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

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Date:	Tue, 7 Jun 2016 09:34:28 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Andrea Arcangeli <aarcange@...hat.com>
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

On Fri 03-06-16 17:10:01, Andrea Arcangeli wrote:
> 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 do agree that this model is quite clever (albeit convoluted). It just
assumes that all other mmap_sem users are behaving the same. Now most
in-kernel users will do get_task_mm() and then lock mmap_sem, but I
haven't checked all of them and it is quite possible that some of those
would like to optimize in a similar way and only increment mm_count.
I might be too pessimistic about the out of mm code but I would feel
much better if the exit path didn't depend on them.

Anyway, if the current model sounds better I will definitely not insist
on my patch. It is more of an idea for simplification than a fix for
anything I have seen happening in the real life.
-- 
Michal Hocko
SUSE Labs