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From: Yu Xu <xuyu@linux.alibaba.com>
To: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	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-MM <linux-mm@kvack.org>,
	mm-commits@vger.kernel.org, Matthew Wilcox <willy@infradead.org>
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Tue, 28 Jul 2020 18:07:44 +0800
Message-ID: <1f04a55e-ebea-beb2-a2d8-7bddbd296ba8@linux.alibaba.com> (raw)
In-Reply-To: <20200728093910.GB706@gaia>

On 7/28/20 5:39 PM, Catalin Marinas wrote:
> On Tue, Jul 28, 2020 at 10:22:20AM +0100, Will Deacon wrote:
>> On Sat, Jul 25, 2020 at 04:58:43PM +0100, Catalin Marinas wrote:
>>> On Fri, Jul 24, 2020 at 06:29:43PM -0700, Linus Torvalds wrote:
>>>> For any architecture that guarantees that a page fault will always
>>>> flush the old TLB entry for this kind of situation, that
>>>> flush_tlb_fix_spurious_fault() thing can be a no-op.
>>>>
>>>> So that's why on x86, we just do
>>>>
>>>>    #define flush_tlb_fix_spurious_fault(vma, address) do { } while (0)
>>>>
>>>> and have no issues.
>>>>
>>>> Note that it does *not* need to do any cross-CPU flushing or anything
>>>> like that. So it's actually wrong (I think) to have that default
>>>> fallback for
>>>>
>>>>     #define flush_tlb_fix_spurious_fault(vma, address)
>>>> flush_tlb_page(vma, address)
>>>>
>>>> because flush_tlb_page() is the serious "do cross CPU etc".
>>>>
>>>> Does the arm64 flush_tlb_page() perhaps do the whole expensive
>>>> cross-CPU thing rather than the much cheaper "just local invalidate"
>>>> version?
>>>
>>> I think it makes sense to have a local-only
>>> flush_tlb_fix_spurious_fault(), but with ptep_set_access_flags() updated
>>> to still issue the full broadcast TLBI. In addition, I added a minor
>>> optimisation to avoid the TLB flush if the old pte was not accessible.
>>> In a read-access fault case (followed by mkyoung), the TLB wouldn't have
>>> cached a non-accessible pte (not sure it makes much difference to Yang's
>>> case). Anyway, from ARMv8.1 onwards, the hardware handles the access
>>> flag automatically.
>>>
>>> I'm not sure the first dsb(nshst) below is of much use in this case. If
>>> we got a spurious fault, the write to the pte happened on a different
>>> CPU (IIUC, we shouldn't return to user with updated ptes without a TLB
>>> flush on the same CPU). Anyway, we can refine this if it solves Yang's
>>> performance regression.
>>>
>>> -------------8<-----------------------
>>> diff --git a/arch/arm64/include/asm/tlbflush.h b/arch/arm64/include/asm/tlbflush.h
>>> index d493174415db..d1401cbad7d4 100644
>>> --- a/arch/arm64/include/asm/tlbflush.h
>>> +++ b/arch/arm64/include/asm/tlbflush.h
>>> @@ -268,6 +268,20 @@ static inline void flush_tlb_page(struct vm_area_struct *vma,
>>>   	dsb(ish);
>>>   }
>>>   
>>> +static inline void local_flush_tlb_page(struct vm_area_struct *vma,
>>> +					unsigned long uaddr)
>>> +{
>>> +	unsigned long addr = __TLBI_VADDR(uaddr, ASID(vma->vm_mm));
>>> +
>>> +	dsb(nshst);
>>> +	__tlbi(vale1, addr);
>>> +	__tlbi_user(vale1, addr);
>>> +	dsb(nsh);
>>> +}
>>> +
>>> +#define flush_tlb_fix_spurious_fault(vma, address) \
>>> +	local_flush_tlb_page(vma, address)
>>
>> Why can't we just have flush_tlb_fix_spurious_fault() be a NOP on arm64?
> 
> Possibly, as long as any other optimisations only defer the TLB flushing
> for relatively short time (the fault is transient, it will get a
> broadcast TLBI eventually).
> 
> Either way, it's worth benchmarking the above patch but with
> flush_tlb_fix_spurious_fault() a no-op (we still need flush_tlb_page()
> in ptep_set_access_flags()). Xu, Yang, could you please give it a try?

If I understand correctly, this should do as good as the patch of Linux
or Yang in will-it-scale page_fault3 testcase, which both avoid doing
flush_tlb_fix_spurious_fault(), in case of FAULT_FLAG_TRIED.

Shouldn't we be concerned about data integrity if to have
flush_tlb_fix_spurious_fault() be a nop on arm64?

Thanks,
Yu

> 
>> Given that the architecture prohibits the TLB from caching invalid entries,
>> then software access/dirty is fine without additional flushing.
> 
> The access fault is fine, the TLB has not cached the entry. For a dirty
> fault, however, the TLB could cache a read-only mapping, so it does need
> flushing. Question is, do we make the pte dirty anywhere without a
> subsequent (broadcast) TLBI?
> 
>> The only
>> problematic case I can think of is on the invalid->valid (i.e. map) path,
>> where we elide the expensive DSB instruction because (a) most CPUs have a
>> walker that can snoop the store buffer and (b) even if they don't, the
>> store buffer tends to drain by the time we get back to userspace. Even
>> if that was a problem, flush_tlb_fix_spurious_fault() wouldn't be the
>> right hook, since the DSB must occur on the CPU that did the pte update.
> 
> I guess the best a CPU can do is attempt the page table walk again, in
> the hope that the write buffer on the other CPU eventually drains.
> 


  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 [this message]
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
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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