From: Catalin Marinas <catalin.marinas@arm.com>
To: 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>,
xuyu@linux.alibaba.com
Subject: Re: [patch 01/15] mm/memory.c: avoid access flag update TLB flush for retried page fault
Date: Tue, 28 Jul 2020 10:39:11 +0100 [thread overview]
Message-ID: <20200728093910.GB706@gaia> (raw)
In-Reply-To: <20200728092220.GA21800@willie-the-truck>
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?
> 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.
--
Catalin
next prev parent reply other threads:[~2020-07-28 9:39 UTC|newest]
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 [this message]
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
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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