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    SubjectRe: [PATCH v3] mm,thp: recheck each page before collapsing file THP
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    > On Nov 5, 2019, at 8:58 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
    >
    > On Sat, 19 Oct 2019 05:24:00 +0000 Song Liu <songliubraving@fb.com> wrote:
    >
    >>> We don't have a ref on that page. After we've released the xarray lock
    >>> we have no business playing with *page at all, correct?
    >>
    >> Yeah, this piece is not just redundant, but also buggy. I am also
    >> including some information about it.
    >>
    >> Updated commit log:
    >>
    >> ============================= 8< =============================
    >>
    >> In collapse_file(), for !is_shmem case, current check cannot guarantee
    >> the locked page is up-to-date. Specifically, xas_unlock_irq() should not
    >> be called before lock_page() and get_page(); and it is necessary to
    >> recheck PageUptodate() after locking the page.
    >>
    >> With this bug and CONFIG_READ_ONLY_THP_FOR_FS=y, madvise(HUGE)'ed .text
    >> may contain corrupted data. This is because khugepaged mistakenly
    >> collapses some not up-to-date sub pages into a huge page, and assumes the
    >> huge page is up-to-date. This will NOT corrupt data in the disk, because
    >> the page is read-only and never written back. Fix this by properly
    >> checking PageUptodate() after locking the page. This check replaces
    >> "VM_BUG_ON_PAGE(!PageUptodate(page), page);".
    >>
    >> Also, move PageDirty() check after locking the page. Current khugepaged
    >> should not try to collapse dirty file THP, because it is limited to
    >> read-only .text. Add a warning with the PageDirty() check as it should
    >> not happen. This warning is added after page_mapping() check, because
    >> if the page is truncated, it might be dirty.
    >
    > I've lost the plot on this patch. I have the v3 patch plus these fixes:
    >
    > http://lkml.kernel.org/r/20191028221414.3685035-1-songliubraving@fb.com
    > http://lkml.kernel.org/r/20191022191006.411277-1-songliubraving@fb.com
    > http://lkml.kernel.org/r/20191030200736.3455046-1-songliubraving@fb.com
    >
    > and there's a v4 which I can't correlate with the above. And there has
    > been discussion about deferring some of the filemap_flush() changes
    > until later.
    >
    > So I think it's best if we just start again. Can you please prepare
    > and send out a v5 (which might be a 2-patch series)?

    Sounds good. Sending v5 soon.

    Thanks,
    Song

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