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Message-Id: <20220907142246.c7d6f55e1b56fba3e16d974b@linux-foundation.org>
Date: Wed, 7 Sep 2022 14:22:46 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yang Shi <shy828301@...il.com>
Cc: david@...hat.com, peterx@...hat.com,
kirill.shutemov@...ux.intel.com, jhubbard@...dia.com,
jgg@...dia.com, hughd@...gle.com, aneesh.kumar@...ux.ibm.com,
linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH 1/2] mm: gup: fix the fast GUP race against THP
collapse
On Wed, 7 Sep 2022 11:01:43 -0700 Yang Shi <shy828301@...il.com> wrote:
> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
> introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer
> sufficient to handle concurrent GUP-fast in all cases, it only handles
> traditional IPI-based GUP-fast correctly. On architectures that send
> an IPI broadcast on TLB flush, it works as expected. But on the
> architectures that do not use IPI to broadcast TLB flush, it may have
> the below race:
>
> CPU A CPU B
> THP collapse fast GUP
> gup_pmd_range() <-- see valid pmd
> gup_pte_range() <-- work on pte
> pmdp_collapse_flush() <-- clear pmd and flush
> __collapse_huge_page_isolate()
> check page pinned <-- before GUP bump refcount
> pin the page
> check PTE <-- no change
> __collapse_huge_page_copy()
> copy data to huge page
> ptep_clear()
> install huge pmd for the huge page
> return the stale page
> discard the stale page
>
> The race could be fixed by checking whether PMD is changed or not after
> taking the page pin in fast GUP, just like what it does for PTE. If the
> PMD is changed it means there may be parallel THP collapse, so GUP
> should back off.
>
> Also update the stale comment about serializing against fast GUP in
> khugepaged.
>
> Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()")
Is this not worth a -stable backport?
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