+ mm-vmscan-always-allow-writeback-during-memcg-reclaim.patch added to mm-new branch

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

The patch titled
     Subject: mm: vmscan: always allow writeback during memcg reclaim
has been added to the -mm mm-new branch.  Its filename is
     mm-vmscan-always-allow-writeback-during-memcg-reclaim.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-vmscan-always-allow-writeback-during-memcg-reclaim.patch

This patch will later appear in the mm-new branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Note, mm-new is a provisional staging ground for work-in-progress
patches, and acceptance into mm-new is a notification for others take
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------------------------------------------------------
From: Deepanshu Kartikey <kartikey406@gmail.com>
Subject: mm: vmscan: always allow writeback during memcg reclaim
Date: Sat, 13 Dec 2025 14:06:39 +0530

When laptop_mode is enabled, may_writepage is set to 0 in
try_to_free_mem_cgroup_pages().  This triggers a warning in MGLRU's
lru_gen_shrink_lruvec():

    VM_WARN_ON_ONCE(!sc->may_writepage || !sc->may_unmap);

The warning occurs because MGLRU expects full reclaim capabilities to
function correctly.  The call path is:

    mem_cgroup_resize_max()
      try_to_free_mem_cgroup_pages()
        do_try_to_free_pages()
          shrink_node()
            shrink_lruvec()
              lru_gen_shrink_lruvec()  <-- WARNING

Unlike kswapd or direct reclaim where laptop_mode's disk-saving behavior
is a reasonable optimization, memcg limit enforcement is a hard
requirement - memory MUST be freed when a cgroup exceeds its limit.  The
may_unmap field is already set unconditionally to 1 in this path,
acknowledging that memcg reclaim needs full capabilities.

Set may_writepage unconditionally to 1 for memcg reclaim to ensure MGLRU
works correctly and memory limits are properly enforced.

Link: https://lkml.kernel.org/r/20251213083639.364539-1-kartikey406@gmail.com
Fixes: bd74fdaea146 ("mm: multi-gen LRU: support page table walks")
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Reported-by: syzbot+90fcab4d88cffed6d0d8@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=90fcab4d88cffed6d0d8
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

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

--- a/mm/vmscan.c~mm-vmscan-always-allow-writeback-during-memcg-reclaim
+++ a/mm/vmscan.c
@@ -6671,7 +6671,7 @@ unsigned long try_to_free_mem_cgroup_pag
 		.reclaim_idx = MAX_NR_ZONES - 1,
 		.target_mem_cgroup = memcg,
 		.priority = DEF_PRIORITY,
-		.may_writepage = !laptop_mode,
+		.may_writepage = 1,
 		.may_unmap = 1,
 		.may_swap = !!(reclaim_options & MEMCG_RECLAIM_MAY_SWAP),
 		.proactive = !!(reclaim_options & MEMCG_RECLAIM_PROACTIVE),
_

Patches currently in -mm which might be from kartikey406@gmail.com are

mm-vmscan-always-allow-writeback-during-memcg-reclaim.patch