Re: [PATCH v3] mm,thp: recheck each page before collapsing file THP

[Johannes Weiner]

On Sat, Oct 19, 2019 at 05:24:00AM +0000, Song Liu wrote:
> 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.
> 
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Song Liu <songliubraving@xxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>