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From: Matthew Wilcox <>
To: Qian Cai <>
Cc: Huang Ying <>,, "Kirill A. Shutemov" <>
Subject: Re: page cache: Store only head pages in i_pages
Date: Sat, 30 Mar 2019 20:23:26 -0700
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, Mar 30, 2019 at 07:10:52AM -0700, Matthew Wilcox wrote:
> On Fri, Mar 29, 2019 at 08:04:32PM -0700, Matthew Wilcox wrote:
> > Excellent!  I'm not comfortable with the rule that you have to be holding
> > the i_pages lock in order to call find_get_page() on a swap address_space.
> > How does this look to the various smart people who know far more about the
> > MM than I do?
> > 
> > The idea is to ensure that if this race does happen, the page will be
> > handled the same way as a pagecache page.  If __delete_from_swap_cache()
> > can be called while the page is still part of a VMA, then this patch
> > will break page_to_pgoff().  But I don't think that can happen ... ?
> Oh, blah, that can totally happen.  reuse_swap_page() calls
> delete_from_swap_cache().  Need a new plan.

I don't see a good solution here that doesn't involve withdrawing this
patch and starting over.  Bad solutions:

 - Take the i_pages lock around each page lookup call in the swap code
   (not just the one you found; there are others like mc_handle_swap_pte()
   in memcontrol.c)
 - Call synchronize_rcu() in __delete_from_swap_cache()
 - Swap the roles of ->index and ->private for swap pages, and then don't
   clear ->index when deleting a page from the swap cache

The first two would be slow and non-scalable.  The third is still prone
to a race where the page is looked up on one CPU, while another CPU
removes it from one swap file then moves it to a different location,
potentially in a different swap file.  Hard to hit, but not a race we
want to introduce.

I believe that the swap code actually never wants to see subpages.  So if
we start again, introducing APIs (eg find_get_head()) which return the
head page, then convert the swap code over to use those APIs, we don't
need to solve the problem of finding the subpage of a swap page while
not holding the page lock.

I'm obviously reluctant to withdraw the patch, but I don't see a better
option.  Your testing has revealed a problem that needs a deeper solution
than just adding a fix patch.

  reply index

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <>
2019-03-23  3:38 ` Matthew Wilcox
2019-03-23 23:50   ` Qian Cai
2019-03-24  2:06     ` Matthew Wilcox
2019-03-24  2:52       ` Qian Cai
2019-03-24  3:04         ` Matthew Wilcox
2019-03-24 15:42           ` Qian Cai
2019-03-27 10:48           ` William Kucharski
2019-03-27 11:50             ` Matthew Wilcox
2019-03-29  1:43           ` Qian Cai
2019-03-29 19:59             ` Matthew Wilcox
2019-03-29 21:25               ` Qian Cai
2019-03-30  3:04                 ` Matthew Wilcox
2019-03-30 14:10                   ` Matthew Wilcox
2019-03-31  3:23                     ` Matthew Wilcox [this message]
2019-04-01  9:18                       ` Kirill A. Shutemov
2019-04-01  9:27                         ` Kirill A. Shutemov
2019-04-04 13:10                           ` Qian Cai
2019-04-04 13:45                             ` Kirill A. Shutemov
2019-04-04 21:28                               ` Qian Cai
2019-04-05 13:37                                 ` Kirill A. Shutemov
2019-04-05 13:51                                   ` Matthew Wilcox

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