linux-kernel - mmap_sem contention issues

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Date:	Mon, 26 Nov 2012 12:12:16 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: mmap_sem contention issues

I'm having serious latency problems due to mmap_sem contention.  I
have a real-time thread that has (soft) page faults on locked pages,
and it blocks for multiple milliseconds on
(call_rwsem_down_read_failed do_page_fault page_fault).  Can this be
fixed?

Some ideas:

1. Drop mmap_sem during the filesystem part of mmap and munmap.
(MAP_POPULATE in particular is a disaster -- using it will easily
increase latency from a few milliseconds to a respectable fraction of
a second.)

2. Come up with some way to lock specific vm_area_structs for read
access without taking mmap_sem at all.  This looks unpleasant with the
current rbtree structure -- something like a radix tree might work
much better if the nodes were to contain their own locks.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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