linux-kernel - Re: [PATCH v1 2/2] mm/hugetlb: fix uffd-wp handling for migration entries in hugetlb_change

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Date:   Thu, 22 Dec 2022 15:20:54 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Xu <peterx@...hat.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Miaohe Lin <linmiaohe@...wei.com>, stable@...r.kernel.org
Subject: Re: [PATCH v1 2/2] mm/hugetlb: fix uffd-wp handling for migration
 entries in hugetlb_change_protection()

On 12/22/22 21:55, David Hildenbrand wrote:
> We have to update the uffd-wp SWP PTE bit independent of the type of
> migration entry. Currently, if we're unlucky and we want to install/clear
> the uffd-wp bit just while we're migrating a read-only mapped hugetlb page,
> we would miss to set/clear the uffd-wp bit.
> 
> Further, if we're processing a readable-exclusive
> migration entry and neither want to set or clear the uffd-wp bit, we
> could currently end up losing the uffd-wp bit. Note that the same would
> hold for writable migrating entries, however, having a writable
> migration entry with the uffd-wp bit set would already mean that
> something went wrong.
> 
> Note that the change from !is_readable_migration_entry ->
> writable_migration_entry is harmless and actually cleaner, as raised by
> Miaohe Lin and discussed in [1].
> 
> [1] https://lkml.kernel.org/r/90dd6a93-4500-e0de-2bf0-bf522c311b0c@huawei.com
> 
> Fixes: 60dfaad65aa9 ("mm/hugetlb: allow uffd wr-protect none ptes")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/hugetlb.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)

Thanks,

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
-- 
Mike Kravetz