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Date: Fri, 20 Nov 2020 15:15:24 +0000 From: Will Deacon <will@...nel.org> To: Peter Zijlstra <peterz@...radead.org> Cc: linux-kernel@...r.kernel.org, kernel-team@...roid.com, Catalin Marinas <catalin.marinas@....com>, Yu Zhao <yuzhao@...gle.com>, Minchan Kim <minchan@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Anshuman Khandual <anshuman.khandual@....com>, linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org Subject: Re: [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty page state On Fri, Nov 20, 2020 at 04:00:23PM +0100, Peter Zijlstra wrote: > On Fri, Nov 20, 2020 at 02:35:55PM +0000, Will Deacon wrote: > > Since commit 0758cd830494 ("asm-generic/tlb: avoid potential double flush"), > > TLB invalidation is elided in tlb_finish_mmu() if no entries were batched > > via the tlb_remove_*() functions. Consequently, the page-table modifications > > performed by clear_refs_write() in response to a write to > > /proc/<pid>/clear_refs do not perform TLB invalidation. Although this is > > fine when simply aging the ptes, in the case of clearing the "soft-dirty" > > state we can end up with entries where pte_write() is false, yet a > > writable mapping remains in the TLB. > > > > Fix this by calling tlb_remove_tlb_entry() for each entry being > > write-protected when cleating soft-dirty. > > > > > @@ -1053,6 +1054,7 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma, > > ptent = pte_wrprotect(old_pte); > > ptent = pte_clear_soft_dirty(ptent); > > ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent); > > + tlb_remove_tlb_entry(tlb, pte, addr); > > } else if (is_swap_pte(ptent)) { > > ptent = pte_swp_clear_soft_dirty(ptent); > > set_pte_at(vma->vm_mm, addr, pte, ptent); > > Oh! > > Yesterday when you had me look at this code; I figured the sane thing > to do was to make it look more like mprotect(). Ah, so you mean ditch the mmu_gather altogether? > Why did you chose to make it work with mmu_gather instead? I'll grant > you that it's probably the smaller patch, but I still think it's weird > to use mmu_gather here. > > Also, is tlb_remote_tlb_entry() actually correct? If you look at > __tlb_remove_tlb_entry() you'll find that Power-Hash-32 will clear the > entry, which might not be what we want here, we want to update the > entrty. Hmm, I didn't spot that, although ptep_modify_prot_start() does actually clear the pte so we could just move this up a few lines. Will > > >
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