[PATCH] netfs: Fix a number of read-retry hangs

David Howells
2025-01-28T00:33:20Z

Hi Ihor, Marc, Steve,

I think this patch should better fix the hangs that have been seen than Ihor's
previously tested fix (which I think isn't actually correct).

David
---
From: David Howells <dhowells@redhat.com>

netfs: Fix a number of read-retry hangs

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.

Reported-by: Ihor Solodrai <ihor.solodrai@pm.me>
Closes: https://lore.kernel.org/r/a7x33d4dnMdGTtRivptq6S1i8btK70SNBP2XyX_xwDAhLvgQoPox6FVBOkifq4eBinfFfbZlIkMZBe3QarlWTxoEtHZwJCZbNKtaqrR7PvI=@pm.me/
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Steve French <stfrench@microsoft.com>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: v9fs@lists.linux.dev
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
---
fs/netfs/read_collect.c | 6 ++++--
fs/netfs/read_retry.c | 40 ++++++++++++++++++++++++++++++----------
include/linux/netfs.h | 2 +-
include/trace/events/netfs.h | 4 +++-
4 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
index f65affa5a9e4..636cc5a98ef5 100644
--- a/fs/netfs/read_collect.c
+++ b/fs/netfs/read_collect.c
@@ -470,7 +470,8 @@ void netfs_read_collection_worker(struct work_struct *work)
*/
void netfs_wake_read_collector(struct netfs_io_request *rreq)
{
- if (test_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &rreq->flags)) {
+ if (test_bit(NETFS_RREQ_OFFLOAD_COLLECTION, &rreq->flags) &&
+ !test_bit(NETFS_RREQ_RETRYING, &rreq->flags)) {
if (!work_pending(&rreq->work)) {
netfs_get_request(rreq, netfs_rreq_trace_get_work);
if (!queue_work(system_unbound_wq, &rreq->work))
@@ -586,7 +587,8 @@ void netfs_read_subreq_terminated(struct netfs_io_subrequest *subreq)
smp_mb__after_atomic(); /* Clear IN_PROGRESS before task state */

/* If we are at the head of the queue, wake up the collector. */
- if (list_is_first(&subreq->rreq_link, &stream->subrequests))
+ if (list_is_first(&subreq->rreq_link, &stream->subrequests) ||
+ test_bit(NETFS_RREQ_RETRYING, &rreq->flags))
netfs_wake_read_collector(rreq);

netfs_put_subrequest(subreq, true, netfs_sreq_trace_put_terminated);
diff --git a/fs/netfs/read_retry.c b/fs/netfs/read_retry.c
index 2290af0d51ac..8316c4533a51 100644
--- a/fs/netfs/read_retry.c
+++ b/fs/netfs/read_retry.c
@@ -14,7 +14,6 @@ static void netfs_reissue_read(struct netfs_io_request *rreq,
{
__clear_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
__set_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags);
- netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
subreq->rreq->netfs_ops->issue_read(subreq);
}

@@ -48,6 +47,7 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
__clear_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
subreq->retry_count++;
netfs_reset_iter(subreq);
+ netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
netfs_reissue_read(rreq, subreq);
}
}
@@ -75,7 +75,7 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
struct iov_iter source;
unsigned long long start, len;
size_t part;
- bool boundary = false;
+ bool boundary = false, subreq_superfluous = false;

/* Go through the subreqs and find the next span of contiguous
* buffer that we then rejig (cifs, for example, needs the
@@ -116,8 +116,10 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
/* Work through the sublist. */
subreq = from;
list_for_each_entry_from(subreq, &stream->subrequests, rreq_link) {
- if (!len)
+ if (!len) {
+ subreq_superfluous = true;
break;
+ }
subreq->source = NETFS_DOWNLOAD_FROM_SERVER;
subreq->start = start - subreq->transferred;
subreq->len = len + subreq->transferred;
@@ -154,19 +156,21 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)

netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);
netfs_reissue_read(rreq, subreq);
- if (subreq == to)
+ if (subreq == to) {
+ subreq_superfluous = false;
break;
+ }
}

/* If we managed to use fewer subreqs, we can discard the
* excess; if we used the same number, then we're done.
*/
if (!len) {
- if (subreq == to)
+ if (!subreq_superfluous)
continue;
list_for_each_entry_safe_from(subreq, tmp,
&stream->subrequests, rreq_link) {
- trace_netfs_sreq(subreq, netfs_sreq_trace_discard);
+ trace_netfs_sreq(subreq, netfs_sreq_trace_superfluous);
list_del(&subreq->rreq_link);
netfs_put_subrequest(subreq, false, netfs_sreq_trace_put_done);
if (subreq == to)
@@ -187,14 +191,12 @@ static void netfs_retry_read_subrequests(struct netfs_io_request *rreq)
subreq->source = NETFS_DOWNLOAD_FROM_SERVER;
subreq->start = start;
subreq->len = len;
- subreq->debug_index = atomic_inc_return(&rreq->subreq_counter);
subreq->stream_nr = stream->stream_nr;
subreq->retry_count = 1;

trace_netfs_sreq_ref(rreq->debug_id, subreq->debug_index,
refcount_read(&subreq->ref),
netfs_sreq_trace_new);
- netfs_get_subrequest(subreq, netfs_sreq_trace_get_resubmit);

list_add(&subreq->rreq_link, &to->rreq_link);
to = list_next_entry(to, rreq_link);
@@ -256,14 +258,32 @@ void netfs_retry_reads(struct netfs_io_request *rreq)
{
struct netfs_io_subrequest *subreq;
struct netfs_io_stream *stream = &rreq->io_streams[0];
+ DEFINE_WAIT(myself);
+
+ set_bit(NETFS_RREQ_RETRYING, &rreq->flags);

/* Wait for all outstanding I/O to quiesce before performing retries as
* we may need to renegotiate the I/O sizes.
*/
list_for_each_entry(subreq, &stream->subrequests, rreq_link) {
- wait_on_bit(&subreq->flags, NETFS_SREQ_IN_PROGRESS,
- TASK_UNINTERRUPTIBLE);
+ if (!test_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags))
+ continue;
+
+ trace_netfs_rreq(rreq, netfs_rreq_trace_wait_queue);
+ for (;;) {
+ prepare_to_wait(&rreq->waitq, &myself, TASK_UNINTERRUPTIBLE);
+
+ if (!test_bit(NETFS_SREQ_IN_PROGRESS, &subreq->flags))
+ break;
+
+ trace_netfs_sreq(subreq, netfs_sreq_trace_wait_for);
+ schedule();
+ trace_netfs_rreq(rreq, netfs_rreq_trace_woke_queue);
+ }
+
+ finish_wait(&rreq->waitq, &myself);
}
+ clear_bit(NETFS_RREQ_RETRYING, &rreq->flags);

trace_netfs_rreq(rreq, netfs_rreq_trace_resubmit);
netfs_retry_read_subrequests(rreq);
diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 071d05d81d38..c86a11cfc4a3 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -278,7 +278,7 @@ struct netfs_io_request {
#define NETFS_RREQ_PAUSE 11 /* Pause subrequest generation */
#define NETFS_RREQ_USE_IO_ITER 12 /* Use ->io_iter rather than ->i_pages */
#define NETFS_RREQ_ALL_QUEUED 13 /* All subreqs are now queued */
-#define NETFS_RREQ_NEED_RETRY 14 /* Need to try retrying */
+#define NETFS_RREQ_RETRYING 14 /* Set if we're in the retry path */
#define NETFS_RREQ_USE_PGPRIV2 31 /* [DEPRECATED] Use PG_private_2 to mark
* write to cache on read */
const struct netfs_request_ops *netfs_ops;
diff --git a/include/trace/events/netfs.h b/include/trace/events/netfs.h
index 6e699cadcb29..f880835f7695 100644
--- a/include/trace/events/netfs.h
+++ b/include/trace/events/netfs.h
@@ -99,7 +99,7 @@
EM(netfs_sreq_trace_limited, "LIMIT") \
EM(netfs_sreq_trace_need_clear, "N-CLR") \
EM(netfs_sreq_trace_partial_read, "PARTR") \
- EM(netfs_sreq_trace_need_retry, "NRTRY") \
+ EM(netfs_sreq_trace_need_retry, "ND-RT") \
EM(netfs_sreq_trace_prepare, "PREP ") \
EM(netfs_sreq_trace_prep_failed, "PRPFL") \
EM(netfs_sreq_trace_progress, "PRGRS") \
@@ -108,7 +108,9 @@
EM(netfs_sreq_trace_short, "SHORT") \
EM(netfs_sreq_trace_split, "SPLIT") \
EM(netfs_sreq_trace_submit, "SUBMT") \
+ EM(netfs_sreq_trace_superfluous, "SPRFL") \
EM(netfs_sreq_trace_terminated, "TERM ") \
+ EM(netfs_sreq_trace_wait_for, "_WAIT") \
EM(netfs_sreq_trace_write, "WRITE") \
EM(netfs_sreq_trace_write_skip, "SKIP ") \
E_(netfs_sreq_trace_write_term, "WTERM")