linux-kernel - Re: [PATCH 0/9] Use RCU to stabilize page counts

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Date:	Fri, 19 Aug 2011 00:53:43 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Shaohua Li <shaohua.li@...el.com>
Subject: Re: [PATCH 0/9] Use RCU to stabilize page counts

Adding Paul - I meant to have him in the original email, but git
send-email filtered him out because I forgot to add <> around his
email. DOH!

On Fri, Aug 19, 2011 at 12:48 AM, Michel Lespinasse <walken@...gle.com> wrote:
> include/linux/pagemap.h describes the protocol one should use to get pages
> from page cache - one can't know if the reference they get will be on the
> desired page, so newly allocated pages might see elevated reference counts,
> but using RCU this effect can be limited in time to one RCU grace period.
>
> For this protocol to work, every call site of get_page_unless_zero() has to
> participate, and this was not previously enforced.
>
> Patches 1-3 convert some get_page_unless_zero() call sites to use the proper
> RCU protocol as described in pagemap.h
>
> Patches 4-5 convert some get_page_unless_zero() call sites to just call
> get_page()
>
> Patch 6 asserts that every remaining get_page_unless_zero() call site should
> participate in the RCU protocol. Well, not actually all of them -
> __isolate_rcu_page() is exempted because it holds the zone LRU lock which
> would prevent the given page from getting entirely freed, and a few others
> related to hwpoison, memory hotplug and memory failure are exempted because
> I haven't been able to figure out what to do.
>
> Patch 7 is a placeholder for an RCU API extension we have been talking about
> with Paul McKenney. The idea is to record an initial time as an opaque cookie,
> and to be able to determine later on if an rcu grace period has elapsed since
> that initial time.
>
> Patch 8 adds wrapper functions to store an RCU cookie into compound pages.
>
> Patch 9 makes use of new RCU API, as well as the prior fixes from patches 1-6,
> to ensure tail page counts are stable while we split THP pages. This fixes a
> (rather theorical, not actually been observed) race condition where THP page
> splitting could result in incorrect page counts if THP page allocation and
> splitting both occur while another thread tries to run get_page_unless_zero
> on a single page that got re-allocated as THP tail page.
>
>
> The patches have received only a limited amount of testing; however I
> believe patches 1-6 to be sane and I would like them to get more
> exposure, maybe as part of andrew's -mm tree.
>
>
> Besides that, this proposal is also to sync up with Paul regarding the RCU
> functionality :)
>
>
> Michel Lespinasse (9):
>  mm: rcu read lock for getting reference on pages in
>    migration_entry_wait()
>  mm: avoid calling get_page_unless_zero() when charging cgroups
>  mm: rcu read lock when getting from tail to head page
>  mm: use get_page in deactivate_page()
>  kvm: use get_page instead of get_page_unless_zero
>  mm: assert that get_page_unless_zero() callers hold the rcu lock
>  rcu: rcu_get_gp_cookie() / rcu_gp_cookie_elapsed() stand-ins
>  mm: add API for setting a grace period cookie on compound pages
>  mm: make sure tail page counts are stable before splitting THP pages
>
>  arch/x86/kvm/mmu.c       |    3 +--
>  include/linux/mm.h       |   38 +++++++++++++++++++++++++++++++++++++-
>  include/linux/mm_types.h |    6 +++++-
>  include/linux/pagemap.h  |    1 +
>  include/linux/rcupdate.h |   35 +++++++++++++++++++++++++++++++++++
>  mm/huge_memory.c         |   33 +++++++++++++++++++++++++++++----
>  mm/hwpoison-inject.c     |    2 +-
>  mm/ksm.c                 |    4 ++++
>  mm/memcontrol.c          |   20 ++++++++++----------
>  mm/memory-failure.c      |    6 +++---
>  mm/memory_hotplug.c      |    2 +-
>  mm/migrate.c             |    3 +++
>  mm/page_alloc.c          |    1 +
>  mm/swap.c                |   22 ++++++++++++++--------
>  mm/vmscan.c              |    7 ++++++-
>  15 files changed, 151 insertions(+), 32 deletions(-)

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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