linux-kernel - MAP_HUGETLB and MPOL

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Message-ID: <CALCETrVXzXLqvB+S6BMXuFbVGftg5Kwk+fGe1=w1G9S67S_ivA@mail.gmail.com>
Date:	Thu, 18 Jul 2013 17:51:06 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: MAP_HUGETLB and MPOL_PREFERRED = SIGBUS

When I mmap anonymous hugepages with MAP_HUGETLB and there are
available (pre-reserved) hugepages available, but only on the wrong
node, things blow up.  The mmap succeeds, as it should (the accounting
here is wrong -- known issue AFAIK, but that's only relevant to
MPOL_BIND or cpusets).  But writing to the resulting page causes a
SIGBUS.

AFAICS the issue is that dequeue_huge_page_vma is calling
huge_zonelist, which returns a single-entry nodemask.  The loop over
allowable zones* will never try other numa zones, and the function
fails.

I'm not sure whether it would be better to try other nodes first or to
try get get a page from the buddy allocator on the preferred node
first, but currently the other nodes' reserved lists are never
checked.  The result is a crash.

Working around this in userspace is going to be a real PITA.  Grr.

* Why is this iterating zones instead of nodes?

--Andy
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