linux-kernel - Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing extra unlock check

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Date:   Thu, 23 Nov 2023 01:32:58 +0800
From:   Kairui Song <ryncsn@...il.com>
To:     Chris Li <chrisl@...nel.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        "Huang, Ying" <ying.huang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/24] mm/swap: avoid setting page lock bit and doing
 extra unlock check

Chris Li <chrisl@...nel.org> 于2023年11月21日周二 01:44写道：
>
> On Mon, Nov 20, 2023 at 3:15 AM Kairui Song <ryncsn@...il.com> wrote:
> > > > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > > > index ac4fa404eaa7..45dd8b7c195d 100644
> > > > --- a/mm/swap_state.c
> > > > +++ b/mm/swap_state.c
> > > > @@ -458,6 +458,8 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > >
> > > You move the mem_cgroup_swapin_charge_folio() inside the for loop:
> > >
> > >
> > >         for (;;) {
> > >                 int err;
> > >                 /*
> > >                  * First check the swap cache.  Since this is normally
> > >                  * called after swap_cache_get_folio() failed, re-calling
> > >                  * that would confuse statistics.
> > >                  */
> > >                 folio = filemap_get_folio(swap_address_space(entry),
> > >                                                 swp_offset(entry));
> > >
> > >
> > > >                                                 mpol, ilx, numa_node_id());
> > > >                 if (!folio)
> > > >                          goto fail_put_swap;
> > > > +               if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry))
> > > > +                       goto fail_put_folio;
> > >
> > > Wouldn't it cause repeat charging of the folio when it is racing
> > > against others in the for loop?
> >
> > The race loser will call folio_put and discharge it?
>
> There are two different charges. Memcg charging and memcg swapin charging.
> The folio_put will do the memcg discharge, the corresponding memcg
> charge is in follio allocation.

Hi Chris,

I didn't get your idea here... By "memcg swapin charge", do you mean
"memory.swap.*"? And "memcg charging" means "memory.*"?. There is no
memcg charge related code in folio allocation (alloc_pages_mpol),
actually the mem_cgroup_swapin_charge_folio here is doing memcg charge
not memcg swapin charge. Swapin path actually need to uncharge
"memory.swap" by mem_cgroup_swapin_uncharge_swap in later part of this
function.


> Memcg swapin charge does things differently, it needs to modify the
> swap relately accounting.
> The memcg uncharge is not a pair for memcg swapin charge.
>
> > > >                 /*
> > > >                  * Swap entry may have been freed since our caller observed it.
> > > > @@ -483,13 +485,9 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > > >         /*
> > > >          * The swap entry is ours to swap in. Prepare the new page.
> > > >          */
> > > > -
> > > >         __folio_set_locked(folio);
> > > >         __folio_set_swapbacked(folio);
> > > >
> > > > -       if (mem_cgroup_swapin_charge_folio(folio, NULL, gfp_mask, entry))
> > > > -               goto fail_unlock;
> > > > -
> > >
> > > The original code makes the charge outside of the for loop. Only the
> > > winner can charge once.
> >
> > Right, this patch may make the charge/dis-charge path more complex for
> > race swapin, I'll re-check this part.
>
> It is more than just complex, it seems to change the behavior of this code.
>
> Chris