[PATCH] fs: break out of iomap_file_buffered_write on fatal signals

[Michal Hocko]

Tetsuo has noticed that an OOM stress test which performs large write
requests can cause the full memory reserves depletion. He has tracked
this down to the following path
	__alloc_pages_nodemask+0x436/0x4d0
	alloc_pages_current+0x97/0x1b0
	__page_cache_alloc+0x15d/0x1a0          mm/filemap.c:728
	pagecache_get_page+0x5a/0x2b0           mm/filemap.c:1331
	grab_cache_page_write_begin+0x23/0x40   mm/filemap.c:2773
	iomap_write_begin+0x50/0xd0             fs/iomap.c:118
	iomap_write_actor+0xb5/0x1a0            fs/iomap.c:190
	? iomap_write_end+0x80/0x80             fs/iomap.c:150
	iomap_apply+0xb3/0x130                  fs/iomap.c:79
	iomap_file_buffered_write+0x68/0xa0     fs/iomap.c:243
	? iomap_write_end+0x80/0x80
	xfs_file_buffered_aio_write+0x132/0x390 [xfs]
	? remove_wait_queue+0x59/0x60
	xfs_file_write_iter+0x90/0x130 [xfs]
	__vfs_write+0xe5/0x140
	vfs_write+0xc7/0x1f0
	? syscall_trace_enter+0x1d0/0x380
	SyS_write+0x58/0xc0
	do_syscall_64+0x6c/0x200
	entry_SYSCALL64_slow_path+0x25/0x25

the oom victim has access to all memory reserves to make a forward
progress to exit easier. But iomap_file_buffered_write loops to complete
the full request. We need to check for fatal signals and back off with a
short write.

Fixes: 68a9f5e7007c ("xfs: implement iomap based buffered write path")
Cc: stable # 4.8+
Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 fs/iomap.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/iomap.c b/fs/iomap.c
index e57b90b5ff37..a22672387549 100644
--- a/fs/iomap.c
+++ b/fs/iomap.c
@@ -238,6 +238,10 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *iter,
 	loff_t pos = iocb->ki_pos, ret = 0, written = 0;
 
 	while (iov_iter_count(iter)) {
+		if (fatal_signal_pending(current)) {
+			ret = -EINTR;
+			break;
+		}
 		ret = iomap_apply(inode, pos, iov_iter_count(iter),
 				IOMAP_WRITE, ops, iter, iomap_write_actor);
 		if (ret <= 0)
-- 
2.11.0


-- 
Michal Hocko
SUSE Labs