From: Yang Shi yang.shi@linux.alibaba.com Subject: mm/memory.c: avoid access flag update TLB flush for retried page fault
Recently we found regression when running will_it_scale/page_fault3 test on ARM64. Over 70% down for the multi processes cases and over 20% down for the multi threads cases. It turns out the regression is caused by commit 89b15332af7c0312a41e50846819ca6613b58b4c ("mm: drop mmap_sem before calling balance_dirty_pages() in write fault").
The test mmaps a memory size file then write to the mapping, this would make all memory dirty and trigger dirty pages throttle, that upstream commit would release mmap_sem then retry the page fault. The retried page fault would see correct PTEs installed by the first try then update dirty bit and clear read-only bit and flush TLBs for ARM. The regression is caused by the excessive TLB flush. It is fine on x86 since x86 doesn't clear read-only bit so there is no need to flush TLB for this case.
The page fault would be retried due to: 1. Waiting for page readahead 2. Waiting for page swapped in 3. Waiting for dirty pages throttling
The first two cases don't have PTEs set up at all, so the retried page fault would install the PTEs, so they don't reach there. But the #3 case usually has PTEs installed, the retried page fault would reach the dirty bit and read-only bit update. But it seems not necessary to modify those bits again for #3 since they should be already set by the first page fault try.
Of course the parallel page fault may set up PTEs, but we just need care about write fault. If the parallel page fault setup a writable and dirty PTE then the retried fault doesn't need do anything extra. If the parallel page fault setup a clean read-only PTE, the retried fault should just call do_wp_page() then return as the below code snippet shows:
if (vmf->flags & FAULT_FLAG_WRITE) { if (!pte_write(entry)) return do_wp_page(vmf); }
With this fix the test result get back to normal.
[yang.shi@linux.alibaba.com: incorporate comment from Will Deacon, update commit log per discussion] Link: http://lkml.kernel.org/r/1594848990-55657-1-git-send-email-yang.shi@linux.al... Link: http://lkml.kernel.org/r/1594148072-91273-1-git-send-email-yang.shi@linux.al... Signed-off-by: Yang Shi yang.shi@linux.alibaba.com Reported-by: Xu Yu xuyu@linux.alibaba.com Debugged-by: Xu Yu xuyu@linux.alibaba.com Tested-by: Xu Yu xuyu@linux.alibaba.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Kirill A. Shutemov kirill.shutemov@linux.intel.com Cc: Josef Bacik josef@toxicpanda.com Cc: Hillf Danton hdanton@sina.com Cc: Hugh Dickins hughd@google.com Cc: Catalin Marinas catalin.marinas@arm.com Cc: Will Deacon will.deacon@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/memory.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
--- a/mm/memory.c~mm-avoid-access-flag-update-tlb-flush-for-retried-page-fault +++ a/mm/memory.c @@ -4241,8 +4241,14 @@ static vm_fault_t handle_pte_fault(struc if (vmf->flags & FAULT_FLAG_WRITE) { if (!pte_write(entry)) return do_wp_page(vmf); - entry = pte_mkdirty(entry); } + + if (vmf->flags & FAULT_FLAG_TRIED) + goto unlock; + + if (vmf->flags & FAULT_FLAG_WRITE) + entry = pte_mkdirty(entry); + entry = pte_mkyoung(entry); if (ptep_set_access_flags(vmf->vma, vmf->address, vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE)) { _