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From: Michal Hocko <mhocko@kernel.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-mm@kvack.org, linux-next@vger.kernel.org,
	linux-kernel@vger.kernel.org, Hugh Dickins <hughd@google.com>
Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup
Date: Tue, 7 Jun 2016 09:34:28 +0200	[thread overview]
Message-ID: <20160607073427.GC12305@dhcp22.suse.cz> (raw)
In-Reply-To: <20160603151001.GG29930@redhat.com>

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

  reply	other threads:[~2016-06-07  7:34 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-01  3:11 linux-next: Tree for Jun 1 Stephen Rothwell
2016-06-02  1:48 ` [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup Sergey Senozhatsky
2016-06-02  9:21   ` Michal Hocko
2016-06-02 12:08     ` Sergey Senozhatsky
2016-06-02 12:21       ` Michal Hocko
2016-06-03 13:51         ` Andrea Arcangeli
2016-06-03 14:46           ` Michal Hocko
2016-06-03 15:10             ` Andrea Arcangeli
2016-06-07  7:34               ` Michal Hocko [this message]
2016-06-08  8:19               ` Vlastimil Babka
2016-06-03  7:15     ` Sergey Senozhatsky
2016-06-03  7:25       ` Michal Hocko
2016-06-03  8:43         ` Sergey Senozhatsky
2016-06-03  9:55           ` Michal Hocko
2016-06-03 10:05             ` Michal Hocko
2016-06-03 13:38               ` Sergey Senozhatsky
2016-06-03 13:45                 ` Michal Hocko
2016-06-03 13:49                   ` Michal Hocko
2016-06-04  7:51                     ` Sergey Senozhatsky
2016-06-06  8:39                       ` Michal Hocko
2016-06-02 13:24   ` Vlastimil Babka
2016-06-02 18:58     ` Ebru Akagunduz
2016-06-03  1:00       ` Sergey Senozhatsky
2016-06-03  1:29         ` Sergey Senozhatsky
2016-06-03  4:14           ` Sergey Senozhatsky
2016-06-03 12:28     ` [PATCH] mm, thp: fix locking inconsistency in collapse_huge_page Ebru Akagunduz
2016-06-06 13:05       ` Vlastimil Babka
2016-06-09  3:51         ` Sergey Senozhatsky

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