Re: [PATCH v3 0/4] mm/vmscan: fix swappiness=max and clean up per-node proactive reclaim

[Barry Song]

On Fri, Aug 14, 2026 at 10:03 AM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
>
>
>
> On 8/14/2026 6:37 AM, Barry Song wrote:
> > On Tue, Jul 28, 2026 at 4:34 PM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> >>
> >>
> >>
> >> On 7/24/2026 7:12 PM, Barry Song wrote:
> >>> On Fri, Jul 24, 2026 at 10:43 AM Ridong Chen <ridong.chen@xxxxxxxxx> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 7/24/2026 8:18 AM, Andrew Morton wrote:
[...]
> >>
> >> So can we add just like?
> >>
> >> if ((sc->proactive && node_page_state_pages(pgdat, NR_SLAB_RECLAIMABLE_B) >
> >> pgdat->min_slab_pages) ||
> >>          node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages ||
> >>          node_page_state_pages(pgdat, NR_SLAB_RECLAIMABLE_B) > pgdat->min_slab_pages) {
> >
> > I feel both pgdat->min_unmapped_pages and
> > pgdat->min_slab_pages are quite broken in mainline.
> >
> > For example, even when the page cache is below
> > min_unmapped_pages, it may still be reclaimed. Similarly, slab may
> > still be reclaimed even when it is below min_slab_pages.
> >
> > Also, when both the page cache and slab are below their respective
> > thresholds, node_reclaim() may reclaim nothing even if we have
> > plenty of anon folios available.
> >
> >          if (node_pagecache_reclaimable(pgdat) <= pgdat->min_unmapped_pages &&
> >              node_page_state_pages(pgdat, NR_SLAB_RECLAIMABLE_B) <=
> >              pgdat->min_slab_pages)
> >                  return 0;
> >
> > For example, if slab > min_slab_pages but the page cache is below
> > min_unmapped_pages, we still reclaim file pages, even though the
> > comment says we should not.
> >
> > So we are not going to introduce another broken mechanism.
> > Maybe we should start by fixing the existing broken protection
> > against reclaiming slab and page cache?
> >
> For example, Maybe we can skip shrink_slab when node_page_state_pages(pgdat,
> NR_SLAB_RECLAIMABLE_B) <= pgdat->min_slab_pages?
>
> And similarly, in get_scan_count, we could avoid reclaiming file page cache if
> node_pagecache_reclaimable(pgdat) <= pgdat->min_unmapped_pages.

Yes, probably something along the lines of what you're saying. Also,
get_type_to_scan() in MGLRU.

It's a bit surprising that nobody has ever reported a bug related to
this. It may have caused performance problems that we simply haven't
realized were related to this broken protection against the
min_unmapped_pages and min_slab_pages thresholds.

I guess this fix could improve performance as well. For example, when
the inode and dentry caches are already extremely low, we shouldn't
keep struggling to reclaim them and waste time there. People have
already complained that shrink_slab() is slow and suggested having
an asynchronous shrinker for it.
Maybe it's related to something going wrong here.

Thanks
Barry