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Fri, 20 Nov 2020 15:27:32 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 006E3304D28; Fri, 20 Nov 2020 16:27:32 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id E09B8202397FB; Fri, 20 Nov 2020 16:27:31 +0100 (CET) Date: Fri, 20 Nov 2020 16:27:31 +0100 From: Peter Zijlstra To: Will Deacon Cc: linux-kernel@vger.kernel.org, kernel-team@android.com, Catalin Marinas , Yu Zhao , Minchan Kim , Linus Torvalds , Anshuman Khandual , linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty page state Message-ID: <20201120152731.GK3021@hirez.programming.kicks-ass.net> References: <20201120143557.6715-1-will@kernel.org> <20201120143557.6715-5-will@kernel.org> <20201120150023.GH3040@hirez.programming.kicks-ass.net> <20201120151523.GA6861@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201120151523.GA6861@willie-the-truck> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Nov 20, 2020 at 03:15:24PM +0000, Will Deacon wrote: > 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//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? Yes. Alternatively, if we decide mmu_gather is 'right', then we should probably look at converting mprotect(). That is, I see no reason why this and mprotect should differ on this point. > > 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. Yes, but hash-entry != pte. If I'm not mistaken (and I could very well be, it's Friday and Power-MMUs being the maze they are), the end result here is an updated PTE but an empty hash-entry.