[PATCH v2] mm: Fix error handling in __filemap_get_folio() with FGP_NOWAIT

Raphael S. Carvalho
2025-02-24T08:14:41Z

original report:
https://lore.kernel.org/all/CAKhLTr1UL3ePTpYjXOx2AJfNk8Ku2EdcEfu+CH1sf3Asr=B-Dw@mail.gmail.com/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=0x…
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=0x…
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.

Fixes: 66dabbb65d67 ("mm: return an ERR_PTR from __filemap_get_folio")
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
---
mm/filemap.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 804d7365680c..d7646e73f481 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1986,8 +1986,12 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,

if (err == -EEXIST)
goto repeat;
- if (err)
+ if (err) {
+ /* Prevents -ENOMEM from escaping to user space with FGP_NOWAIT */
+ 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.
--
2.48.1