+ mm-filemap-retain-mapped-dropbehind-folios.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: filemap: retain mapped dropbehind folios
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-filemap-retain-mapped-dropbehind-folios.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-retain-mapped-dropbehind-folios.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: Wenjie Qi <qiwenjie@xiaomi.com>
Subject: mm: filemap: retain mapped dropbehind folios
Date: Sun, 30 Aug 2026 01:36:12 +0800

Fault-around can map ready dropbehind folios without going through the
normal page-cache lookup that clears dropbehind.  A mapping represents a
competing cached user, so retain the folio instead of forcibly unmapping
it when writeback completes.

For a mapped folio, folio_unmap_invalidate() can call
unmap_mapping_folio(), which takes i_mmap_rwsem and may sleep.  Retaining
mapped folios avoids this path when folio_end_dropbehind() runs in
non-preemptible task context.

Unmapped dropbehind folios continue through the existing invalidation path.

Link: https://lore.kernel.org/4aba05e1a2c3b61cb337d373eb9b7a8db4ddd822.1788024049.git.qiwenjie@xiaomi.com
Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Tal Zussman <tz2294@columbia.edu>
Cc: Barry Song <baohua@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/filemap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/filemap.c~mm-filemap-retain-mapped-dropbehind-folios
+++ a/mm/filemap.c
@@ -1616,7 +1616,7 @@ static void filemap_end_dropbehind(struc
 		return;
 	if (!folio_test_clear_dropbehind(folio))
 		return;
-	if (mapping)
+	if (mapping && !folio_mapped(folio))
 		folio_unmap_invalidate(mapping, folio, 0);
 }
 
_

Patches currently in -mm which might be from qiwenjie@xiaomi.com are

mm-filemap-retain-mapped-dropbehind-folios.patch