From: Yu Zhao <yuzhao@google.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Andy Lutomirski <luto@amacapital.net>,
Andy Lutomirski <luto@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Xu <peterx@redhat.com>, linux-mm <linux-mm@kvack.org>,
lkml <linux-kernel@vger.kernel.org>,
Pavel Emelyanov <xemul@openvz.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Mike Rapoport <rppt@linux.vnet.ibm.com>,
stable <stable@vger.kernel.org>, Minchan Kim <minchan@kernel.org>,
Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Wed, 23 Dec 2020 20:34:00 -0700 [thread overview]
Message-ID: <X+QMKC7jPEeThjB1@google.com> (raw)
In-Reply-To: <3A6A1049-24C6-4B2D-8C59-21B549F742B4@gmail.com>
On Wed, Dec 23, 2020 at 07:09:10PM -0800, Nadav Amit wrote:
> > On Dec 23, 2020, at 6:00 PM, Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > On Wed, Dec 23, 2020 at 05:21:43PM -0800, Andy Lutomirski wrote:
> >> I don’t love this as a long term fix. AFAICT we can have mm_tlb_flush_pending set for quite a while — mprotect seems like it can wait in IO while splitting a huge page, for example. That gives us a window in which every write fault turns into a TLB flush.
> >
> > mprotect can't run concurrently with a page fault in the first place.
> >
> > One other near zero cost improvement easy to add if this would be "if
> > (vma->vm_flags & (VM_SOFTDIRTY|VM_UFFD_WP))" and it could be made
> > conditional to the two config options too.
> >
> > Still I don't mind doing it in some other way, uffd-wp has much easier
> > time doing it in another way in fact.
> >
> > Whatever performs better is fine, but queuing up pending invalidate
> > ranges don't look very attractive since it'd be a fixed cost that we'd
> > always have to pay even when there's no fault (and there can't be any
> > fault at least for mprotect).
>
> I think there are other cases in which Andy’s concern is relevant
> (MADV_PAGEOUT).
That patch only demonstrate a rough idea and I should have been
elaborate: if we ever decide to go that direction, we only need to
worry about "jumping through hoops", because the final patch (set)
I have in mind would not only have the build time optimization Andrea
suggested but also include runtime optimizations like skipping
do_swap_page() path and (!PageAnon() || page_mapcount > 1). Rest
assured, the performance impact on do_wp_page() from occasionally an
additional TLB flush on top of a page copy is negligible.
> Perhaps holding some small bitmap based on part of the deferred flushed
> pages (e.g., bits 12-17 of the address or some other kind of a single
> hash-function bloom-filter) would be more performant to avoid (most)
> unnecessary TLB flushes. It will be cleared before a TLB flush and set while
> holding the PTL.
>
> Checking if a flush is needed, under the PTL, would require a single memory
> access (although potentially cache miss). It will however require one atomic
> operation for each page-table whose PTEs’ flushes are deferred - in contrast
> to the current scheme which requires two atomic operations for the *entire*
> operation.
>
next prev parent reply other threads:[~2020-12-24 3:34 UTC|newest]
Thread overview: 120+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-19 4:30 [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect Nadav Amit
2020-12-19 19:15 ` Andrea Arcangeli
[not found] ` <EDC00345-B46E-4396-8379-98E943723809@gmail.com>
2020-12-19 22:06 ` Nadav Amit
2020-12-20 2:20 ` Andrea Arcangeli
2020-12-21 4:36 ` Nadav Amit
2020-12-21 5:12 ` Yu Zhao
2020-12-21 5:25 ` Nadav Amit
2020-12-21 5:39 ` Nadav Amit
2020-12-21 7:29 ` Yu Zhao
2020-12-22 20:34 ` Andy Lutomirski
2020-12-22 20:58 ` Nadav Amit
2020-12-22 21:34 ` Andrea Arcangeli
2020-12-20 2:01 ` Andy Lutomirski
2020-12-20 2:49 ` Andrea Arcangeli
2020-12-20 5:08 ` Andy Lutomirski
2020-12-21 18:03 ` Andrea Arcangeli
2020-12-21 18:22 ` Andy Lutomirski
2020-12-20 6:05 ` Yu Zhao
2020-12-20 8:06 ` Nadav Amit
2020-12-20 9:54 ` Yu Zhao
2020-12-21 3:33 ` Nadav Amit
2020-12-21 4:44 ` Yu Zhao
2020-12-21 17:27 ` Peter Xu
2020-12-21 18:31 ` Nadav Amit
2020-12-21 19:16 ` Yu Zhao
2020-12-21 19:55 ` Linus Torvalds
2020-12-21 20:21 ` Yu Zhao
2020-12-21 20:25 ` Linus Torvalds
2020-12-21 20:23 ` Nadav Amit
2020-12-21 20:26 ` Linus Torvalds
2020-12-21 21:24 ` Yu Zhao
2020-12-21 21:49 ` Nadav Amit
2020-12-21 22:30 ` Peter Xu
2020-12-21 22:55 ` Nadav Amit
2020-12-21 23:30 ` Linus Torvalds
2020-12-21 23:46 ` Nadav Amit
2020-12-22 19:44 ` Andrea Arcangeli
2020-12-22 20:19 ` Nadav Amit
2020-12-22 21:17 ` Andrea Arcangeli
2020-12-21 23:12 ` Yu Zhao
2020-12-21 23:33 ` Linus Torvalds
2020-12-22 0:00 ` Yu Zhao
2020-12-22 0:11 ` Linus Torvalds
2020-12-22 0:24 ` Yu Zhao
2020-12-21 23:22 ` Linus Torvalds
2020-12-22 3:19 ` Andy Lutomirski
2020-12-22 4:16 ` Linus Torvalds
2020-12-22 20:19 ` Andy Lutomirski
2021-01-05 15:37 ` Peter Zijlstra
2021-01-05 18:03 ` Andrea Arcangeli
2021-01-12 16:20 ` Peter Zijlstra
2021-01-12 11:43 ` Vinayak Menon
2021-01-12 15:47 ` Laurent Dufour
2021-01-12 16:57 ` Peter Zijlstra
2021-01-12 19:02 ` Laurent Dufour
2021-01-12 19:15 ` Nadav Amit
2021-01-12 19:56 ` Yu Zhao
2021-01-12 20:38 ` Nadav Amit
2021-01-12 20:49 ` Yu Zhao
2021-01-12 21:43 ` Will Deacon
2021-01-12 22:29 ` Nadav Amit
2021-01-12 22:46 ` Will Deacon
2021-01-13 0:31 ` Andy Lutomirski
2021-01-17 4:41 ` Yu Zhao
2021-01-17 7:32 ` Nadav Amit
2021-01-17 9:16 ` Yu Zhao
2021-01-17 10:13 ` Nadav Amit
2021-01-17 19:25 ` Yu Zhao
2021-01-18 2:49 ` Nadav Amit
2020-12-22 9:38 ` Nadav Amit
2020-12-22 19:31 ` Andrea Arcangeli
2020-12-22 20:15 ` Matthew Wilcox
2020-12-22 20:26 ` Andrea Arcangeli
2020-12-22 21:14 ` Yu Zhao
2020-12-22 22:02 ` Andrea Arcangeli
2020-12-22 23:39 ` Yu Zhao
2020-12-22 23:50 ` Linus Torvalds
2020-12-23 0:01 ` Linus Torvalds
2020-12-23 0:23 ` Yu Zhao
2020-12-23 2:17 ` Andrea Arcangeli
2020-12-23 9:44 ` Linus Torvalds
2020-12-23 10:06 ` Yu Zhao
2020-12-23 16:24 ` Peter Xu
2020-12-23 18:51 ` Andrea Arcangeli
2020-12-23 18:55 ` Andrea Arcangeli
2020-12-23 19:12 ` Yu Zhao
2020-12-23 19:32 ` Peter Xu
2020-12-23 0:20 ` Linus Torvalds
2020-12-23 2:56 ` Andrea Arcangeli
2020-12-23 3:36 ` Yu Zhao
2020-12-23 15:52 ` Peter Xu
2020-12-23 21:07 ` Andrea Arcangeli
2020-12-23 21:39 ` Andrea Arcangeli
2020-12-23 22:29 ` Yu Zhao
2020-12-23 23:04 ` Andrea Arcangeli
2020-12-24 1:21 ` Andy Lutomirski
2020-12-24 2:00 ` Andrea Arcangeli
2020-12-24 3:09 ` Nadav Amit
2020-12-24 3:30 ` Nadav Amit
2020-12-24 3:34 ` Yu Zhao [this message]
2020-12-24 4:01 ` Andrea Arcangeli
2020-12-24 5:18 ` Nadav Amit
2020-12-24 18:49 ` Andrea Arcangeli
2020-12-24 19:16 ` Andrea Arcangeli
2020-12-24 4:37 ` Nadav Amit
2020-12-24 3:31 ` Andrea Arcangeli
2020-12-23 23:39 ` Linus Torvalds
2020-12-24 1:01 ` Andrea Arcangeli
2020-12-22 21:14 ` Nadav Amit
2020-12-22 12:40 ` Nadav Amit
2020-12-22 18:30 ` Yu Zhao
2020-12-22 19:20 ` Nadav Amit
2020-12-23 16:23 ` Will Deacon
2020-12-23 19:04 ` Nadav Amit
2020-12-23 22:05 ` Andrea Arcangeli
2020-12-23 22:45 ` Nadav Amit
2020-12-23 23:55 ` Andrea Arcangeli
2020-12-21 21:55 ` Peter Xu
2020-12-21 23:13 ` Linus Torvalds
2020-12-21 19:53 ` Peter Xu
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