linux-kernel - Re: [PATCH RESEND v5 3/4] mm/mlock: check for THP missing the mlock in try_to_unmap

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Message-ID: <CAK1f24n=fT6MDCkN_2ycCmxV3fTkdqRdg7iiq_aXX+AvQ7hXmQ@mail.gmail.com>
Date: Tue, 14 May 2024 15:46:44 +0800
From: Lance Yang <ioworker0@...il.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org, 
	maskray@...gle.com, ziy@...dia.com, ryan.roberts@....com, david@...hat.com, 
	21cnbao@...il.com, mhocko@...e.com, fengwei.yin@...el.com, zokeefe@...gle.com, 
	shy828301@...il.com, xiehuan09@...il.com, libang.li@...group.com, 
	wangkefeng.wang@...wei.com, songmuchun@...edance.com, peterx@...hat.com, 
	minchan@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v5 3/4] mm/mlock: check for THP missing the mlock
 in try_to_unmap_one()

On Tue, May 14, 2024 at 2:41 PM Baolin Wang
<baolin.wang@...ux.alibaba.com> wrote:
>
>
>
> On 2024/5/13 15:47, Lance Yang wrote:
> > The TTU_SPLIT_HUGE_PMD will no longer perform immediately, so we might
> > encounter a PMD-mapped THP missing the mlock in the VM_LOCKED range
> > during the pagewalk. It's likely necessary to mlock this THP to prevent
> > it from being picked up during page reclaim.
> >
> > Suggested-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
>
> IMO, this patch should be merged into patch 2, otherwise patch 2 is
> buggy. Quote the description in the 'submission patches.rst' document:
>
> "When dividing your change into a series of patches, take special care
> to ensure that the kernel builds and runs properly after each patch in
> the series.  Developers using ``git bisect`` to track down a problem can
> end up splitting your patch series at any point; they will not thank you
> if you introduce bugs in the middle."

Thanks for bringing this up!

I completely agree that this patch should be merged into patch2.

Thanks,
Lance

>
> > ---
> >   mm/rmap.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 4c4d14325f2e..08a93347f283 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1671,7 +1671,8 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
> >               if (!(flags & TTU_IGNORE_MLOCK) &&
> >                   (vma->vm_flags & VM_LOCKED)) {
> >                       /* Restore the mlock which got missed */
> > -                     if (!folio_test_large(folio))
> > +                     if (!folio_test_large(folio) ||
> > +                         (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)))
> >                               mlock_vma_folio(folio, vma);
> >                       goto walk_done_err;
> >               }