linux-kernel - Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write

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Date:	Fri, 3 Jun 2016 17:10:01 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>, linux-mm@...ck.org,
	linux-next@...r.kernel.org, linux-kernel@...r.kernel.org,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit
 rwsem_down_write_failed lockup

Hello Michal,

CC'ed Hugh,

On Fri, Jun 03, 2016 at 04:46:00PM +0200, Michal Hocko wrote:
> What do you think about the external dependencies mentioned above. Do
> you think this is a sufficient argument wrt. occasional higher
> latencies?

It's a tradeoff and both latencies would be short and uncommon so it's
hard to tell.

There's also mmput_async for paths that may care about mmput
latencies. Exit itself cannot use it, it's mostly for people taking
the mm_users pin that may not want to wait for mmput to run. It also
shouldn't happen that often, it's a slow path.

The whole model inherited from KSM is to deliberately depend only on
the mmap_sem + test_exit + mm_count, and never on mm_users, which to
me in principle doesn't sound bad. I consider KSM version a
"finegrined" implementation but I never thought it would be a problem
to wait a bit in exit() in case the slow path hits. I thought it was
more of a problem if exit() runs, the parent then start a new task but
the memory wasn't freed yet.

So I would suggest Hugh to share his view on the down_write/up_write
that may temporarily block mmput (until the next test_exit bailout
point) vs higher latency in reaching exit_mmap for a real exit(2) that
would happen with the proposed change.

Thanks!
Andrea