[PATCH 3/4] netfs: Fix netfs_release_folio() to say no if folio dirty

[David Howells]

Fix netfs_release_folio() to say no (ie. return false) if the folio is
dirty (analogous with iomap's behaviour).  Without this, it will say yes to
the release of a dirty page by split_huge_page_to_list_to_order(), which
will result in the loss of untruncated data in the folio.

Without this, the generic/075 and generic/112 xfstests (both fsx-based
tests) fail with minimum folio size patches applied[1].

Fixes: c1ec4d7c2e13 ("netfs: Provide invalidate_folio and release_folio calls")
Signed-off-by: David Howells <dhowells at redhat.com>
cc: Matthew Wilcox (Oracle) <willy at infradead.org>
cc: Pankaj Raghav <p.raghav at samsung.com>
cc: Jeff Layton <jlayton at kernel.org>
cc: Marc Dionne <marc.dionne at auristor.com>
cc: linux-afs at lists.infradead.org
cc: netfs at lists.linux.dev
cc: linux-mm at kvack.org
cc: linux-fsdevel at vger.kernel.org
Link: https://lore.kernel.org/r/20240815090849.972355-1-kernel@pankajraghav.com/ [1]
---
 fs/netfs/misc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/netfs/misc.c b/fs/netfs/misc.c
index 554a1a4615ad..69324761fcf7 100644
--- a/fs/netfs/misc.c
+++ b/fs/netfs/misc.c
@@ -161,6 +161,9 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 	struct netfs_inode *ctx = netfs_inode(folio_inode(folio));
 	unsigned long long end;
 
+	if (folio_test_dirty(folio))
+		return false;
+
 	end = folio_pos(folio) + folio_size(folio);
 	if (end > ctx->zero_point)
 		ctx->zero_point = end;