linux-kernel - Re: [v3 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent

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Date:   Wed, 27 Apr 2022 16:16:41 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Song Liu <songliubraving@...com>,
        Rik van Riel <riel@...riel.com>, Zi Yan <ziy@...dia.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [v3 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent

On Wed, Apr 27, 2022 at 1:59 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Apr 04, 2022 at 05:48:49PM -0700, Yang Shi wrote:
> > When khugepaged collapses file THPs, its behavior is not consistent.
> > It is kind of "random luck" for khugepaged to see the file vmas (see
> > report: https://lore.kernel.org/linux-mm/00f195d4-d039-3cf2-d3a1-a2c88de397a0@suse.cz/)
> > since currently the vmas are registered to khugepaged when:
> >   - Anon huge pmd page fault
> >   - VMA merge
> >   - MADV_HUGEPAGE
> >   - Shmem mmap
> >
> > If the above conditions are not met, even though khugepaged is enabled
> > it won't see any file vma at all.  MADV_HUGEPAGE could be specified
> > explicitly to tell khugepaged to collapse this area, but when
> > khugepaged mode is "always" it should scan suitable vmas as long as
> > VM_NOHUGEPAGE is not set.
>
> I don't see that as being true at all.  The point of this hack was that
> applications which really knew what they were doing could enable it.
> It makes no sense to me that setting "always" by the sysadmin for shmem
> also force-enables ROTHP, even for applications which aren't aware of it.
>
> Most telling, I think, is that Song Liu hasn't weighed in on this at
> all.  It's clearly not important to the original author.

Song Liu already acked the series, please see
https://lore.kernel.org/linux-mm/96F2D93B-2043-44C3-8062-C639372A0212@fb.com/