linux-kernel - Re: [v2 PATCH 0/8] Make khugepaged collapse readonly FS THP more consistent

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Date:   Fri, 18 Mar 2022 01:10:30 +0000
From:   Song Liu <songliubraving@...com>
To:     Yang Shi <shy828301@...il.com>
CC:     Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "linmiaohe@...wei.com" <linmiaohe@...wei.com>,
        Rik van Riel <riel@...riel.com>,
        Matthew Wilcox <willy@...radead.org>, Zi Yan <ziy@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "tytso@....edu" <tytso@....edu>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "darrick.wong@...cle.com" <darrick.wong@...cle.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [v2 PATCH 0/8] Make khugepaged collapse readonly FS THP more
 consistent



> On Mar 17, 2022, at 4:48 PM, Yang Shi <shy828301@...il.com> wrote:
> 
> 
> Changelog
> v2: * Collected reviewed-by tags from Miaohe Lin.
>    * Fixed build error for patch 4/8.
> 
> 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:
>  - 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 readonly FS vmas 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 readonly FS vmas are registered to khugepaged to make the
> behavior more consistent.
> 
> Registering the vmas in mmap path seems more preferred from performance
> point of view since page fault path is definitely hot path.
> 
> 
> The patch 1 ~ 7 are minor bug fixes, clean up and preparation patches.
> The patch 8 converts ext4 and xfs.  We may need convert more filesystems,
> but I'd like to hear some comments before doing that.
> 
> 
> Tested with khugepaged test in selftests and the testcase provided by
> Vlastimil Babka in https://lore.kernel.org/lkml/df3b5d1c-a36b-2c73-3e27-99e74983de3a@suse.cz/
> by commenting out MADV_HUGEPAGE call.

LGTM. For the series:

Acked-by: Song Liu <song@...nel.org>

> 
> 
> b/fs/ext4/file.c                 |    4 +++
> b/fs/xfs/xfs_file.c              |    4 +++
> b/include/linux/huge_mm.h        |    9 +++++++
> b/include/linux/khugepaged.h     |   69 +++++++++++++++++++++----------------------------------------
> b/include/linux/sched/coredump.h |    3 +-
> b/kernel/fork.c                  |    4 ---
> b/mm/huge_memory.c               |   15 +++----------
> b/mm/khugepaged.c                |   71 ++++++++++++++++++++++++++++++++++++++++++++-------------------
> b/mm/shmem.c                     |   14 +++---------
> 9 files changed, 102 insertions(+), 91 deletions(-)
>