Re: [PATCH v2] thp: use is_zero_pfn only after pte_present check

[Greg Kroah-Hartman]

On Tue, Oct 13, 2015 at 11:38:38AM +0900, Minchan Kim wrote:
> Use is_zero_pfn on pteval only after pte_present check on pteval
> (It might be better idea to introduce is_zero_pte where checks
> pte_present first). Otherwise, it could work with swap or
> migration entry and if pte_pfn's result is equal to zero_pfn
> by chance, we lose user's data in __collapse_huge_page_copy.
> So if you're luck, the application is segfaulted and finally you
> could see below message when the application is exit.
> 
> BUG: Bad rss-counter state mm:ffff88007f099300 idx:2 val:3
> 
> Cc: <stable@xxxxxxxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> Hello Greg,
> 
> This patch should go to -stable but when you will apply it
> after merging of linus tree, it will be surely conflicted due
> to userfaultfd part.
> 
> I want to know how to handle it.

You will get an automated email saying it didn't apply and then you
provide a backported version.  Or you send a properly backported version
to stable@xxxxxxxxxxxxxxx before then, with the git commit id of the
patch in Linus's tree.

thanks,

greg k-h
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