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Date:   Wed, 21 Nov 2018 21:01:59 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Yu Zhao <yuzhao@google.com>
Cc:     Hugh Dickins <hughd@google.com>,
        Matthew Wilcox <willy@infradead.org>, stable@vger.kernel.org,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] mm: use swp_offset as key in shmem_replace_page()
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In-Reply-To: <20181121215442.138545-1-yuzhao@google.com>
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        <20181121215442.138545-1-yuzhao@google.com>
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On Wed, 21 Nov 2018 14:54:42 -0700 Yu Zhao <yuzhao@google.com> wrote:

> We changed key of swap cache tree from swp_entry_t.val to
> swp_offset. Need to do so in shmem_replace_page() as well.

What are the user-visible effects of this change?

> Fixes: f6ab1f7f6b2d ("mm, swap: use offset of swap entry as key of swap cache")
> Cc: stable@vger.kernel.org # v4.9+

Please always provide the user-impact information when fixing bugs.  This
becomes especially important when proposing -stable backporting.

Hugh said

: shmem_replace_page() has been wrong since the day I wrote it: good
: enough to work on swap "type" 0, which is all most people ever use
: (especially those few who need shmem_replace_page() at all), but broken
: once there are any non-0 swp_type bits set in the higher order bits.

but we still don't have a description of "broken".

Thanks.