Re: [PATCH v2 4/4] mm/mglru: fix anon-only reclaim evicting file pages when swappiness=max

[Ridong Chen]

On 7/20/2026 10:48 AM, Qi Zheng wrote:
> On 7/18/26 9:43 PM, Barry Song wrote:
> > On Sat, Jul 18, 2026 at 5:54 PM Ridong Chen <ridong.chen@xxxxxxxxx> 
> > wrote:
> > > From: Ridong Chen <chenridong@xxxxxxxxxx>
> > >
> > > The previous patch fixes this issue for the traditional LRU, which also
> > > exists in MGLRU [1]. Fix this by checking whether swappiness is
> > > SWAPPINESS_ANON_ONLY in get_swappiness() first, and returning 0 from
> > > get_nr_to_scan() when swappiness is SWAPPINESS_ANON_ONLY and anon pages
> > > cannot be reclaimed, to avoid useless work.
> >
> > Is this intended to avoid unnecessary work, or to prevent file folios
> > from being reclaimed incorrectly?
>
> Based on the test results below, it seems primarily intended to prevent
> file pages from being mistakenly reclaimed. Also, bailing out early in
> the SWAPPINESS_ANON_ONLY + !can_reclaim_anon_pages case is to avoid
> unnecessary work.
>
> Hi Ridong, perhaps the commit message could be clearer. ;)

Thanks for the feedback. Let me clarify:

When SWAPPINESS_ANON_ONLY is set, get_swappiness() returns the 
swappiness value, and get_type_to_scan() can always return LRU_GEN_ANON. 
So if anonymous pages cannot be reclaimed, we bail out early to avoid 
unnecessary work. In that case, get_nr_to_scan() just returns 0.

If we do not return early in get_nr_to_scan() under the 
SWAPPINESS_ANON_ONLY + !can_reclaim_anon_pages condition, then in 
isolate_folios(), it may fall back to scanning file pages, but this only 
happens when scanned = 0, which is not the usual case.


```
shrink_one
try_to_shrink_lruvec
  get_swappiness // return SWAPPINESS_ANON_ONLY if swappiness=max
  get_nr_to_scan // return 0 if anon can't be reclaimed, skip
  evict_folios
    isolate_folios // !scanned falling back to anyther type if !scanned
```

I'll update the commit message to make this clearer in the next version.

> > > The test result:
> > > Before fix:
> > >
> > >    # cat /sys/kernel/mm/lru_gen/enabled
> > >    0x0007
> > >    # cat memory.stat
> > >    anon 204800
> > >    file 67108864
> > >    ...
> > >    pgsteal_proactive 0
> > >    pgscan_proactive 0
> > >
> > >    # echo "64M swappiness=max" > memory.reclaim
> > >    # cat memory.stat
> > >    anon 208896
> > >    file 0
> > >    ...
> > >    pgsteal_proactive 16384
> > >    pgscan_proactive 16384
> > >
> > > After fix:
> > >
> > >    # cat memory.stat
> > >    anon 188416
> > >    file 67215360
> > >    kernel 1970176
> > >    ...
> > >    pgsteal_proactive 0
> > >    pgscan_proactive 0
> > >
> > >    # echo "64M swappiness=max" > memory.reclaim
> > >    -bash: echo: write error: Resource temporarily unavailable
> > >    # cat memory.stat
> > >    anon 204800
> > >    file 67215360
> > >    ...
> > >    pgsteal_proactive 0
> > >    pgscan_proactive 0
> > >
> > > [1] https://sashiko.dev/#/patchset/20260717113300.214717-1- 
> > > ridong.chen@xxxxxxxxx
> > >
> > > Fixes: 68a1436bde00 ("mm: add swappiness=max arg to memory.reclaim 
> > > for only anon reclaim")
> > > Signed-off-by: Ridong Chen <chenridong@xxxxxxxxxx>
> > > ---
> > >   mm/vmscan.c | 14 +++++++++++++-
> > >   1 file changed, 13 insertions(+), 1 deletion(-)
>
> Overall, this looks good to me.
>
> Acked-by: Qi Zheng <qi.zheng@xxxxxxxxx>
>
> Thanks,
> Qi

--
Best regards
Ridong