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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Hugh Dickins <hughd@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>,
	"Kirill A. Shutemov" <kirill.shutemov@intel.linux.com>,
	Michel Lespinasse <walken@google.com>,
	akpm@linux-foundation.org, riel@redhat.com, mgorman@suse.de,
	peterz@infradead.org, mingo@kernel.org,
	linux-kernel@vger.kernel.org, dbueso@suse.de, linux-mm@kvack.org
Subject: Re: [PATCH 08/10] mm/mremap: share the i_mmap_rwsem
Date: Tue, 4 Nov 2014 14:29:01 +0200	[thread overview]
Message-ID: <20141104122901.GA28274@node.dhcp.inet.fi> (raw)
In-Reply-To: <alpine.LSU.2.11.1411032148230.15596@eggly.anvils>

On Mon, Nov 03, 2014 at 10:04:24PM -0800, Hugh Dickins wrote:
> I'm glad to see this series back, and nicely presented: thank you.
> Not worth respinning them, but consider 1,2,3,4,5,6,7 and 9 as
> Acked-by: Hugh Dickins <hughd@google.com>
> 
> On Thu, 30 Oct 2014, Davidlohr Bueso wrote:
> 
> > As per the comment in move_ptes(), we only require taking the
> > anon vma and i_mmap locks to ensure that rmap will always observe
> > either the old or new ptes, in the case of need_rmap_lock=true.
> > No modifications to the tree itself, thus share the i_mmap_rwsem.
> > 
> > Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@intel.linux.com>
> 
> But this one is Nacked by me.  I don't understand how you and Kirill
> could read Michel's painstaking comment on need_rmap_locks, then go
> go ahead and remove the exclusion of rmap_walk().
> 
> I agree the code here does not modify the interval tree, but the
> comment explains how we're moving a pte from one place in the tree
> to another, and in some cases there's a danger that the rmap walk
> might miss the pte from both places (which doesn't matter much to
> most of its uses, but is critical in page migration).
> 
> Or am I the one missing something?

You're completely right.

I've seen the comment (and I've added the missed need_rmap_locks case for
move_huge_pmd() before). What happened is I've over-extrapolated my
experience of rmap walk in case of split_huge_page(), which takes exclusive
anon_vma lock, to the rest of rmap use-cases. This of course was hugely
wrong.

I'm ashamed and feel really bad about the situation. Sorry.

-- 
 Kirill A. Shutemov

  reply	other threads:[~2014-11-04 12:31 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-10-30 19:34 [PATCH v2 -next 00/10] mm: improve usage of the i_mmap lock Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 01/10] mm,fs: introduce helpers around the i_mmap_mutex Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 02/10] mm: use new helper functions " Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 03/10] mm: convert i_mmap_mutex to rwsem Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 04/10] mm/rmap: share the i_mmap_rwsem Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 05/10] uprobes: " Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 06/10] mm/xip: " Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 07/10] mm/memory-failure: " Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 08/10] mm/mremap: " Davidlohr Bueso
2014-11-04  6:04   ` Hugh Dickins
2014-11-04 12:29     ` Kirill A. Shutemov [this message]
2014-10-30 19:34 ` [PATCH 09/10] mm/nommu: " Davidlohr Bueso
2014-10-30 19:34 ` [PATCH 10/10] mm/hugetlb: " Davidlohr Bueso
2014-11-04  6:35   ` Hugh Dickins
2014-11-05  0:59     ` Davidlohr Bueso
2014-11-05 16:04       ` Hugh Dickins
2014-11-10 16:22     ` Mel Gorman
2014-11-03  8:17 ` [PATCH v2 -next 00/10] mm: improve usage of the i_mmap lock Peter Zijlstra
2014-11-10 16:28 ` Mel Gorman
  -- strict thread matches above, loose matches on Subject: below --
2014-10-24 22:06 [PATCH " Davidlohr Bueso
2014-10-24 22:06 ` [PATCH 08/10] mm/mremap: share the i_mmap_rwsem Davidlohr Bueso

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