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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Linux-MM <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Amir Goldstein <amir73il@gmail.com>
Subject: Re: [PATCH 0/4] Some more lock_page work..
Date: Thu, 15 Oct 2020 12:43:44 +0300	[thread overview]
Message-ID: <20201015094344.pmvg2jxrb2bsoanr@box> (raw)
In-Reply-To: <CAHk-=wjXBv0ZKqH4muuo2j4bH2km=7wedrEeQJxY6g2JcdOZSQ@mail.gmail.com>

On Wed, Oct 14, 2020 at 09:53:35AM -0700, Linus Torvalds wrote:
> In particular, what I _think_ we could do is:
> 
>  - lock the page tables
> 
>  - check that the page isn't locked
> 
>  - increment the page mapcount (atomic and ordered)
> 
>  - check that the page still isn't locked
> 
>  - insert pte
> 
> without taking the page lock. And the reason that's safe is that *if*
> we're racing with something that is about the remove the page mapping,
> then *that* code will
> 
>  (a) hold the page lock
> 
>  (b) before it removes the mapping, it has to remove it from any
> existing mappings, which involves checking the mapcount and going off
> and getting the page table locks to remove any ptes.
> 
> but my patch did *not* do that, because you have to re-organize things a bit.

Okay, I see what you propose.

But I don't think it addresses race with try_to_unmap():

	CPU0				CPU1
filemap_map_pages()
  take ptl
  PageLocked() == false
 				lock_page()
				try_to_unmap()
				  rwc->done()
				    page_mapcount_is_zero() == true
				  rwc->done() == true, skip full rmap walk
				  never take ptl taken by CPU0
				try_to_unmap() == true
				...
				unlock_page()
  increment mapcount
  PageLocked() == false
  insert PTE

Are we willing to give up rwc->done() optimization?
Or do I miss some other serialization point?

-- 
 Kirill A. Shutemov


  parent reply	other threads:[~2020-10-15  9:43 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-13 19:59 Linus Torvalds
2020-10-13 20:03 ` Linus Torvalds
2020-10-14 13:05   ` Kirill A. Shutemov
2020-10-14 16:53     ` Linus Torvalds
2020-10-14 18:15       ` Matthew Wilcox
2020-10-15 10:41         ` Kirill A. Shutemov
2020-10-15  9:43       ` Kirill A. Shutemov [this message]
2020-10-15 16:44         ` Linus Torvalds
2020-10-14  5:50 ` Hugh Dickins
     [not found] ` <4794a3fa3742a5e84fb0f934944204b55730829b.camel@lca.pw>
2020-10-15  2:44   ` Linus Torvalds
2020-10-15 15:16     ` Possible deadlock in fuse write path (Was: Re: [PATCH 0/4] Some more lock_page work..) Vivek Goyal
2020-10-15 19:55       ` Vivek Goyal
2020-10-15 21:21         ` Linus Torvalds
2020-10-16 10:02           ` Miklos Szeredi
2020-10-16 12:27             ` Matthew Wilcox
2020-10-20 20:42             ` Vivek Goyal
2020-10-21  7:40               ` Miklos Szeredi
2020-10-21 20:12                 ` Vivek Goyal
2020-10-28 20:29                   ` Miklos Szeredi
2021-02-09 10:01                     ` Miklos Szeredi
2021-02-09 19:09                       ` Vivek Goyal
2020-10-16 18:19           ` Vivek Goyal
2020-10-16 18:24             ` Linus Torvalds
2020-10-16 18:24               ` Linus Torvalds
2020-10-16 23:03             ` Dave Chinner

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