From: Vlastimil Babka <vbabka@suse.cz>
To: Hugh Dickins <hughd@google.com>,
Mel Gorman <mgorman@techsingularity.net>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Li Wang <liwang@redhat.com>,
Alex Shi <alex.shi@linux.alibaba.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH] mm, page_alloc: capture page in task context only
Date: Fri, 12 Jun 2020 12:30:53 +0200 [thread overview]
Message-ID: <01287865-153d-42e7-afd8-1178ec6bc5b9@suse.cz> (raw)
In-Reply-To: <alpine.LSU.2.11.2006101342250.4607@eggly.anvils>
On 6/10/20 10:48 PM, Hugh Dickins wrote:
> While stressing compaction, one run oopsed on NULL capc->cc in
> __free_one_page()'s task_capc(zone): compact_zone_order() had been
> interrupted, and a page was being freed in the return from interrupt.
>
> Though you would not expect it from the source, both gccs I was using
> (a 4.8.1 and a 7.5.0) had chosen to compile compact_zone_order() with
> the ".cc = &cc" implemented by mov %rbx,-0xb0(%rbp) immediately before
> callq compact_zone - long after the "current->capture_control = &capc".
> An interrupt in between those finds capc->cc NULL (zeroed by an earlier
> rep stos).
Ugh, nasty. Same here with gcc 10.
> This could presumably be fixed by a barrier() before setting
> current->capture_control in compact_zone_order(); but would also need
> more care on return from compact_zone(), in order not to risk leaking
> a page captured by interrupt just before capture_control is reset.
I was hoping a WRITE_ONCE(current->capture_control) would be enough,
but apparently it's not (I tried).
> Maybe that is the preferable fix, but I felt safer for task_capc() to
> exclude the rather surprising possibility of capture at interrupt time.
> Fixes: 5e1f0f098b46 ("mm, compaction: capture a page under direct compaction")
> Cc: stable@vger.kernel.org # 5.1+
> Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
But perhaps I would also make sure that we don't expose the half initialized
capture_control and run into this problem again later. It's not like this is a
fast path where barriers hurt. Something like this then? (with added comments)
diff --git a/mm/compaction.c b/mm/compaction.c
index fd988b7e5f2b..c89e26817278 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2316,15 +2316,17 @@ static enum compact_result compact_zone_order(struct zone *zone, int order,
.page = NULL,
};
- current->capture_control = &capc;
+ barrier();
+
+ WRITE_ONCE(current->capture_control, &capc);
ret = compact_zone(&cc, &capc);
VM_BUG_ON(!list_empty(&cc.freepages));
VM_BUG_ON(!list_empty(&cc.migratepages));
- *capture = capc.page;
- current->capture_control = NULL;
+ WRITE_ONCE(current->capture_control, NULL);
+ *capture = READ_ONCE(capc.page);
return ret;
}
next prev parent reply other threads:[~2020-06-12 10:30 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-10 20:48 [PATCH] mm, page_alloc: capture page in task context only Hugh Dickins
2020-06-11 15:43 ` Mel Gorman
2020-06-12 10:30 ` Vlastimil Babka [this message]
2020-06-15 21:03 ` Hugh Dickins
2020-06-16 7:45 ` Vlastimil Babka
2020-06-16 8:26 ` [PATCH 1/2] mm, compaction: make capture control handling safe wrt interrupts Vlastimil Babka
2020-06-16 8:26 ` [PATCH 2/2] mm, page_alloc: use unlikely() in task_capc() Vlastimil Babka
2020-06-16 20:29 ` Hugh Dickins
2020-06-17 9:55 ` Vlastimil Babka
2020-06-22 8:58 ` Mel Gorman
2020-06-16 20:18 ` [PATCH 1/2] mm, compaction: make capture control handling safe wrt interrupts Hugh Dickins
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