From: Matthew Wilcox <willy@infradead.org>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
John Hubbard <jhubbard@nvidia.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux-MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
Peter Xu <peterx@redhat.com>, Pavel Emelyanov <xemul@openvz.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
Minchan Kim <minchan@kernel.org>, Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Hugh Dickins <hughd@google.com>, Oleg Nesterov <oleg@redhat.com>,
Jann Horn <jannh@google.com>, Kees Cook <keescook@chromium.org>,
Leon Romanovsky <leonro@nvidia.com>,
Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
Kirill Tkhai <ktkhai@virtuozzo.com>,
Nadav Amit <nadav.amit@gmail.com>, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 0/1] mm: restore full accuracy in COW page reuse
Date: Wed, 13 Jan 2021 12:55:32 +0000 [thread overview]
Message-ID: <20210113125532.GN35215@casper.infradead.org> (raw)
In-Reply-To: <20210113123232.62vv6xsrpitne7hc@box>
On Wed, Jan 13, 2021 at 03:32:32PM +0300, Kirill A. Shutemov wrote:
> On Tue, Jan 12, 2021 at 07:31:07PM -0800, Linus Torvalds wrote:
> > On Tue, Jan 12, 2021 at 6:16 PM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > The thing about the speculative page cache references is that they can
> > > temporarily bump a refcount on a page which _used_ to be in the page
> > > cache and has now been reallocated as some other kind of page.
> >
> > Oh, and thinking about this made me think we might actually have a
> > serious bug here, and it has nothing what-so-ever to do with COW, GUP,
> > or even the page count itself.
> >
> > It's unlikely enough that I think it's mostly theoretical, but tell me
> > I'm wrong.
> >
> > PLEASE tell me I'm wrong:
> >
> > CPU1 does page_cache_get_speculative under RCU lock
> >
> > CPU2 frees and re-uses the page
> >
> > CPU1 CPU2
> > ---- ----
> >
> > page = xas_load(&xas);
> > if (!page_cache_get_speculative(page))
> > goto repeat;
> > .. succeeds ..
> >
> > remove page from XA
> > release page
> > reuse for something else
>
> How can it be reused if CPU1 hold reference to it?
Yes, Linus mis-stated it:
page = xas_load(&xas);
remove page from XA
release page
reuse for something else
if (!page_cache_get_speculative(page))
goto repeat;
if (unlikely(page != xas_reload(&xas))) {
put_page(page);
... but as David pointed out, I fixed this in e320d3012d25
next prev parent reply other threads:[~2021-01-13 12:57 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-10 0:44 [PATCH 0/1] mm: restore full accuracy in COW page reuse Andrea Arcangeli
2021-01-10 0:44 ` [PATCH 1/1] " Andrea Arcangeli
2021-01-10 2:54 ` Andrea Arcangeli
2021-01-11 14:11 ` Kirill A. Shutemov
2021-01-10 0:55 ` [PATCH 0/1] " Linus Torvalds
2021-01-10 1:19 ` Linus Torvalds
2021-01-10 1:37 ` Linus Torvalds
2021-01-10 3:24 ` Andrea Arcangeli
2021-01-10 2:51 ` Andrea Arcangeli
2021-01-10 3:51 ` Linus Torvalds
2021-01-10 19:30 ` Linus Torvalds
2021-01-11 1:18 ` Jason Gunthorpe
2021-01-11 7:26 ` John Hubbard
2021-01-11 12:42 ` Matthew Wilcox
2021-01-11 16:05 ` Jason Gunthorpe
2021-01-11 16:15 ` Michal Hocko
2021-01-11 19:19 ` Linus Torvalds
2021-01-11 22:18 ` Linus Torvalds
2021-01-12 17:07 ` Andy Lutomirski
2021-01-12 23:51 ` Jerome Glisse
2021-01-13 2:16 ` Matthew Wilcox
2021-01-13 2:43 ` Linus Torvalds
2021-01-13 3:31 ` Linus Torvalds
2021-01-13 8:52 ` David Hildenbrand
2021-01-13 8:57 ` David Hildenbrand
2021-01-13 12:32 ` Kirill A. Shutemov
2021-01-13 12:55 ` Matthew Wilcox [this message]
2021-01-13 19:54 ` Linus Torvalds
2021-01-13 23:54 ` Peter Xu
2021-01-11 15:52 ` Jason Gunthorpe
2021-01-15 8:59 ` David Hildenbrand
2021-01-15 18:37 ` Jason Gunthorpe
2021-01-15 19:46 ` David Hildenbrand
2021-01-15 19:53 ` Jason Gunthorpe
2021-01-16 3:40 ` John Hubbard
2021-01-16 11:42 ` David Hildenbrand
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