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From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Matthew Wilcox <willy@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Hugh Dickins <hughd@google.com>, 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 13:41:39 +0300	[thread overview]
Message-ID: <20201015104139.ryllzhzdxevrjrbg@box> (raw)
In-Reply-To: <20201014181509.GU20115@casper.infradead.org>

On Wed, Oct 14, 2020 at 07:15:09PM +0100, Matthew Wilcox wrote:
> 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*
> [...]
> > And they aren't necessarily a _lot_ more involved. In fact, I think we
> > may already hold the page table lock due to doing that
> > "pte_alloc_one_map()" thing over all of filemap_map_pages(). So I
> > think the only _real_ problem is that I think we increment the
> > page_mapcount() too late in alloc_set_pte().
> 
> I'm not entirely sure why we require the page lock to be held in
> page_add_file_rmap():
> 
>         } else {
>                 if (PageTransCompound(page) && page_mapping(page)) {
>                         VM_WARN_ON_ONCE(!PageLocked(page));
> 
>                         SetPageDoubleMap(compound_head(page));
>                         if (PageMlocked(page))
>                                 clear_page_mlock(compound_head(page));
>                 }
> 
> We have a reference to the page, so compound_head() isn't going
> to change.  SetPageDoubleMap() is atomic.  PageMlocked() is atomic.
> clear_page_mlock() does TestClearPageMlocked() as its first thing,
> so that's atomic too.  What am I missing?  (Kirill added it, so I
> assume he remembers ;-)

In general (with current scheme), we need page lock here to serialize
against try_to_unmap() and other rmap walkers.

For file THP, we also serialize against follow_trans_huge_pmd(FOLL_MLOCK).

-- 
 Kirill A. Shutemov


  reply	other threads:[~2020-10-15 10:41 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 [this message]
2020-10-15  9:43       ` Kirill A. Shutemov
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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