linux-kernel - Re: [PATCH v2 06/10] mm/thp: unmap_mapping_page() to fix THP truncate_cleanup

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Date:   Wed, 9 Jun 2021 14:11:19 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Yang Shi <shy828301@...il.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Wang Yugui <wangyugui@...-tech.com>,
        Matthew Wilcox <willy@...radead.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>, Zi Yan <ziy@...dia.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>, Jue Wang <juew@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jan Kara <jack@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/10] mm/thp: unmap_mapping_page() to fix THP
 truncate_cleanup_page()

On Wed, 9 Jun 2021, Yang Shi wrote:
> On Tue, Jun 8, 2021 at 9:19 PM Hugh Dickins <hughd@...gle.com> wrote:
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1361,7 +1361,17 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
> >                         else if (zap_huge_pmd(tlb, vma, pmd, addr))
> >                                 goto next;
> >                         /* fall through */
> > +               } else if (details && details->single_page &&
> > +                          PageTransCompound(details->single_page) &&
> > +                          next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) {
> > +                       /*
> > +                        * Take and drop THP pmd lock so that we cannot return
> > +                        * prematurely, while zap_huge_pmd() has cleared *pmd,
> > +                        * but not yet decremented compound_mapcount().
> > +                        */
> > +                       spin_unlock(pmd_lock(tlb->mm, pmd));
> 
> Just a nit, why not follow the style of patch #3 to have lock and
> unlock with separate lines?

Good point.  Doing it on one line is my own preferred style (particularly
with the "take and drop lock" comment just above), carried forward from
before.  I simply didn't think to change this one when I agreed to change
the other.  You're right to question it: v3 of just this 06/10 follows.

And thank you to Kirill and to you for these rapid further reviews (and
for silently forgiving my screwup in omitting to Cc linux-mm and lkml
on those early ones).

Hugh