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From: Nicholas Piggin <npiggin@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Hillf Danton <hdanton@sina.com>, Hugh Dickins <hughd@google.com>,
	Josef Bacik <josef@toxicpanda.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	mm-commits@vger.kernel.org, Will Deacon <will.deacon@arm.com>,
	Matthew Wilcox <willy@infradead.org>,
	Yu Xu <xuyu@linux.alibaba.com>,
	Yang Shi <yang.shi@linux.alibaba.com>
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Wed, 29 Jul 2020 08:53:48 +1000
Message-ID: <1595974242.esf9644sf3.astroid@bobo.none> (raw)
In-Reply-To: <CAHk-=wgrgRqeEo-YUgec7yQNkN+_+sHBP-NtCnfktCFEuPHTDQ@mail.gmail.com>

Excerpts from Linus Torvalds's message of July 29, 2020 5:02 am:
> On Tue, Jul 28, 2020 at 3:53 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>>
>> The quirk is a problem with coprocessor where it's supposed to
>> invalidate the translation after a fault but it doesn't, so we can get a
>> read-only TLB stuck after something else does a RO->RW upgrade on the
>> TLB. Something like that IIRC.  Coprocessors have their own MMU which
>> lives in the nest not the core, so you need a global TLB flush to
>> invalidate that thing.
> 
> So I assumed, but it does seem confused.
> 
> Why? Because if there are stale translations on the co-processor,
> there's no guarantee that one of the CPU's will have them and take a
> fault.
> 
> So I'm not seeing why a core CPU doing spurious TLB invalidation would
> follow from "stale TLB in the Nest".

If the nest MMU access faults, it sends an interrupt to the CPU and
the driver tries to handle the page fault for it (I think that's how
it works).

> If anything, I think "we have a coprocessor that needs to never have
> stale TLB entries" would impact the _regular_ TLB invalidates (by
> update_mmu_cache()) and perhaps make those more aggressive, exactly
> because the coprocessor may not handle the fault as gracefully.

It could be done that way... Hmm although we do have something similar 
also in radix__ptep_set_access_flags for the relaxing permissions case
so maybe this is required for not-present faults as well? I'm not 
actually sure now.

But it's a bit weird and awkward because it's working around quirks in
the hardware which aren't regular, not because we're _completely_ 
confused (I hope).

Thanks,
Nick


  reply index

Thread overview: 69+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-24  4:14 incoming Andrew Morton
2020-07-24  4:15 ` [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault Andrew Morton
2020-07-24  4:38   ` Yang Shi
2020-07-24  4:56     ` Andrew Morton
2020-07-24 19:27   ` Linus Torvalds
2020-07-24 20:22     ` Linus Torvalds
2020-07-25  0:36       ` Yang Shi
2020-07-25  1:29         ` Linus Torvalds
2020-07-25 15:58           ` Catalin Marinas
2020-07-28  9:22             ` Will Deacon
2020-07-28  9:39               ` Catalin Marinas
2020-07-28 10:07                 ` Yu Xu
2020-07-28 11:46                   ` Catalin Marinas
2020-07-28 10:21                 ` Will Deacon
2020-07-28 18:28                 ` Linus Torvalds
2020-07-27 17:52           ` Yang Shi
2020-07-27 18:04             ` Linus Torvalds
2020-07-27 18:42               ` Catalin Marinas
2020-07-27 20:56                 ` Linus Torvalds
2020-07-27 22:34               ` Yang Shi
2020-07-27  7:31       ` Yu Xu
2020-07-27 11:05         ` Catalin Marinas
2020-07-27 17:01           ` Linus Torvalds
2020-07-28 11:19             ` Catalin Marinas
2020-07-27 17:12           ` Yu Xu
2020-07-27 18:04             ` Yang Shi
2020-07-27 18:37               ` Linus Torvalds
2020-07-27 22:43                 ` Yang Shi
2020-07-28  0:38                   ` Linus Torvalds
2020-07-28  0:13                 ` Yu Xu
2020-07-28 10:53                 ` Nicholas Piggin
2020-07-28 19:02                   ` Linus Torvalds
2020-07-28 22:53                     ` Nicholas Piggin [this message]
2020-07-29 13:58                       ` Michael Ellerman
2020-07-28  6:41             ` Yu Xu
2020-07-24  4:15 ` [patch 02/15] mm/mmap.c: close race between munmap() and expand_upwards()/downwards() Andrew Morton
2020-07-24  4:15 ` [patch 03/15] vfs/xattr: mm/shmem: kernfs: release simple xattr entry in a right way Andrew Morton
2020-07-24  4:15 ` [patch 04/15] mm: initialize return of vm_insert_pages Andrew Morton
2020-07-24  4:15 ` [patch 05/15] mm/memcontrol: fix OOPS inside mem_cgroup_get_nr_swap_pages() Andrew Morton
2020-07-24  4:15 ` [patch 06/15] mm/memcg: fix refcount error while moving and swapping Andrew Morton
2020-07-24 13:41   ` Alex Shi
2020-07-24  4:15 ` [patch 07/15] mm: memcg/slab: fix memory leak at non-root kmem_cache destroy Andrew Morton
2020-07-24  4:15 ` [patch 08/15] mm/hugetlb: avoid hardcoding while checking if cma is enabled Andrew Morton
2020-07-24  4:15 ` [patch 09/15] khugepaged: fix null-pointer dereference due to race Andrew Morton
2020-07-24  4:15 ` [patch 10/15] mailmap: add entry for Mike Rapoport Andrew Morton
2020-07-24  4:15 ` [patch 11/15] squashfs: fix length field overlap check in metadata reading Andrew Morton
2020-07-24  4:15 ` [patch 12/15] scripts/decode_stacktrace: strip basepath from all paths Andrew Morton
2020-07-24  4:15 ` [patch 13/15] io-mapping: indicate mapping failure Andrew Morton
2020-07-24  4:15 ` [patch 14/15] MAINTAINERS: add KCOV section Andrew Morton
2020-07-24  4:15 ` [patch 15/15] scripts/gdb: fix lx-symbols 'gdb.error' while loading modules Andrew Morton
2020-07-28  1:19 ` mmotm 2020-07-27-18-18 uploaded Andrew Morton
2020-07-28  2:14   ` Stephen Rothwell
2020-07-28  3:22   ` mmotm 2020-07-27-18-18 uploaded (drivers/scsi/ufs/: SCSI_UFS_EXYNOS) Randy Dunlap
2020-07-28  8:23     ` Alim Akhtar
2020-07-28 12:33   ` mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c) Randy Dunlap
2020-07-28 21:55     ` Andrew Morton
2020-07-28 22:20       ` Stephen Rothwell
2020-07-28 22:31         ` Andrew Morton
2020-07-29 14:18           ` Michael S. Tsirkin
2020-07-29 14:38             ` David Hildenbrand
2020-07-29 16:14               ` David Hildenbrand
2020-07-29 17:29                 ` Randy Dunlap
2020-07-28 22:39       ` Randy Dunlap
2020-07-29  1:43         ` Nathan Chancellor
2020-07-29  1:44         ` Andrew Morton
2020-07-29  2:04           ` Randy Dunlap
2020-07-29 14:09           ` make oldconfig (Re: mmotm 2020-07-27-18-18 uploaded (mm/page_alloc.c)) Alexey Dobriyan
2020-07-31 23:46 ` mmotm 2020-07-31-16-45 uploaded Andrew Morton
2020-08-01  5:24   ` mmotm 2020-07-31-16-45 uploaded (drivers/staging/vc04_services/) Randy Dunlap

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