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From: Matthew Wilcox <willy@infradead.org>
To: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Michal Hocko <mhocko@kernel.org>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: mmap: unmap large mapping by section
Date: Wed, 21 Mar 2018 15:15:02 -0700	[thread overview]
Message-ID: <20180321221502.GA3969@bombadil.infradead.org> (raw)
In-Reply-To: <f057a634-7e0a-1b51-eede-dcb6f128b18e@linux.alibaba.com>

On Wed, Mar 21, 2018 at 02:45:44PM -0700, Yang Shi wrote:
> On 3/21/18 10:29 AM, Matthew Wilcox wrote:
> > On Wed, Mar 21, 2018 at 09:31:22AM -0700, Yang Shi wrote:
> > > On 3/21/18 6:08 AM, Michal Hocko wrote:
> > > > Yes, this definitely sucks. One way to work that around is to split the
> > > > unmap to two phases. One to drop all the pages. That would only need
> > > > mmap_sem for read and then tear down the mapping with the mmap_sem for
> > > > write. This wouldn't help for parallel mmap_sem writers but those really
> > > > need a different approach (e.g. the range locking).
> > > page fault might sneak in to map a page which has been unmapped before?
> > > 
> > > range locking should help a lot on manipulating small sections of a large
> > > mapping in parallel or multiple small mappings. It may not achieve too much
> > > for single large mapping.
> > I don't think we need range locking.  What if we do munmap this way:
> > 
> > Take the mmap_sem for write
> > Find the VMA
> >    If the VMA is large(*)
> >      Mark the VMA as deleted
> >      Drop the mmap_sem
> >      zap all of the entries
> >      Take the mmap_sem
> >    Else
> >      zap all of the entries
> > Continue finding VMAs
> > Drop the mmap_sem
> > 
> > Now we need to change everywhere which looks up a VMA to see if it needs
> > to care the the VMA is deleted (page faults, eg will need to SIGBUS; mmap
> 
> Marking vma as deleted sounds good. The problem for my current approach is
> the concurrent page fault may succeed if it access the not yet unmapped
> section. Marking deleted vma could tell page fault the vma is not valid
> anymore, then return SIGSEGV.
> 
> > does not care; munmap will need to wait for the existing munmap operation
> 
> Why mmap doesn't care? How about MAP_FIXED? It may fail unexpectedly, right?

Oh, I forgot about MAP_FIXED.  Yes, MAP_FIXED should wait for the munmap
to finish.  But a regular mmap can just pretend that it happened before
the munmap call and avoid the deleted VMAs.

  reply	other threads:[~2018-03-21 22:15 UTC|newest]

Thread overview: 41+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-20 21:31 [RFC PATCH 0/8] Drop mmap_sem during unmapping large map Yang Shi
2018-03-20 21:31 ` [RFC PATCH 1/8] mm: mmap: unmap large mapping by section Yang Shi
2018-03-21 13:08   ` Michal Hocko
2018-03-21 16:31     ` Yang Shi
2018-03-21 17:29       ` Matthew Wilcox
2018-03-21 21:45         ` Yang Shi
2018-03-21 22:15           ` Matthew Wilcox [this message]
2018-03-21 22:40             ` Yang Shi
2018-03-21 22:46           ` Matthew Wilcox
2018-03-22 15:32             ` Laurent Dufour
2018-03-22 15:40               ` Matthew Wilcox
2018-03-22 15:54                 ` Laurent Dufour
2018-03-22 16:05                   ` Matthew Wilcox
2018-03-22 16:18                     ` Laurent Dufour
2018-03-22 16:46                       ` Yang Shi
2018-03-23 13:03                         ` Laurent Dufour
2018-03-22 16:51                       ` Matthew Wilcox
2018-03-22 16:49                     ` Yang Shi
2018-03-22 17:34         ` Yang Shi
2018-03-22 18:48           ` Matthew Wilcox
2018-03-24 18:24         ` Jerome Glisse
2018-03-21 13:14   ` Michal Hocko
2018-03-21 16:50     ` Yang Shi
2018-03-21 17:16       ` Yang Shi
2018-03-21 21:23         ` Michal Hocko
2018-03-21 22:36           ` Yang Shi
2018-03-22  9:10             ` Michal Hocko
2018-03-22 16:06               ` Yang Shi
2018-03-22 16:12                 ` Michal Hocko
2018-03-22 16:13                 ` Matthew Wilcox
2018-03-22 16:28                   ` Laurent Dufour
2018-03-22 16:36                     ` David Laight
2018-03-20 21:31 ` [RFC PATCH 2/8] mm: mmap: pass atomic parameter to do_munmap() call sites Yang Shi
2018-03-20 21:31 ` [RFC PATCH 3/8] mm: mremap: pass atomic parameter to do_munmap() Yang Shi
2018-03-20 21:31 ` [RFC PATCH 4/8] mm: nommu: add " Yang Shi
2018-03-20 21:31 ` [RFC PATCH 5/8] ipc: shm: pass " Yang Shi
2018-03-20 21:31 ` [RFC PATCH 6/8] fs: proc/vmcore: " Yang Shi
2018-03-20 21:31 ` [RFC PATCH 7/8] x86: mpx: " Yang Shi
2018-03-20 22:35   ` Thomas Gleixner
2018-03-21 16:53     ` Yang Shi
2018-03-20 21:31 ` [RFC PATCH 8/8] x86: vma: " Yang Shi

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