Re: [PATCH] mm: make page pfmemalloc check more robust

[Mel Gorman <mgorman@suse.de>]

On Thu, Aug 13, 2015 at 10:58:54AM +0200, 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
> and that leads to hangs when the nfs client waits for a response from
> the server which has been dropped and thus never arrive.
> 
> The struct page is already heavily packed so rather than finding
> another hole to put it in, let's do a trick instead. We can reuse the
> index again but define it to an impossible value (-1UL). This is the
> page index so it should never see the value that large. Replace all
> direct users of page->pfmemalloc by page_is_pfmemalloc which will
> hide this nastiness from unspoiled eyes.
> 
> The information will get lost if somebody wants to use page->index
> obviously but that was the case before and the original code expected
> that the information should be persisted somewhere else if that is
> really needed (e.g. what SLAB and SLUB do).
> 
> Fixes: c48a11c7ad26 ("netvm: propagate page->pfmemalloc to skb")
> Cc: stable # 3.6+
> Debugged-by: Vlastimil Babka <vbabka@suse.com>
> Debugged-by: Jiri Bohac <jbohac@suse.com>
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Acked-by: Mel Gorman <mgorman@suse.de>

-- 
Mel Gorman
SUSE Labs