Re: [PATCH] mm/vmscan: fix missing NR_ISOLATED counter update in MGLRU reclaim path

[Hui Zhu]

> 
> On Tue, Aug 11, 2026 at 03:17:03PM +0800, Hui Zhu wrote:
> 
> > 
> > From: Hui Zhu <zhuhui@xxxxxxxxxx>
> >  
> >  The MGLRU evict_folios() isolates folios from the LRU without updating
> >  NR_ISOLATED_ANON/FILE counters, unlike the legacy shrink_inactive_list()
> >  path.
> >  This causes compaction's too_many_isolated() check and the OOMkiller to
> >  under-count isolated pages when MGLRU reclaim is active, potentially
> >  leading to unnecessary compaction attempts or incorrect OOM decisions
> >  under memory pressure.
> > 
> This patch still doesn't cover the OOM problem, too_many_isolated is not
> used for MGLRU. I prefer to keep unifying them like what we did previously
> with writeback.

I will address this in the v2.

> 
> It might work for compaction though, did you test it?

I didn't test it, I just came to the conclusion through code analysis.
isolate_migratepages_block() calls too_many_isolated()
which reads NR_ISOLATED counters, and since MGLRU doesn't update them, the
check may under-count isolated pages and fail to throttle when it should.


> 
> > 
> > Add NR_ISOLATED counter updates in evict_folios(): increment after
> >  isolate_folios() and decrement after all retry passes complete, using
> >  the original isolated count saved before retry.


The current patch is based on mm-stable. I will rebase on mm-unstable
and use the existing nr_isolated variable.

Best,
Hui

> >  
> >  Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
> >  ---
> >  mm/vmscan.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >  
> >  diff --git a/mm/vmscan.c b/mm/vmscan.c
> >  index bc324e37c5f1..723b513574b3 100644
> >  --- a/mm/vmscan.c
> >  +++ b/mm/vmscan.c
> >  @@ -4817,6 +4817,7 @@ static int evict_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
> >  struct lru_gen_mm_walk *walk;
> >  int scanned, reclaimed;
> >  int isolated = 0, type, type_scanned;
> >  + int isolated_orig = 0;
> > 
> The variable name looks a bit confusing, and what is your based commit?
> In latest mm-unstable there is already a nr_isolated you can use.
>