linux-kernel - Re: [PATCH v2] mm/shmem: fix THP allocation and fallback loop

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Message-ID: <CAGsJ_4yidWp9BqJbTa44uLWpJU-d5A+76r8QeErtL+Au+CEqZw@mail.gmail.com>
Date: Thu, 23 Oct 2025 19:05:09 +1300
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Hugh Dickins <hughd@...gle.com>, 
	Dev Jain <dev.jain@....com>, David Hildenbrand <david@...hat.com>, 
	Liam Howlett <liam.howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Mariano Pache <npache@...hat.com>, Matthew Wilcox <willy@...radead.org>, 
	Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH v2] mm/shmem: fix THP allocation and fallback loop

On Thu, Oct 23, 2025 at 6:58 PM Kairui Song <ryncsn@...il.com> wrote:
>
> On Thu, Oct 23, 2025 at 1:51 PM Barry Song <21cnbao@...il.com> wrote:
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index b50ce7dbc84a..7559773ebb30 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -1895,10 +1895,11 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
> > >                 order = highest_order(suitable_orders);
> > >                 while (suitable_orders) {
> > >                         pages = 1UL << order;
> > > -                       index = round_down(index, pages);
> > > -                       folio = shmem_alloc_folio(gfp, order, info, index);
> > > -                       if (folio)
> > > +                       folio = shmem_alloc_folio(gfp, order, info, round_down(index, pages));
> > > +                       if (folio) {
> > > +                               index = round_down(index, pages);
> > >                                 goto allocated;
> > > +                       }
> >
> > Could this be a temporary variable to store round_down(index, pages)?
>
> Right we can do that, but the generated code should be the same, the
> compiler is smart enough, I just checked the generated code with gcc /
> clang.
>
> Do you think the code will be cleaner with a temporary variable? I can
> send a V3 if anyone suggests, it's really a trivial change.

Personally, I think a temporary variable makes the code cleaner, but I don’t
have any strong preference.

The fix looks correct to me.

Thanks
Barry