linux-kernel - Re: [PATCH v2] mm/ksm: ignore STABLE_FLAG of rmap_item->address in rmap_walk

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Date:   Wed, 9 May 2018 16:31:01 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jia He <hejianet@...il.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Minchan Kim <minchan@...nel.org>,
        Claudio Imbrenda <imbrenda@...ux.vnet.ibm.com>,
        Arvind Yadav <arvind.yadav.cs@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jia.he@...-semitech.com,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2] mm/ksm: ignore STABLE_FLAG of rmap_item->address in
 rmap_walk_ksm

On Fri,  4 May 2018 11:11:46 +0800 Jia He <hejianet@...il.com> wrote:

> In our armv8a server(QDF2400), I noticed lots of WARN_ON caused by PAGE_SIZE
> unaligned for rmap_item->address under memory pressure tests(start 20 guests
> and run memhog in the host).
> 
> ...
> 
> In rmap_walk_ksm, the rmap_item->address might still have the STABLE_FLAG,
> then the start and end in handle_hva_to_gpa might not be PAGE_SIZE aligned.
> Thus it will cause exceptions in handle_hva_to_gpa on arm64.
> 
> This patch fixes it by ignoring(not removing) the low bits of address when
> doing rmap_walk_ksm.
> 
> Signed-off-by: jia.he@...-semitech.com

I assumed you wanted this patch to be committed as
From:jia.he@...-semitech.com rather than From:hejianet@...il.com, so I
made that change.  Please let me know if this was inappropriate.

You can do this yourself by adding an explicit From: line to the very
start of the patch's email text.

Also, a storm of WARN_ONs is pretty poor behaviour.  Is that the only
misbehaviour which this bug causes?  Do you think the fix should be
backported into earlier kernels?