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SubjectRe: [PATCH v3] mm,thp: recheck each page before collapsing file THP
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)?

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