The patch titled
Subject: mm: fix error handling in __filemap_get_folio() with FGP_NOWAIT
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-fix-error-handling-in-__filemap_get_folio-with-fgp_nowait.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-fix-error-handling-in-__filemap_get_folio-with-fgp_nowait.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: "Raphael S. Carvalho" <raphaelsc@xxxxxxxxxxxx>
Subject: mm: fix error handling in __filemap_get_folio() with FGP_NOWAIT
Date: Mon, 24 Feb 2025 11:37:00 -0300
original report:
https://lore.kernel.org/all/CAKhLTr1UL3ePTpYjXOx2AJfNk8Ku2EdcEfu+CH1sf3Asr=B-Dw@xxxxxxxxxxxxxx/T/
When doing buffered writes with FGP_NOWAIT, under memory pressure, the
system returned ENOMEM despite there being plenty of available memory, to
be reclaimed from page cache. The user space used io_uring interface,
which in turn submits I/O with FGP_NOWAIT (the fast path).
retsnoop pointed to iomap_get_folio:
00:34:16.180612 -> 00:34:16.180651 TID/PID 253786/253721
(reactor-1/combined_tests):
entry_SYSCALL_64_after_hwframe+0x76
do_syscall_64+0x82
__do_sys_io_uring_enter+0x265
io_submit_sqes+0x209
io_issue_sqe+0x5b
io_write+0xdd
xfs_file_buffered_write+0x84
iomap_file_buffered_write+0x1a6
32us [-ENOMEM] iomap_write_begin+0x408
iter=&{.inode=0xffff8c67aa031138,.len=4096,.flags=33,.iomap={.addr=0xffffffffffffffff,.length=4096,.type=1,.flags=3,.bdev=0xâ?¦
pos=0 len=4096 foliop=0xffffb32c296b7b80
! 4us [-ENOMEM] iomap_get_folio
iter=&{.inode=0xffff8c67aa031138,.len=4096,.flags=33,.iomap={.addr=0xffffffffffffffff,.length=4096,.type=1,.flags=3,.bdev=0xâ?¦
pos=0 len=4096
This is likely a regression caused by 66dabbb65d67 ("mm: return an ERR_PTR
from __filemap_get_folio"), which moved error handling from
io_map_get_folio() to __filemap_get_folio(), but broke FGP_NOWAIT
handling, so ENOMEM is being escaped to user space. Had it correctly
returned -EAGAIN with NOWAIT, either io_uring or user space itself would
be able to retry the request.
It's not enough to patch io_uring since the iomap interface is the one
responsible for it, and pwritev2(RWF_NOWAIT) and AIO interfaces must
return the proper error too.
The patch was tested with scylladb test suite (its original reproducer),
and the tests all pass now when memory is pressured.
Link: https://lkml.kernel.org/r/20250224143700.23035-1-raphaelsc@xxxxxxxxxxxx
Fixes: 66dabbb65d67 ("mm: return an ERR_PTR from __filemap_get_folio")
Signed-off-by: Raphael S. Carvalho <raphaelsc@xxxxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: "Darrick J. Wong" <djwong@xxxxxxxxxx>
Cc: Matthew Wilcow (Oracle) <willy@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
--- a/mm/filemap.c~mm-fix-error-handling-in-__filemap_get_folio-with-fgp_nowait
+++ a/mm/filemap.c
@@ -1986,8 +1986,19 @@ no_page:
if (err == -EEXIST)
goto repeat;
- if (err)
+ if (err) {
+ /*
+ * When NOWAIT I/O fails to allocate folios this could
+ * be due to a nonblocking memory allocation and not
+ * because the system actually is out of memory.
+ * Return -EAGAIN so that there caller retries in a
+ * blocking fashion instead of propagating -ENOMEM
+ * to the application.
+ */
+ if ((fgp_flags & FGP_NOWAIT) && err == -ENOMEM)
+ err = -EAGAIN;
return ERR_PTR(err);
+ }
/*
* filemap_add_folio locks the page, and for mmap
* we expect an unlocked page.
_
Patches currently in -mm which might be from raphaelsc@xxxxxxxxxxxx are
mm-fix-error-handling-in-__filemap_get_folio-with-fgp_nowait.patch
