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

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Date:   Mon, 4 Apr 2022 17:48:49 -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 Mon, Apr 4, 2022 at 5:16 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, Apr 04, 2022 at 01:02:42PM -0700, Yang Shi wrote:
> > The readonly FS THP relies on khugepaged to collapse THP for suitable
> > vmas.  But it is kind of "random luck" for khugepaged to see the
> > readonly FS 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:
>
> I still don't see the point.  The effort should be put into
> supporting large folios, not in making this hack work better.

The series makes sense even though the hack is replaced by large
folios IMHO. The problem is the file VMAs may be not registered by
khugepaged consistently for some THP modes, for example, always,
regardless of whether it's readonly or the hack is gone or not. IIUC
even though the hack is replaced by the large folios, we still have
khugepaged to collapse pmd-mappable huge pages for both anonymous vmas
and file vmas, right? Or are you thinking about killing khugepaged
soon with supporting large folios?

Anyway it may make things clearer if the cover letter is rephrased to:

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.

So make sure file vmas are registered to khugepaged to make the
behavior more consistent.