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

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Date:	Sat, 4 Jun 2016 16:51:14 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrea Arcangeli <aarcange@...hat.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
Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit
 rwsem_down_write_failed lockup

Hello,

On (06/03/16 15:49), Michal Hocko wrote:
> __khugepaged_exit is called during the final __mmput and it employs a
> complex synchronization dances to make sure it doesn't race with the
> khugepaged which might be scanning this mm at the same time. This is
> all caused by the fact that khugepaged doesn't pin mm_users. Things
> would simplify considerably if we simply check the mm at
> khugepaged_scan_mm_slot and if mm_users was already 0 then we know it
> is dead and we can unhash the mm_slot and move on to another one. This
> will also guarantee that __khugepaged_exit cannot race with khugepaged
> and so we can free up the slot if it is still hashed.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

with this patch and
http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-make-swapin-readahead-under-down_read-of-mmap_sem-fix-2.patch

I saw no problems during my tests (well, may be didn't test hard
enough).

	-ss