From: Andrea Arcangeli <aarcange@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Xu <peterx@redhat.com>, Yu Zhao <yuzhao@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
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>,
Andy Lutomirski <luto@kernel.org>, Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
Date: Tue, 22 Dec 2020 14:44:08 -0500 [thread overview]
Message-ID: <X+JMiHv+EktzyZgr@redhat.com> (raw)
In-Reply-To: <B8095F3C-81E3-4AF9-A6A5-F597D51264BD@gmail.com>
On Mon, Dec 21, 2020 at 02:55:12PM -0800, Nadav Amit wrote:
> wouldn’t mmap_write_downgrade() be executed before mprotect_fixup() (so
I assume you mean "in" mprotect_fixup, after change_protection.
If you would downgrade the mmap_lock to read there, then it'd severely
slowdown the non contention case, if there's more than vma that needs
change_protection.
You'd need to throw away the prev->vm_next info and you'd need to do a
new find_vma after droping the mmap_lock for reading and re-taking the
mmap_lock for writing at every iteration of the loop.
To do less harm to the non-contention case you could perhaps walk
vma->vm_next and check if it's outside the mprotect range and only
downgrade in such case. So let's assume we intend to optimize with
mmap_write_downgrade only the last vma.
The problem is once you had to take mmap_lock for writing, you already
stalled for I/O and waited all concurrent page faults and blocked them
as well for the vma allocations in split_vma, so that extra boost in
SMP scalability you get is lost in the noise there at best.
And the risk is that at worst that extra locked op of
mmap_write_downgrade() will hurt SMP scalability because it would
increase the locked ops of mprotect on the hottest false-shared
cacheline by 50% and that may outweight the benefit from unblocking
the page faults half a usec sooner on large systems.
But the ultimate reason why mprotect cannot do mmap_write_downgrade()
while userfaultfd_writeprotect can do mmap_read_lock and avoid the
mmap_write_lock altogether, is that mprotect leaves no mark in the
pte/hugepmd that allows to detect when the TLB is stale in order to
redirect the page fault in a dead end (handle_userfault() or
do_numa_page) until after the TLB has been flushed as it happens in
the the 4 cases below:
/*
* STALE_TLB_WARNING: while the uffd_wp bit is set, the TLB
* can be stale. We cannot allow do_wp_page to proceed or
* it'll wrongly assume that nobody can still be writing to
* the page if !pte_write.
*/
if (userfaultfd_pte_wp(vma, *vmf->pte)) {
/*
* STALE_TLB_WARNING: while the uffd_wp bit is set,
* the TLB can be stale. We cannot allow wp_huge_pmd()
* to proceed or it'll wrongly assume that nobody can
* still be writing to the page if !pmd_write.
*/
if (userfaultfd_huge_pmd_wp(vmf->vma, orig_pmd))
/*
* STALE_TLB_WARNING: if the pte is NUMA protnone the TLB can
* be stale.
*/
if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
/*
* STALE_TLB_WARNING: if the pmd is NUMA
* protnone the TLB can be stale.
*/
if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
Thanks,
Andrea
next prev parent reply other threads:[~2020-12-22 19:44 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 [this message]
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
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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