From: Andy Lutomirski <luto@kernel.org>
To: Michel Lespinasse <michel@lespinasse.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
Andy Lutomirski <luto@kernel.org>, Linux-MM <linux-mm@kvack.org>,
Laurent Dufour <ldufour@linux.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Michal Hocko <mhocko@suse.com>,
Matthew Wilcox <willy@infradead.org>,
Rik van Riel <riel@surriel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Suren Baghdasaryan <surenb@google.com>,
Joel Fernandes <joelaf@google.com>,
Rom Lemarchand <romlem@google.com>,
Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault().
Date: Wed, 28 Apr 2021 17:05:17 -0700 [thread overview]
Message-ID: <CALCETrWybk8k8Z=9+x3Ns7zMUPMzganzPY47pqOJCbB3LkfQ+A@mail.gmail.com> (raw)
In-Reply-To: <20210429000225.GC10973@lespinasse.org>
On Wed, Apr 28, 2021 at 5:02 PM Michel Lespinasse <michel@lespinasse.org> wrote:
>
> On Wed, Apr 28, 2021 at 09:11:08AM -0700, Paul E. McKenney wrote:
> > On Wed, Apr 28, 2021 at 08:13:53AM -0700, Andy Lutomirski wrote:
> > > On Wed, Apr 28, 2021 at 8:05 AM Michel Lespinasse <michel@lespinasse.org> wrote:
> > > >
> > > > On Wed, Apr 07, 2021 at 08:36:01AM -0700, Andy Lutomirski wrote:
> > > > > On 4/6/21 6:44 PM, Michel Lespinasse wrote:
> > > > > > The page table tree is walked with local irqs disabled, which prevents
> > > > > > page table reclamation (similarly to what fast GUP does). The logic is
> > > > > > otherwise similar to the non-speculative path, but with additional
> > > > > > restrictions: in the speculative path, we do not handle huge pages or
> > > > > > wiring new pages tables.
> > > > >
> > > > > Not on most architectures. Quoting the actual comment in mm/gup.c:
> > > > >
> > > > > > * Before activating this code, please be aware that the following assumptions
> > > > > > * are currently made:
> > > > > > *
> > > > > > * *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to
> > > > > > * free pages containing page tables or TLB flushing requires IPI broadcast.
> > > > >
> > > > > On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the
> > > > > assumption that it is safe to dereference a pointer in a page table just
> > > > > because irqs are off. You need RCU protection, too.
> > > > >
> > > > > You have the same error in the cover letter.
> > > >
> > > > Hi Andy,
> > > >
> > > > Thanks for your comment. At first I thought did not matter, because we
> > > > only enable ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT on selected
> > > > architectures, and I thought MMU_GATHER_RCU_TABLE_FREE is not set on
> > > > these. But I was wrong - MMU_GATHER_RCU_TABLE_FREE is enabled on X86
> > > > with paravirt. So I took another look at fast GUP to make sure I
> > > > actually understand it.
> > > >
> > > > This brings a question about lockless_pages_from_mm() - I see it
> > > > disabling interrupts, which it explains is necessary for disabling THP
> > > > splitting IPIs, but I do not see it taking an RCU read lock as would
> > > > be necessary for preventing paga table freeing on
> > > > MMU_GATHER_RCU_TABLE_FREE configs. I figure local_irq_save()
> > > > indirectly takes an rcu read lock somehow ? I think this is something
> > > > I should also mention in my explanation, and I have not seen a good
> > > > description of this on the fast GUP side...
> > >
> > > Sounds like a bug! That being said, based on my extremely limited
> > > understanding of how the common RCU modes work, local_irq_save()
> > > probably implies an RCU lock in at least some cases. Hi Paul!
> >
> > In modern kernels, local_irq_save() does have RCU reader semantics,
> > meaning that synchronize_rcu() will wait for pre-exiting irq-disabled
> > regions. It will also wait for pre-existing bh-disable, preempt-disable,
> > and of course rcu_read_lock() sections of code.
>
> Thanks Paul for confirming / clarifying this. BTW, it would be good to
> add this to the rcu header files, just so people have something to
> reference to when they depend on such behavior (like fast GUP
> currently does).
Or, even better, fast GUP could add an explicit RCU read lock.
>
> Going back to my patch. I don't need to protect against THP splitting
> here, as I'm only handling the small page case. So when
> MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with
> using only an rcu read lock, instead of disabling interrupts which
> implicitly creates the rcu read lock. I'm not sure which way to go -
> fast GUP always disables interrupts regardless of the
> MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be
> made for following the fast GUP stes rather than trying to be smarter.
How about adding some little helpers:
lockless_page_walk_begin();
lockless_page_walk_end();
these turn into RCU read locks if MMU_GATHER_RCU_TABLE_FREE and into
irqsave otherwise. And they're somewhat self-documenting.
--Andy
next prev parent reply other threads:[~2021-04-29 0:05 UTC|newest]
Thread overview: 85+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-07 1:44 [RFC PATCH 00/37] Speculative page faults Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 01/37] mmap locking API: mmap_lock_is_contended returns a bool Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 02/37] mmap locking API: name the return values Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 03/37] do_anonymous_page: use update_mmu_tlb() Michel Lespinasse
2021-04-07 2:06 ` Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 04/37] do_anonymous_page: reduce code duplication Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 05/37] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 06/37] x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 07/37] mm: add FAULT_FLAG_SPECULATIVE flag Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 08/37] mm: add do_handle_mm_fault() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 09/37] mm: add per-mm mmap sequence counter for speculative page fault handling Michel Lespinasse
2021-04-07 14:47 ` Peter Zijlstra
2021-04-07 20:50 ` Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 10/37] mm: rcu safe vma freeing Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 11/37] x86/mm: attempt speculative mm faults first Michel Lespinasse
2021-04-07 14:48 ` Peter Zijlstra
2021-04-07 15:35 ` Matthew Wilcox
2021-04-07 20:32 ` Michel Lespinasse
2021-04-07 20:14 ` Michel Lespinasse
2021-04-07 20:18 ` Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 12/37] mm: refactor __handle_mm_fault() / handle_pte_fault() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault() Michel Lespinasse
2021-04-07 15:36 ` Andy Lutomirski
2021-04-28 14:58 ` Michel Lespinasse
2021-04-28 15:13 ` Andy Lutomirski
2021-04-28 16:11 ` Paul E. McKenney
2021-04-29 0:02 ` Michel Lespinasse
2021-04-29 0:05 ` Andy Lutomirski [this message]
2021-04-29 16:12 ` Matthew Wilcox
2021-04-29 18:04 ` Andy Lutomirski
2021-04-29 19:14 ` Michel Lespinasse
2021-04-29 19:34 ` Matthew Wilcox
2021-04-29 23:56 ` Michel Lespinasse
2021-04-29 15:52 ` Paul E. McKenney
2021-04-29 18:34 ` Paul E. McKenney
2021-04-29 18:49 ` Matthew Wilcox
2021-05-03 3:14 ` Paul E. McKenney
2021-04-29 21:17 ` Michel Lespinasse
2021-05-03 3:40 ` Paul E. McKenney
2021-05-03 4:34 ` Michel Lespinasse
2021-05-03 16:32 ` Paul E. McKenney
2021-04-07 1:44 ` [RFC PATCH 14/37] mm: add pte_map_lock() and pte_spinlock() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 15/37] mm: implement speculative handling in do_anonymous_page() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 16/37] mm: enable speculative fault handling through do_anonymous_page() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 17/37] mm: implement speculative handling in do_numa_page() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 18/37] mm: enable speculative fault " Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 19/37] mm: implement speculative handling in wp_page_copy() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 20/37] mm: implement and enable speculative fault handling in handle_pte_fault() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 21/37] mm: implement speculative handling in do_swap_page() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 22/37] mm: enable speculative fault handling through do_swap_page() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 23/37] mm: rcu safe vma->vm_file freeing Michel Lespinasse
2021-04-08 5:12 ` [mm] 87b1c39af4: nvml.blk_rw_mt_TEST0_check_pmem_debug.fail kernel test robot
2021-04-07 1:44 ` [RFC PATCH 24/37] mm: implement speculative handling in __do_fault() Michel Lespinasse
2021-04-07 2:35 ` Matthew Wilcox
2021-04-07 2:53 ` Michel Lespinasse
2021-04-07 3:01 ` Matthew Wilcox
2021-04-07 14:40 ` Peter Zijlstra
2021-04-07 21:20 ` Michel Lespinasse
2021-04-07 21:27 ` Matthew Wilcox
2021-04-08 7:00 ` Peter Zijlstra
2021-04-08 7:13 ` Matthew Wilcox
2021-04-08 8:18 ` Peter Zijlstra
2021-04-08 8:37 ` Michel Lespinasse
2021-04-08 11:28 ` Matthew Wilcox
2021-04-07 1:44 ` [RFC PATCH 25/37] mm: implement speculative handling in filemap_fault() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 26/37] mm: implement speculative fault handling in finish_fault() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 27/37] mm: implement speculative handling in do_fault_around() Michel Lespinasse
2021-04-07 2:37 ` Matthew Wilcox
2021-04-07 1:44 ` [RFC PATCH 28/37] mm: implement speculative handling in filemap_map_pages() Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 29/37] fs: list file types that support speculative faults Michel Lespinasse
2021-04-07 2:39 ` Matthew Wilcox
2021-04-07 1:44 ` [RFC PATCH 30/37] mm: enable speculative fault handling for supported file types Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 31/37] ext4: implement speculative fault handling Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 32/37] f2fs: " Michel Lespinasse
2021-04-07 1:44 ` [RFC PATCH 33/37] mm: enable speculative fault handling only for multithreaded user space Michel Lespinasse
2021-04-07 2:48 ` Matthew Wilcox
2021-04-07 1:44 ` [RFC PATCH 34/37] mm: rcu safe vma freeing " Michel Lespinasse
2021-04-07 2:50 ` Matthew Wilcox
2021-04-08 7:53 ` Michel Lespinasse
2021-04-07 1:45 ` [RFC PATCH 35/37] mm: spf statistics Michel Lespinasse
2021-04-07 1:45 ` [RFC PATCH 36/37] arm64/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Michel Lespinasse
2021-04-07 1:45 ` [RFC PATCH 37/37] arm64/mm: attempt speculative mm faults first Michel Lespinasse
2021-04-21 1:44 ` [RFC PATCH 00/37] Speculative page faults Chinwen Chang
2021-06-28 22:14 ` Axel Rasmussen
2021-07-21 11:33 ` vjitta
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