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Date:   Tue, 5 Nov 2019 20:58:34 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Song Liu <songliubraving@fb.com>
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        "Johannes Weiner" <hannes@cmpxchg.org>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP
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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)?