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Date:   Thu, 15 Dec 2022 07:47:40 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Jann Horn <jannh@google.com>
Cc:     stable@vger.kernel.org
Subject: Re: [PATCH v2 stable 4.19 1/2] mm/khugepaged: fix GUP-fast
 interaction by sending IPI
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On Mon, Dec 12, 2022 at 06:32:35PM +0100, Jann Horn wrote:
> Since commit 70cbc3cc78a99 ("mm: gup: fix the fast GUP race against THP
> collapse"), the lockless_pages_from_mm() fastpath rechecks the pmd_t to
> ensure that the page table was not removed by khugepaged in between.
> 
> However, lockless_pages_from_mm() still requires that the page table is
> not concurrently freed.  Fix it by sending IPIs (if the architecture uses
> semi-RCU-style page table freeing) before freeing/reusing page tables.
> 
> Link: https://lkml.kernel.org/r/20221129154730.2274278-2-jannh@google.com
> Link: https://lkml.kernel.org/r/20221128180252.1684965-2-jannh@google.com
> Link: https://lkml.kernel.org/r/20221125213714.4115729-2-jannh@google.com
> Fixes: ba76149f47d8 ("thp: khugepaged")
> Signed-off-by: Jann Horn <jannh@google.com>
> Reviewed-by: Yang Shi <shy828301@gmail.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> [manual backport: two of the three places in khugepaged that can free
> ptes were refactored into a common helper between 5.15 and 6.0;
> TLB flushing was refactored between 5.4 and 5.10;
> TLB flushing was refactored between 4.19 and 5.4;
> pmd collapse for PTE-mapped THP was only added in 5.4;
> ugly hack needed in <=4.19 for s390]
> Signed-off-by: Jann Horn <jannh@google.com>

All now queued up, thanks for the backports!

greg k-h