[PATCH] netfs: Fix off-by-one in the flush range in netfs_page_mkwrite()

[Fredric Cover <fredric.cover.lkernel@gmail.com>]

When a page fault wants to make a folio writable but the folio belongs
to a different netfs group (e.g. a ceph snap context) than the one the
write is being made under, netfs_page_mkwrite() flushes the folio and
retries the fault.  It does so with:

	filemap_fdatawrite_range(mapping,
				 folio_pos(folio),
				 folio_next_pos(folio));

filemap_fdatawrite_range() takes an inclusive end offset, but
folio_next_pos() is the position of the first byte of the following
folio.  The range therefore covers one byte too many, and writeback is
also run on whichever folio contains that byte.

filemap_fdatawrite_range() is a WB_SYNC_ALL operation documented as
waiting upon dirty or in-writeback pages in the range, so the faulting
task can stall behind I/O on a neighbouring folio that has nothing to
do with the fault, and a neighbouring dirty folio is written out early.

The off-by-one dates from the original implementation, which passed
folio_pos(folio) + folio_size(folio) as the end of a
filemap_fdatawait_range() call, which also takes an inclusive end.  It
was carried over when the call was changed to
filemap_fdatawrite_range().

The flush of the faulting folio itself is unaffected, so this does not
cause data loss or incorrect results, only unneeded writeback and
latency.

Fix it by passing folio_next_pos(folio) - 1, as the flush_content path
in netfs_perform_write() already does with fpos + flen - 1.

Assisted-by: LLM
Fixes: 102a7e2c598c ("netfs: Allow buffered shared-writeable mmap through netfs_page_mkwrite()")
Signed-off-by: Fredric Cover <fredric.cover.lkernel@gmail.com>
---
 fs/netfs/buffered_write.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/netfs/buffered_write.c b/fs/netfs/buffered_write.c
index ecf119b4f9166..b398c3bef6f3c 100644
--- a/fs/netfs/buffered_write.c
+++ b/fs/netfs/buffered_write.c
@@ -579,7 +579,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 		folio_unlock(folio);
 		err = filemap_fdatawrite_range(mapping,
 					       folio_pos(folio),
-					       folio_next_pos(folio));
+					       folio_next_pos(folio) - 1);
 		switch (err) {
 		case 0:
 			ret = VM_FAULT_RETRY;
-- 
2.53.0