From: Vlastimil Babka <vbabka@suse.cz>
To: mhocko@kernel.org, linux-mm@kvack.org
Cc: Mel Gorman <mgorman@suse.de>, Jiri Bohac <jbohac@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
"David S. Miller" <davem@davemloft.net>,
Eric Dumazet <eric.dumazet@gmail.com>,
LKML <linux-kernel@vger.kernel.org>,
netdev@vger.kernel.org, Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH] mm: make page pfmemalloc check more robust
Date: Thu, 13 Aug 2015 11:13:04 +0200 [thread overview]
Message-ID: <55CC5FA0.300@suse.cz> (raw)
In-Reply-To: <1439456364-4530-1-git-send-email-mhocko@kernel.org>
On 08/13/2015 10:58 AM, mhocko@kernel.org wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> The patch c48a11c7ad26 ("netvm: propagate page->pfmemalloc to skb")
> added the checks for page->pfmemalloc to __skb_fill_page_desc():
>
> if (page->pfmemalloc && !page->mapping)
> skb->pfmemalloc = true;
>
> It assumes page->mapping == NULL implies that page->pfmemalloc can be
> trusted. However, __delete_from_page_cache() can set set page->mapping
> to NULL and leave page->index value alone. Due to being in union, a
> non-zero page->index will be interpreted as true page->pfmemalloc.
>
> So the assumption is invalid if the networking code can see such a
> page. And it seems it can. We have encountered this with a NFS over
> loopback setup when such a page is attached to a new skbuf. There is no
> copying going on in this case so the page confuses __skb_fill_page_desc
> which interprets the index as pfmemalloc flag and the network stack
> drops packets that have been allocated using the reserves unless they
> are to be queued on sockets handling the swapping which is the case here
^ not ?
The full story (according to Jiri Bohac and my understanding, I don't
know much about netdev) is that the __skb_fill_page_desc() is invoked
here during *sending* and normally the skb->pfmemalloc would be ignored
in the end. But because it is a localhost connection, the receiving code
will think it was a memalloc allocation during receive, and then do the
socket restriction.
Given that this apparently isn't the first case of this localhost issue,
I wonder if network code should just clear skb->pfmemalloc during send
(or maybe just send over localhost). That would be probably easier than
distinguish the __skb_fill_page_desc() callers for send vs receive.
> and that leads to hangs when the nfs client waits for a response from
> the server which has been dropped and thus never arrive.
next prev parent reply other threads:[~2015-08-13 9:13 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-08-13 8:58 [PATCH] mm: make page pfmemalloc check more robust mhocko
2015-08-13 9:13 ` Vlastimil Babka [this message]
2015-08-13 9:31 ` Michal Hocko
2015-08-13 14:40 ` Eric Dumazet
2015-08-14 13:26 ` Vlastimil Babka
2015-08-13 10:00 ` Mel Gorman
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