linux-kernel - Re: [PATCH 2/2] memory-hotplug: don't replace lowmem pages with highmem

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Message-ID: <xa1tk3vzh6m6.fsf@mina86.com>
Date:	Wed, 12 Sep 2012 12:34:41 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	Minchan Kim <minchan@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Minchan Kim <minchan@...nel.org>,
	Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Wen Congyang <wency@...fujitsu.com>
Subject: Re: [PATCH 2/2] memory-hotplug: don't replace lowmem pages with highmem

On Wed, Sep 12 2012, Minchan Kim wrote:
> [1] reporeted that lowmem pages could be replaced by
> highmem pages during migration of CMA and fixed.
>
> Quote from [1]'s description
> "
>     The filesystem layer expects pages in the block device's mapping to not
>     be in highmem (the mapping's gfp mask is set in bdget()), but CMA can
>     currently replace lowmem pages with highmem pages, leading to crashes in
>     filesystem code such as the one below:
>
>       Unable to handle kernel NULL pointer dereference at virtual address 00000400
>       pgd = c0c98000
>       [00000400] *pgd=00c91831, *pte=00000000, *ppte=00000000
>       Internal error: Oops: 817 [#1] PREEMPT SMP ARM
>       CPU: 0    Not tainted  (3.5.0-rc5+ #80)
>       PC is at __memzero+0x24/0x80
>       ...
>       Process fsstress (pid: 323, stack limit = 0xc0cbc2f0)
>       Backtrace:
>       [<c010e3f0>] (ext4_getblk+0x0/0x180) from [<c010e58c>] (ext4_bread+0x1c/0x98)
>       [<c010e570>] (ext4_bread+0x0/0x98) from [<c0117944>] (ext4_mkdir+0x160/0x3bc)
>        r4:c15337f0
>       [<c01177e4>] (ext4_mkdir+0x0/0x3bc) from [<c00c29e0>] (vfs_mkdir+0x8c/0x98)
>       [<c00c2954>] (vfs_mkdir+0x0/0x98) from [<c00c2a60>] (sys_mkdirat+0x74/0xac)
>        r6:00000000 r5:c152eb40 r4:000001ff r3:c14b43f0
>       [<c00c29ec>] (sys_mkdirat+0x0/0xac) from [<c00c2ab8>] (sys_mkdir+0x20/0x24)
>        r6:beccdcf0 r5:00074000 r4:beccdbbc
>       [<c00c2a98>] (sys_mkdir+0x0/0x24) from [<c000e3c0>] (ret_fast_syscall+0x0/0x30)
> "
>
> Memory-hotplug has same problem with CMA so [1]'s fix could be applied
> with memory-hotplug, too.
>
> Fix it by reusing.
>
> [1] 6a6dccba2, mm: cma: don't replace lowmem pages with highmem
>
> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
> Cc: Michal Nazarewicz <mina86@...a86.com>

Looks legit:

Acked-by: Michal Nazarewicz <mina86@...a86.com>

I also like how both of the patches cumulatively have negative delta. :]

> Cc: Marek Szyprowski <m.szyprowski@...sung.com>
> Cc: Wen Congyang <wency@...fujitsu.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
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