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Date: Tue, 1 Feb 2022 17:16:43 +0000
From: Liam Howlett <liam.howlett@...cle.com>
To: Michel Lespinasse <michel@...pinasse.org>
CC: Linux-MM <linux-mm@...ck.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
"kernel-team@...com" <kernel-team@...com>,
Laurent Dufour <ldufour@...ux.ibm.com>,
Jerome Glisse <jglisse@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Michal Hocko <mhocko@...e.com>,
Vlastimil Babka <vbabka@...e.cz>,
Davidlohr Bueso <dave@...olabs.net>,
Matthew Wilcox <willy@...radead.org>,
Rik van Riel <riel@...riel.com>,
Paul McKenney <paulmck@...nel.org>,
Song Liu <songliubraving@...com>,
Suren Baghdasaryan <surenb@...gle.com>,
Minchan Kim <minchan@...gle.com>,
Joel Fernandes <joelaf@...gle.com>,
David Rientjes <rientjes@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2 13/35] x86/mm: attempt speculative mm faults first
* Michel Lespinasse <michel@...pinasse.org> [220128 08:10]:
> Attempt speculative mm fault handling first, and fall back to the
> existing (non-speculative) code if that fails.
>
> The speculative handling closely mirrors the non-speculative logic.
> This includes some x86 specific bits such as the access_error() call.
> This is why we chose to implement the speculative handling in arch/x86
> rather than in common code.
>
> The vma is first looked up and copied, under protection of the rcu
> read lock. The mmap lock sequence count is used to verify the
> integrity of the copied vma, and passed to do_handle_mm_fault() to
> allow checking against races with mmap writers when finalizing the fault.
>
> Signed-off-by: Michel Lespinasse <michel@...pinasse.org>
> ---
> arch/x86/mm/fault.c | 44 +++++++++++++++++++++++++++++++++++
> include/linux/mm_types.h | 5 ++++
> include/linux/vm_event_item.h | 4 ++++
> mm/vmstat.c | 4 ++++
> 4 files changed, 57 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index d0074c6ed31a..99b0a358154e 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1226,6 +1226,10 @@ void do_user_addr_fault(struct pt_regs *regs,
> struct mm_struct *mm;
> vm_fault_t fault;
> unsigned int flags = FAULT_FLAG_DEFAULT;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + struct vm_area_struct pvma;
> + unsigned long seq;
> +#endif
>
> tsk = current;
> mm = tsk->mm;
> @@ -1323,6 +1327,43 @@ void do_user_addr_fault(struct pt_regs *regs,
> }
> #endif
>
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + count_vm_event(SPF_ATTEMPT);
> + seq = mmap_seq_read_start(mm);
> + if (seq & 1)
> + goto spf_abort;
> + rcu_read_lock();
> + vma = __find_vma(mm, address);
> + if (!vma || vma->vm_start > address) {
This fits the vma_lookup() pattern - although you will have to work
around the locking issue still. This is the same for the other
platforms too; they fit the pattern also.
> + rcu_read_unlock();
> + goto spf_abort;
> + }
> + pvma = *vma;
> + rcu_read_unlock();
> + if (!mmap_seq_read_check(mm, seq))
> + goto spf_abort;
> + vma = &pvma;
> + if (unlikely(access_error(error_code, vma)))
> + goto spf_abort;
> + fault = do_handle_mm_fault(vma, address,
> + flags | FAULT_FLAG_SPECULATIVE, seq, regs);
> +
> + if (!(fault & VM_FAULT_RETRY))
> + goto done;
> +
> + /* Quick path to respond to signals */
> + if (fault_signal_pending(fault, regs)) {
> + if (!user_mode(regs))
> + kernelmode_fixup_or_oops(regs, error_code, address,
> + SIGBUS, BUS_ADRERR,
> + ARCH_DEFAULT_PKEY);
> + return;
> + }
> +
> +spf_abort:
> + count_vm_event(SPF_ABORT);
> +#endif
> +
> /*
> * Kernel-mode access to the user address space should only occur
> * on well-defined single instructions listed in the exception
> @@ -1419,6 +1460,9 @@ void do_user_addr_fault(struct pt_regs *regs,
> }
>
> mmap_read_unlock(mm);
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +done:
> +#endif
> if (likely(!(fault & VM_FAULT_ERROR)))
> return;
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index b6678578a729..305f05d2a4bc 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -370,6 +370,11 @@ struct anon_vma_name {
> * per VM-area/task. A VM area is any part of the process virtual memory
> * space that has a special rule for the page-fault handlers (ie a shared
> * library, the executable area etc).
> + *
> + * Note that speculative page faults make an on-stack copy of the VMA,
> + * so the structure size matters.
> + * (TODO - it would be preferable to copy only the required vma attributes
> + * rather than the entire vma).
> */
> struct vm_area_struct {
> /* The first cache line has the info for VMA tree walking. */
> diff --git a/include/linux/vm_event_item.h b/include/linux/vm_event_item.h
> index 7b2363388bfa..f00b3e36ff39 100644
> --- a/include/linux/vm_event_item.h
> +++ b/include/linux/vm_event_item.h
> @@ -133,6 +133,10 @@ enum vm_event_item { PGPGIN, PGPGOUT, PSWPIN, PSWPOUT,
> #ifdef CONFIG_X86
> DIRECT_MAP_LEVEL2_SPLIT,
> DIRECT_MAP_LEVEL3_SPLIT,
> +#endif
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + SPF_ATTEMPT,
> + SPF_ABORT,
> #endif
> NR_VM_EVENT_ITEMS
> };
> diff --git a/mm/vmstat.c b/mm/vmstat.c
> index 4057372745d0..dbb0160e5558 100644
> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1390,6 +1390,10 @@ const char * const vmstat_text[] = {
> "direct_map_level2_splits",
> "direct_map_level3_splits",
> #endif
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + "spf_attempt",
> + "spf_abort",
> +#endif
> #endif /* CONFIG_VM_EVENT_COUNTERS || CONFIG_MEMCG */
> };
> #endif /* CONFIG_PROC_FS || CONFIG_SYSFS || CONFIG_NUMA || CONFIG_MEMCG */
> --
> 2.20.1
>
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