linux-kernel - Re: [PATCH] mm: fix pfn spans two sections in has_unmovable

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Message-ID: <57305AD8.9090202@suse.cz>
Date:	Mon, 9 May 2016 11:39:36 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Xishi Qiu <qiuxishi@...wei.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Michal Hocko <mhocko@...e.com>,
	David Rientjes <rientjes@...gle.com>,
	"'Kirill A . Shutemov'" <kirill.shutemov@...ux.intel.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Taku Izumi <izumi.taku@...fujitsu.com>,
	Alexander Duyck <alexander.h.duyck@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>
Cc:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix pfn spans two sections in has_unmovable_pages()

On 05/09/2016 10:34 AM, Xishi Qiu wrote:
> If the pfn is not aligned to pageblock, the check pfn may access a next
> pageblcok, and the next pageblock may belong to a next section. Because
> struct page has not been alloced in the next section, so kernel panic.
>
> I find the caller of has_unmovable_pages() has passed a aligned pfn, so it
> doesn't have this problem. But the earlier kernel version(e.g. v3.10) has.
> e.g. echo xxx > /sys/devices/system/memory/soft_offline_page could trigger
> it. The following log is from RHEL v7.1

I think has_unmovable_pages() is wrong layer where to fix such problem, 
as I'll explain below.

> [14111.611492] Stack:
> [14111.611494] ffffffff8115d952 0000000000000000 01ff880c393ebe40 ffff880c7ffd9000
> [14111.611500] ffffea0061ffffc0 ffff880c7ffd9068 0000000000000286 0000000000000001
> [14111.611505] ffff880c393ebe10 ffffffff811c265a 000000000187ffff 0000000000000200
> [14111.611511] Call Trace:
> [14111.611516] [<ffffffff8115d952>] ? has_unmovable_pages+0xd2/0x130
> [14111.611521] [<ffffffff811c265a>] set_migratetype_isolate+0xda/0x170
> [14111.611526] [<ffffffff811c187a>] soft_offline_page+0x9a/0x590
> [14111.611530] [<ffffffff812e7cab>] ? _kstrtoull+0x3b/0xa0
> [14111.611535] [<ffffffff813e158f>] store_soft_offline_page+0xaf/0xf0
> [14111.611539] [<ffffffff813cae18>] dev_attr_store+0x18/0x30
> [14111.611544] [<ffffffff8123c046>] sysfs_write_file+0xc6/0x140
> [14111.611548] [<ffffffff811c5b5d>] vfs_write+0xbd/0x1e0
> [14111.611551] [<ffffffff811c65a8>] SyS_write+0x58/0xb0
> [14111.611556] [<ffffffff8160f509>] system_call_fastpath+0x16/0x1b
> [14111.611559] Code: 66 66 66 90 48 83 e0 fd 0c a0 5d c3 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 66 66 66 90 48 83 c8 42 0c a0 5d c3 90 66 66 66 66 90 <8b> 07 25 00 c0 00 00 75 02 f3 c3 48 8b 07 f6 c4 80 75 0f 48 81
> [14111.611594] RIP [<ffffffff81199fc5>] PageHuge+0x5/0x40
> [14111.611598] RSP <ffff880c393ebd80>
> [14111.611600] CR2: ffffea0062000000
> [14111.611604] ---[ end trace 9f780ed1def334c6 ]---
> [14111.678586] Kernel panic - not syncing: Fatal exception
>
> Signed-off-by: Xishi Qiu <qiuxishi@...wei.com>

It's not CC'd stable, so how will this patch fix the older kernels? Also 
you should determine which upstream kernel versions are affected, not a 
RHEL derivative.
Also is the current upstream broken or not?

> ---
>   mm/page_alloc.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 59de90d..9afc1bc 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -6842,6 +6842,7 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>   		return false;
>
>   	pfn = page_to_pfn(page);
> +	pfn = pfn & ~(pageblock_nr_pages - 1);

I think it's wrong that has_unmovable_pages() would silently correct 
wrong input. See e.g. the call path from start_isolate_page_range -> 
set_migratetype_isolate -> has_unmovable_pages. In 
start_isolate_page_range() there are BUG_ON's to check the alignment. 
That would be more appropriate here as well (but use VM_BUG_ON please).

One danger of the self-correction is that the adjusted pfn might be of
a different zone, so let's not go there. If there's a call stack that 
passes unaligned page, it has to be fixed higher in the stack IMHO.

>   	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
>   		unsigned long check = pfn + iter;
>
>