linux-kernel - Re: [PATCH 2/2] mm: multi-gen lru: fix stat count

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Message-ID: <CAOUHufYiu-5wEkNnrt+HdnwHTZ+5FytqBB-j3nuHS5kgY+c3ew@mail.gmail.com>
Date:   Wed, 18 Oct 2023 20:39:28 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Huan Yang <link@...o.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
        bpf@...r.kernel.org, opensource.kernel@...o.com
Subject: Re: [PATCH 2/2] mm: multi-gen lru: fix stat count

On Wed, Oct 18, 2023 at 8:17 PM Huan Yang <link@...o.com> wrote:
>
> Hi Yu Zhao,
>
> Thanks for your reply.
>
> 在 2023/10/19 0:21, Yu Zhao 写道:
> > On Wed, Oct 18, 2023 at 2:22 AM Huan Yang <link@...o.com> wrote:
> >> For multi-gen lru reclaim in evict_folios, like shrink_inactive_list,
> >> gather folios which isolate to reclaim, and invoke shirnk_folio_list.
> >>
> >> But, when complete shrink, it not gather shrink reclaim stat into sc,
> >> we can't get info like nr_dirty\congested in reclaim, and then
> >> control writeback, dirty number and mark as LRUVEC_CONGESTED, or
> >> just bpf trace shrink and get correct sc stat.
> >>
> >> This patch fix this by simple copy code from shrink_inactive_list when
> >> end of shrink list.
> > MGLRU doesn't try to write back dirt file pages in the reclaim path --
> > it filters them out in sort_folio() and leaves them to the page
> Nice to know this,  sort_folio() filters some folio indeed.
> But, I want to know, if we touch some folio in shrink_folio_list(), may some
> folio become dirty or writeback even if sort_folio() filter then?

Good question: in that case MGLRU still doesn't try to write those
folios back because isolate_folio() cleared PG_reclaim and
shrink_folio_list() checks PG_reclaim:

if (folio_test_dirty(folio)) {
/*
* Only kswapd can writeback filesystem folios
* to avoid risk of stack overflow. But avoid
* injecting inefficient single-folio I/O into
* flusher writeback as much as possible: only
* write folios when we've encountered many
* dirty folios, and when we've already scanned
* the rest of the LRU for clean folios and see
* the same dirty folios again (with the reclaim
* flag set).
*/
if (folio_is_file_lru(folio) &&
    (!current_is_kswapd() ||
     !folio_test_reclaim(folio) ||
     !test_bit(PGDAT_DIRTY, &pgdat->flags))) {