Re: [PATCH v2 20/20] powerpc/mm: Add speculative page fault
From: Anshuman Khandual <hidden>
Date: 2017-08-21 06:59:48
Also in:
linux-mm, linuxppc-dev
On 08/18/2017 03:35 AM, Laurent Dufour wrote:
This patch enable the speculative page fault on the PowerPC architecture. This will try a speculative page fault without holding the mmap_sem, if it returns with WM_FAULT_RETRY, the mmap_sem is acquired and the
s/WM_FAULT_RETRY/VM_FAULT_RETRY/
traditional page fault processing is done. Support is only provide for BOOK3S_64 currently because: - require CONFIG_PPC_STD_MMU because checks done in set_access_flags_filter()
What checks are done in set_access_flags_filter() ? We are just adding the code block in do_page_fault().
quoted hunk ↗ jump to hunk
- require BOOK3S because we can't support for book3e_hugetlb_preload() called by update_mmu_cache() Signed-off-by: Laurent Dufour <redacted> --- arch/powerpc/include/asm/book3s/64/pgtable.h | 5 +++++ arch/powerpc/mm/fault.c | 30 +++++++++++++++++++++++++++- 2 files changed, 34 insertions(+), 1 deletion(-)diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h index 818a58fc3f4f..897f8b9f67e6 100644 --- a/arch/powerpc/include/asm/book3s/64/pgtable.h +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h@@ -313,6 +313,11 @@ extern unsigned long pci_io_base; /* Advertise support for _PAGE_SPECIAL */ #define __HAVE_ARCH_PTE_SPECIAL +/* Advertise that we call the Speculative Page Fault handler */ +#if defined(CONFIG_PPC_BOOK3S_64) +#define __HAVE_ARCH_CALL_SPF +#endif + #ifndef __ASSEMBLY__ /*diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c index 4c422632047b..7b3cc4c30eab 100644 --- a/arch/powerpc/mm/fault.c +++ b/arch/powerpc/mm/fault.c@@ -291,9 +291,36 @@ int do_page_fault(struct pt_regs *regs, unsigned long address, if (is_write && is_user) store_update_sp = store_updates_sp(regs); - if (is_user) + if (is_user) { flags |= FAULT_FLAG_USER; +#if defined(__HAVE_ARCH_CALL_SPF) + /* let's try a speculative page fault without grabbing the + * mmap_sem. + */ + + /* + * flags is set later based on the VMA's flags, for the common + * speculative service, we need some flags to be set. + */ + if (is_write) + flags |= FAULT_FLAG_WRITE; + + fault = handle_speculative_fault(mm, address, flags); + if (!(fault & VM_FAULT_RETRY || fault & VM_FAULT_ERROR)) { + perf_sw_event(PERF_COUNT_SW_SPF_DONE, 1, + regs, address); + goto done;
Why we should retry with classical page fault on VM_FAULT_ERROR ? We should always return VM_FAULT_RETRY in case there is a clear collision some where which requires retry with classical method and return VM_FAULT_ERROR in cases where we know that it cannot be retried and fail for good. Should not handle_speculative_fault() be changed to accommodate this ?
+ } + + /* + * Resetting flags since the following code assumes + * FAULT_FLAG_WRITE is not set. + */ + flags &= ~FAULT_FLAG_WRITE; +#endif /* defined(__HAVE_ARCH_CALL_SPF) */
Setting and resetting of FAULT_FLAG_WRITE seems confusing. Why you say that some flags need to be set for handle_speculative_fault() function. Could you elaborate on this ?