From: Mel Gorman <mgorman@techsingularity.net>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Florian Weimer <fw@deneb.enyo.de>,
Dave Chinner <david@fromorbit.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH] mm, compaction: fix wrong pfn handling in __reset_isolation_pfn()
Date: Tue, 8 Oct 2019 16:51:56 +0100 [thread overview]
Message-ID: <20191008155156.GD3321@techsingularity.net> (raw)
In-Reply-To: <20191008152915.24704-1-vbabka@suse.cz>
On Tue, Oct 08, 2019 at 05:29:15PM +0200, Vlastimil Babka wrote:
> Florian and Dave reported [1] a NULL pointer dereference in
> __reset_isolation_pfn(). While the exact cause is unclear, staring at the code
> revealed two bugs, which might be related.
>
I think the fix is a good fit. Even if the problem still occurs, it
eliminates an important possibility.
> One bug is that if zone starts in the middle of pageblock, block_page might
> correspond to different pfn than block_pfn, and then the pfn_valid_within()
> checks will check different pfn's than those accessed via struct page. This
> might result in acessing an unitialized page in CONFIG_HOLES_IN_ZONE configs.
>
s/acessing/accessing/
Aside from HOLES_IN_ZONE, the patch addresses an issue if the start
of the zone is not pageblock-aligned. While this is common, it's not
guaranteed. I don't think this needs to be clarified in the changelog as
your example is valid. I'm commenting in case someone decides not to try
the patch because they feel HOLES_IN_ZONE is required.
> The other bug is that end_page refers to the first page of next pageblock and
> not last page of current pageblock. The online and valid check is then wrong
> and with sections, the while (page < end_page) loop might wander off actual
> struct page arrays.
>
> [1] https://lore.kernel.org/linux-xfs/87o8z1fvqu.fsf@mid.deneb.enyo.de/
>
> Reported-by: Florian Weimer <fw@deneb.enyo.de>
> Reported-by: Dave Chinner <david@fromorbit.com>
> Fixes: 6b0868c820ff ("mm/compaction.c: correct zone boundary handling when resetting pageblock skip hints")
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Just one minor irrelevant note below.
> ---
> mm/compaction.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index ce08b39d85d4..672d3c78c6ab 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -270,14 +270,15 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
>
> /* Ensure the start of the pageblock or zone is online and valid */
> block_pfn = pageblock_start_pfn(pfn);
> - block_page = pfn_to_online_page(max(block_pfn, zone->zone_start_pfn));
> + block_pfn = max(block_pfn, zone->zone_start_pfn);
> + block_page = pfn_to_online_page(block_pfn);
> if (block_page) {
> page = block_page;
> pfn = block_pfn;
> }
>
> /* Ensure the end of the pageblock or zone is online and valid */
> - block_pfn += pageblock_nr_pages;
> + block_pfn = pageblock_end_pfn(pfn) - 1;
> block_pfn = min(block_pfn, zone_end_pfn(zone) - 1);
> end_page = pfn_to_online_page(block_pfn);
> if (!end_page)
This is fine and is definetly fixing a potential issue.
> @@ -303,7 +304,7 @@ __reset_isolation_pfn(struct zone *zone, unsigned long pfn, bool check_source,
>
> page += (1 << PAGE_ALLOC_COSTLY_ORDER);
> pfn += (1 << PAGE_ALLOC_COSTLY_ORDER);
> - } while (page < end_page);
> + } while (page <= end_page);
>
> return false;
> }
I think this is also ok as it's appropriate for PFN walkers in general of
this style. However, I think it's unlikely to fix anything given that we
are walking in steps of (1 << PAGE_ALLOC_COSTLY_ORDER) and the final page
is not necessarily aligned on that boundary. Still, it's an improvement.
Thanks
--
Mel Gorman
SUSE Labs
prev parent reply other threads:[~2019-10-08 15:52 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-08 15:29 [PATCH] mm, compaction: fix wrong pfn handling in __reset_isolation_pfn() Vlastimil Babka
2019-10-08 15:51 ` Mel Gorman [this message]
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