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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
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 09:44:51 -0700	[thread overview]
Message-ID: <CAHk-=wgNooZpC8G4=0dB-OBT5PYuv9d=sqzUXS9Ea_acjrAi_A@mail.gmail.com> (raw)
In-Reply-To: <20201015094344.pmvg2jxrb2bsoanr@box>

On Thu, Oct 15, 2020 at 2:43 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
>
> Okay, I see what you propose.
>
> But I don't think it addresses race with try_to_unmap():

I don't think it needs to.

Remember: the map_pages() thing is called only for when the page
tables are empty.

So try_to_unmap() will never see the pte entry, and there is nothing
to race with.

So sure, it can "race" with try_to_unmap like you say, but who cares?
The "race" is no different from taking the page lock _after_
try_to_unmap() already ran (and didn't see anything because the page
hadn't been mapped yet).

IOW I don't think try_to_unmap() really matters. The race you outline
can already happen with the "trylock()" - no different from the
trylock just succeeding after the unlock_page().

So you can think of map_pages() as all happening after try_to_unmap()
has already succeeded - and didn't see the new pte that hasn't been
filled in yet.

I do think there is a real race, but it is is with "__remove_mapping()".

That still happens under the page lock, but it doesn't actually
_depend_ on the page lock as far as I can tell. Because I think the
real protection there is that

        if (!page_ref_freeze(page, refcount))
                goto cannot_free;

it that code sees "oh, somebody else has a reference to the page, we
can't remove the mapping".

But it's entirely possible that I don't understand your worry, and I
overlooked something. If so, can you explain using smaller words,
please ;)

             Linus


  reply	other threads:[~2020-10-15 16:45 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
2020-10-15 16:44         ` Linus Torvalds [this message]
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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