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From: Matthew Wilcox <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michel Lespinasse <michel@lespinasse.org>,
	Linux-MM <linux-mm@kvack.org>,
	linux-kernel@vger.kernel.org, kernel-team@fb.com,
	Laurent Dufour <ldufour@linux.ibm.com>,
	Jerome Glisse <jglisse@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Michal Hocko <mhocko@suse.com>, Vlastimil Babka <vbabka@suse.cz>,
	Davidlohr Bueso <dave@stgolabs.net>,
	Liam Howlett <liam.howlett@oracle.com>,
	Rik van Riel <riel@surriel.com>,
	Paul McKenney <paulmck@kernel.org>,
	Song Liu <songliubraving@fb.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Minchan Kim <minchan@google.com>,
	Joel Fernandes <joelaf@google.com>,
	David Rientjes <rientjes@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v2 00/35] Speculative page faults
Date: Tue, 1 Feb 2022 02:20:39 +0000	[thread overview]
Message-ID: <YfiY9zRm8BhSp7eA@casper.infradead.org> (raw)
In-Reply-To: <20220131171434.89870a8f1ae294912e7ff19e@linux-foundation.org>

On Mon, Jan 31, 2022 at 05:14:34PM -0800, Andrew Morton wrote:
> On Fri, 28 Jan 2022 05:09:31 -0800 Michel Lespinasse <michel@lespinasse.org> wrote:
> > The first step of a speculative page fault is to look up the vma and
> > read its contents (currently by making a copy of the vma, though in
> > principle it would be sufficient to only read the vma attributes that
> > are used in page faults). The mmap sequence count is used to verify
> > that there were no mmap writers concurrent to the lookup and copy steps.
> > Note that walking rbtrees while there may potentially be concurrent
> > writers is not an entirely new idea in linux, as latched rbtrees
> > are already doing this. This is safe as long as the lookup is
> > followed by a sequence check to verify that concurrency did not
> > actually occur (and abort the speculative fault if it did).
> 
> I'm surprised that descending the rbtree locklessly doesn't flat-out
> oops the kernel.  How are we assured that every pointer which is
> encountered actually points at the right thing?  Against things
> which tear that tree down?

It doesn't necessarily point at the _right_ thing.  You may get
entirely the wrong node in the tree if you race with a modification,
but, as Michel says, you check the seqcount before you even look at
the VMA (and if the seqcount indicates a modification, you throw away
the result and fall back to the locked version).  The rbtree always
points to other rbtree nodes, so you aren't going to walk into some
completely wrong data structure.

> > The next step is to walk down the existing page table tree to find the
> > current pte entry. This is done with interrupts disabled to avoid
> > races with munmap().
> 
> Sebastian, could you please comment on this from the CONFIG_PREEMPT_RT
> point of view?

I am not a fan of this approach.  For other reasons, I think we want to
switch to RCU-freed page tables, and then we can walk the page tables
with the RCU lock held.  Some architectures already RCU-free the page
tables, so I think it's just a matter of converting the rest.

  reply	other threads:[~2022-02-01  2:20 UTC|newest]

Thread overview: 76+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-28 13:09 [PATCH v2 00/35] Speculative page faults Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 01/35] mm: export dump_mm Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 02/35] mmap locking API: mmap_lock_is_contended returns a bool Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 03/35] mmap locking API: name the return values Michel Lespinasse
2022-01-31 16:17   ` Liam Howlett
2022-02-07 17:39     ` Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 04/35] do_anonymous_page: use update_mmu_tlb() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 05/35] do_anonymous_page: reduce code duplication Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 06/35] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 07/35] x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 08/35] mm: add FAULT_FLAG_SPECULATIVE flag Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 09/35] mm: add do_handle_mm_fault() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 10/35] mm: add per-mm mmap sequence counter for speculative page fault handling Michel Lespinasse
2022-08-25 11:23   ` Pavan Kondeti
2022-01-28 13:09 ` [PATCH v2 11/35] mm: rcu safe vma freeing Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 12/35] mm: separate mmap locked assertion from find_vma Michel Lespinasse
2022-01-29  0:08   ` kernel test robot
2022-01-29  0:08     ` kernel test robot
2022-01-29  0:33     ` Michel Lespinasse
2022-01-29  0:33       ` Michel Lespinasse
2022-01-31 14:44   ` Matthew Wilcox
2022-02-04 22:41     ` Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 13/35] x86/mm: attempt speculative mm faults first Michel Lespinasse
2022-02-01 17:16   ` Liam Howlett
2022-02-07 17:39     ` Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 14/35] mm: add speculative_page_walk_begin() and speculative_page_walk_end() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 15/35] mm: refactor __handle_mm_fault() / handle_pte_fault() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 16/35] mm: implement speculative handling in __handle_mm_fault() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 17/35] mm: add pte_map_lock() and pte_spinlock() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 18/35] mm: implement speculative handling in do_anonymous_page() Michel Lespinasse
2022-01-28 21:03   ` kernel test robot
2022-01-28 21:03     ` kernel test robot
2022-01-28 22:08     ` Michel Lespinasse
2022-01-28 22:08       ` Michel Lespinasse
2022-01-30  2:54   ` [mm] fa5331bae2: canonical_address#:#[##] kernel test robot
2022-01-30  2:54     ` kernel test robot
2022-01-30  5:08     ` Michel Lespinasse
2022-01-30  5:08       ` Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 19/35] mm: enable speculative fault handling through do_anonymous_page() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 20/35] mm: implement speculative handling in do_numa_page() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 21/35] mm: enable speculative fault " Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 22/35] percpu-rwsem: enable percpu_sem destruction in atomic context Michel Lespinasse
2022-01-29 12:13   ` Hillf Danton
2022-01-31 18:04     ` Suren Baghdasaryan
2022-02-01  2:09       ` Hillf Danton
2022-02-07 19:31         ` Suren Baghdasaryan
2022-02-08  0:20           ` Hillf Danton
2022-02-08  1:31             ` Suren Baghdasaryan
2022-01-28 13:09 ` [PATCH v2 23/35] mm: add mmu_notifier_lock Michel Lespinasse
2022-07-27  7:34   ` Pavan Kondeti
2022-07-27 20:30     ` Suren Baghdasaryan
2022-01-28 13:09 ` [PATCH v2 24/35] mm: write lock mmu_notifier_lock when registering mmu notifiers Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 25/35] mm: add mmu_notifier_trylock() and mmu_notifier_unlock() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 26/35] mm: implement speculative handling in wp_page_copy() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 27/35] mm: implement and enable speculative fault handling in handle_pte_fault() Michel Lespinasse
2022-01-28 13:09 ` [PATCH v2 28/35] mm: disable speculative faults for single threaded user space Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 29/35] mm: disable rcu safe vma freeing " Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 30/35] mm: create new include/linux/vm_event.h header file Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 31/35] mm: anon spf statistics Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 32/35] arm64/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 33/35] arm64/mm: attempt speculative mm faults first Michel Lespinasse
2022-01-30  9:13   ` Mike Rapoport
2022-01-31  8:07     ` Michel Lespinasse
2022-02-01  8:58       ` Mike Rapoport
2022-02-07 17:39         ` Michel Lespinasse
2022-02-08  9:07           ` Mike Rapoport
2022-01-28 13:10 ` [PATCH v2 34/35] powerpc/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2022-01-28 13:10 ` [PATCH v2 35/35] powerpc/mm: attempt speculative mm faults first Michel Lespinasse
2022-01-31  9:56 ` [PATCH v2 00/35] Speculative page faults David Hildenbrand
2022-01-31 17:00   ` Suren Baghdasaryan
2022-02-01  1:14 ` Andrew Morton
2022-02-01  2:20   ` Matthew Wilcox [this message]
2022-02-07 17:39     ` Michel Lespinasse
2022-02-01 17:17   ` Sebastian Andrzej Siewior
2022-02-23 16:11 ` Mel Gorman
2022-03-08  5:37   ` Suren Baghdasaryan

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