-up code to wake up the wait queue rather than requeuing the
     request's work item.

     Note that this flag replaces the NETFS_RREQ_NEED_RETRY flag which is
     no longer used.

 (5) Whilst not strictly anything to do with the hang,
     netfs_retry_read_subrequests() was also doubly incrementing the
     subreq_counter and re-setting the debug index, leaving a gap in the
     trace.  This is also fixed.

One of these hangs was observed with 9p and with cifs.  Others were forced
by manual code injection into fs/afs/file.c.  Firstly, afs_prepare_read()
was created to provide an changing pattern of maximum subrequest sizes:

	static int afs_prepare_read(struct netfs_io_subrequest *subreq)
	{
		struct netfs_io_request *rreq = subreq->rreq;
		if (!S_ISREG(subreq->rreq->inode->i_mode))
			return 0;
		if (subreq->retry_count < 20)
			rreq->io_streams[0].sreq_max_len =
				umax(200, 2222 - subreq->retry_count * 40);
		else
			rreq->io_streams[0].sreq_max_len = 3333;
		return 0;
	}

and pointed to by afs_req_ops.  Then the following:

	struct netfs_io_subrequest *subreq = op->fetch.subreq;
	if (subreq->error == 0 &&
	    S_ISREG(subreq->rreq->inode->i_mode) &&
	    subreq->retry_count < 20) {
		subreq->transferred = subreq->already_done;
		__clear_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
		__set_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags);
		afs_fetch_data_notify(op);
		return;
	}

was inserted into afs_fetch_data_success() at the beginning and struct
netfs_io_subrequest given an extra field, "already_done" that was set to
the value in "subreq->transferred" by netfs_reissue_read().

When reading a 4K file, the subrequests would get gradually smaller, a new
subrequest would be allocated around the 3rd retry and then eventually be
rendered superfluous when the 20th retry was hit and the limit on the first
subrequest was eased.

Fixes: e2d46f2ec332 ("netfs: Change the read result collector to only use one work item")
Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20250212222402.3618494-2-dhowells@redhat.com
Tested-by: Marc Dionne 
Tested-by: 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: 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: Add functions to build/clean a buffer in a folio_queue

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

Add two netfslib functions to build up or clean up a buffer in a
folio_queue.  The first, netfs_alloc_folioq_buffer() will add folios to a
buffer, extending up at least to the given size.  If it can, it will add
multipage folios.  The folios are optionally have the mapping set and will
have the index set according to the distance from the front of the folio
queue.

The second function will free up a folio queue and put any folios in the
queue that have the first mark set.

The netfs_folio tracepoint is also altered to cope with folios that have a
NULL mapping, and the folios being added/put will have trace lines emitted
and will be accounted in the stats.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-19-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: Drop the was_async arg from netfs_read_subreq_terminated()

2024-12-20T21:34:03+00:00

Drop the was_async argument from netfs_read_subreq_terminated().  Almost
every caller is either in process context and passes false.  Some
filesystems delegate the call to a workqueue to avoid doing the work in
their network message queue parsing thread.

The only exception is netfs_cache_read_terminated() which handles
completion in the cache - which is usually a callback from the backing
filesystem in softirq context, though it can be from process context if an
error occurred.  In this case, delegate to a workqueue.

Suggested-by: Linus Torvalds 
Link: https://lore.kernel.org/r/CAHk-=wiVC5Cgyz6QKXFu6fTaA6h4CjexDR-OV9kL6Vo5x9v8=A@mail.gmail.com/
Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-10-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Drop the error arg from netfs_read_subreq_terminated()

2024-12-20T21:34:03+00:00

Drop the error argument from netfs_read_subreq_terminated() in favour of
passing the value in subreq->error.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-9-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.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: Use a folio_queue allocation and free functions

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

Provide and use folio_queue allocation and free functions to combine the
allocation, initialisation and stat (un)accounting steps that are repeated
in several places.

Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/20241216204124.3752367-4-dhowells@redhat.com
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner

netfs: Fix is-caching check in read-retry

2024-12-20T21:07:58+00:00

netfs: Fix is-caching check in read-retry

The read-retry code checks the NETFS_RREQ_COPY_TO_CACHE flag to determine
if there might be failed reads from the cache that need turning into reads
from the server, with the intention of skipping the complicated part if it
can.  The code that set the flag, however, got lost during the read-side
rewrite.

Fix the check to see if the cache_resources are valid instead.  The flag
can then be removed.

Fixes: ee4cdf7ba857 ("netfs: Speed up buffered reading")
Signed-off-by: David Howells 
Link: https://lore.kernel.org/r/3752048.1734381285@warthog.procyon.org.uk
cc: Jeff Layton 
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner