Re: [PATCH v10] mm: vmscan: try to reclaim swapcache pages if no swap space

[Huang, Ying]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Tue 28-11-23 11:19:20, Huang, Ying wrote:
>> Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
>> 
>> > On Mon, Nov 27, 2023 at 1:32 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>> >>
>> >> On Mon, Nov 27, 2023 at 12:22:59AM -0800, Chris Li wrote:
>> >> > On Mon, Nov 27, 2023 at 12:14 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >> > > >  I agree with Ying that anonymous pages typically have different page
>> >> > > > access patterns than file pages, so we might want to treat them
>> >> > > > differently to reclaim them effectively.
>> >> > > > One random idea:
>> >> > > > How about we put the anonymous page in a swap cache in a different LRU
>> >> > > > than the rest of the anonymous pages. Then shrinking against those
>> >> > > > pages in the swap cache would be more effective.Instead of having
>> >> > > > [anon, file] LRU, now we have [anon not in swap cache, anon in swap
>> >> > > > cache, file] LRU
>> >> > >
>> >> > > I don't think that it is necessary.  The patch is only for a special use
>> >> > > case.  Where the swap device is used up while some pages are in swap
>> >> > > cache.  The patch will kill performance, but it is used to avoid OOM
>> >> > > only, not to improve performance.  Per my understanding, we will not use
>> >> > > up swap device space in most cases.  This may be true for ZRAM, but will
>> >> > > we keep pages in swap cache for long when we use ZRAM?
>> >> >
>> >> > I ask the question regarding how many pages can be freed by this patch
>> >> > in this email thread as well, but haven't got the answer from the
>> >> > author yet. That is one important aspect to evaluate how valuable is
>> >> > that patch.
>> >>
>> >> Exactly. Since swap cache has different life time with page cache, they
>> >> would be usually dropped when pages are unmapped(unless they are shared
>> >> with others but anon is usually exclusive private) so I wonder how much
>> >> memory we can save.
>> >
>> > I think the point of this patch is not saving memory, but rather
>> > avoiding an OOM condition that will happen if we have no swap space
>> > left, but some pages left in the swap cache. Of course, the OOM
>> > avoidance will come at the cost of extra work in reclaim to swap those
>> > pages out.
>> >
>> > The only case where I think this might be harmful is if there's plenty
>> > of pages to reclaim on the file LRU, and instead we opt to chase down
>> > the few swap cache pages. So perhaps we can add a check to only set
>> > sc->swapcache_only if the number of pages in the swap cache is more
>> > than the number of pages on the file LRU or similar? Just make sure we
>> > don't chase the swapcache pages down if there's plenty to scan on the
>> > file LRU?
>> 
>> The swap cache pages can be divided to 3 groups.
>> 
>> - group 1: pages have been written out, at the tail of inactive LRU, but
>>   not reclaimed yet.
>> 
>> - group 2: pages have been written out, but were failed to be reclaimed
>>   (e.g., were accessed before reclaiming)
>> 
>> - group 3: pages have been swapped in, but were kept in swap cache.  The
>>   pages may be in active LRU.
>> 
>> The main target of the original patch should be group 1.  And the pages
>> may be cheaper to reclaim than file pages.
>
> Thanks this is really useful summary. And it begs question. How are we
> telling those different types from each other? vmstat counter is
> certainly not sufficient and that means we might be scanning a lot
> without actually making any progress. And doing that repeatedly.

We don't have counters for pages in individual groups.  Pages in group 1
and some pages in group 2 are always at the tail of inactive LRU.  So,
we can identify them relatively easily.  So a simple method could be, if
there are swap cache, try scan the tail of inactive LRU to free pages in
group 1 and move pages in group 2.  If we found some pages aren't in
swap cache, there may be no pages in group 1.  Then, we may give up
scanning if the memory pressure isn't too large.

One possible issue is that some pages (map count == 1) may be swapped
out, swapped in, then deleted from swap cache.  So, some pages not in
swap cache may be at the end of inactive LRU too.

--
Best Regards,
Huang, Ying