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

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 [off-list ref]
Debugged-by: Jiri Bohac [off-list ref]
Signed-off-by: Michal Hocko <mhocko@suse.com>

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

-- 
Mel Gorman
SUSE Labs

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