linux-kernel - Re: [RFC PATCH v2 2/3] mm: handle large folio when large folio in VM

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Message-ID: <6573e671-62e-b7b9-1aae-64336c32bf1@google.com>
Date:   Thu, 13 Jul 2023 19:21:14 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Yin Fengwei <fengwei.yin@...el.com>
cc:     Yu Zhao <yuzhao@...gle.com>, hughd@...gle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        willy@...radead.org, david@...hat.com, ryan.roberts@....com,
        shy828301@...il.com
Subject: Re: [RFC PATCH v2 2/3] mm: handle large folio when large folio in
 VM_LOCKED VMA range

On Wed, 12 Jul 2023, Yin Fengwei wrote:
> On 7/12/23 14:23, Yu Zhao wrote:
> > On Wed, Jul 12, 2023 at 12:02 AM Yin Fengwei <fengwei.yin@...el.com> wrote:
> >> --- a/mm/internal.h
> >> +++ b/mm/internal.h
> >> @@ -643,7 +643,8 @@ static inline void mlock_vma_folio(struct folio *folio,
> >>          *    still be set while VM_SPECIAL bits are added: so ignore it then.
> >>          */
> >>         if (unlikely((vma->vm_flags & (VM_LOCKED|VM_SPECIAL)) == VM_LOCKED) &&
> >> -           (compound || !folio_test_large(folio)))
> >> +           (compound || !folio_test_large(folio) ||
> >> +           folio_in_range(folio, vma, vma->vm_start, vma->vm_end)))
> >>                 mlock_folio(folio);
> >>  }
> > 
> > This can be simplified:
> > 1. remove the compound parameter
> Yes. There is not difference here for pmd mapping of THPs and pte mappings of THPs
> if the only condition need check is whether the folio is within VMA range or not.
> 
> But let me add Huge for confirmation.

I'm not sure what it is that you need me to confirm: if the folio fits
within the vma, then the folio fits within the vma, pmd-mapped or not.

(And I agree with Yu that it's better to drop the folio_test_large()
check too.)

This idea, of counting the folio as mlocked according to whether the
whole folio fits within the vma, does seem a good idea to me: worth
pursuing.  But whether the implementation adds up and works out, I
have not checked.  It was always difficult to arrive at a satisfactory
compromise in mlocking compound pages: I hope this way does work out.

Hugh