linux-kernel - Re: [PATCH 21/24] swap: make swapin_readahead result checking argument mandatory

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Message-ID: <CAMgjq7CMvT-YLQoeRKCaNUDLyzuLzCi7s8rzgsNxpncc=7AWcg@mail.gmail.com>
Date:   Fri, 24 Nov 2023 16:14:49 +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 21/24] swap: make swapin_readahead result checking
 argument mandatory

Chris Li <chrisl@...nel.org> 于2023年11月22日周三 13:18写道：
>
> On Sun, Nov 19, 2023 at 11:49 AM Kairui Song <ryncsn@...il.com> wrote:
> >
> > From: Kairui Song <kasong@...cent.com>
> >
> > This is only one caller now in page fault path, make the result return
> > argument mandatory.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> >  mm/swap_state.c | 17 +++++++----------
> >  1 file changed, 7 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 6f39aa8394f1..0433a2586c6d 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -913,7 +913,6 @@ static struct page *swapin_no_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >  struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >                               struct vm_fault *vmf, enum swap_cache_result *result)
> >  {
> > -       enum swap_cache_result cache_result;
> >         struct swap_info_struct *si;
> >         struct mempolicy *mpol;
> >         void *shadow = NULL;
> > @@ -928,29 +927,27 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >
> >         folio = swap_cache_get_folio(entry, vmf, &shadow);
> >         if (folio) {
> > +               *result = SWAP_CACHE_HIT;
> >                 page = folio_file_page(folio, swp_offset(entry));
> > -               cache_result = SWAP_CACHE_HIT;
> >                 goto done;
> >         }
> >
> >         mpol = get_vma_policy(vmf->vma, vmf->address, 0, &ilx);
> >         if (swap_use_no_readahead(si, swp_offset(entry))) {
> > +               *result = SWAP_CACHE_BYPASS;
>
> Each of this "*result" will compile into memory store instructions.
> The compiler most likely can't optimize and combine them together
> because the store can cause segfault from the compiler's point of
> view. The multiple local variable assignment can be compiled into a
> few registers assignment so it does not cost as much as multiple
> memory stores.
>
> >                 page = swapin_no_readahead(entry, gfp_mask, mpol, ilx, vmf->vma->vm_mm);
> > -               cache_result = SWAP_CACHE_BYPASS;
> >                 if (shadow)
> >                         workingset_refault(page_folio(page), shadow);
> > -       } else if (swap_use_vma_readahead(si)) {
> > -               page = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
> > -               cache_result = SWAP_CACHE_MISS;
> >         } else {
> > -               page = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
> > -               cache_result = SWAP_CACHE_MISS;
> > +               *result = SWAP_CACHE_MISS;
> > +               if (swap_use_vma_readahead(si))
> > +                       page = swap_vma_readahead(entry, gfp_mask, mpol, ilx, vmf);
> > +               else
> > +                       page = swap_cluster_readahead(entry, gfp_mask, mpol, ilx);
>
> I recall you introduce or heavy modify this function in previous patch before.
> Consider combine some of the patch and present the final version sooner.
> From the reviewing point of view, don't need to review so many
> internal version which get over written any way.
>
> >         }
> >         mpol_cond_put(mpol);
> >  done:
> >         put_swap_device(si);
> > -       if (result)
> > -               *result = cache_result;
>
> The original version with check and assign it at one place is better.
> Safer and produce better code.

Yes, that's less error-prone indeed, saving a "if" seems doesn't worth
all the potential trouble, will drop this.