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

[Ridong Chen <ridong.chen@linux.dev>]

On 8/14/2026 10:23 AM, Barry Song wrote:
> On Fri, Aug 14, 2026 at 10:03 AM Ridong Chen <ridong.chen@linux.dev> wrote:
>>
>>
>>
>> On 8/14/2026 6:37 AM, Barry Song wrote:
>>> On Tue, Jul 28, 2026 at 4:34 PM Ridong Chen <ridong.chen@linux.dev> wrote:
>>>>
>>>>
>>>>
>>>> On 7/24/2026 7:12 PM, Barry Song wrote:
>>>>> On Fri, Jul 24, 2026 at 10:43 AM Ridong Chen <ridong.chen@linux.dev> 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.
> 
Yeah, good point. I'll give it a try and run some tests to see if it actually helps.

-- 
Best regards
Ridong