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From: Matthew Wilcox <willy@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	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: Wed, 14 Oct 2020 19:15:09 +0100	[thread overview]
Message-ID: <20201014181509.GU20115@casper.infradead.org> (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*
[...]
> 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 ;-)


  reply	other threads:[~2020-10-14 18:15 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 [this message]
2020-10-15 10:41         ` Kirill A. Shutemov
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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