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From: Minchan Kim <minchan@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Yu Zhao <yuzhao@google.com>, Will Deacon <will@kernel.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	kernel-team@android.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty page state
Date: Fri, 20 Nov 2020 07:55:14 -0800	[thread overview]
Message-ID: <20201120155514.GA3377168@google.com> (raw)
In-Reply-To: <20201120150023.GH3040@hirez.programming.kicks-ass.net>

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().
> 
> 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.

I agree. The reason why clear_refs_write used the gather API was [1] and
seems like to overkill to me.

We could just do like [inc|dec]_tlb_flush_pending with flush_tlb_mm at
right before dec_tlb_flush_pending instead of gather.

thought?

[1] b3a81d0841a95, mm: fix KSM data corruption

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WARNING: multiple messages have this Message-ID (diff)
From: Minchan Kim <minchan@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org, kernel-team@android.com,
	Catalin Marinas <catalin.marinas@arm.com>,
	Yu Zhao <yuzhao@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	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
Date: Fri, 20 Nov 2020 07:55:14 -0800	[thread overview]
Message-ID: <20201120155514.GA3377168@google.com> (raw)
In-Reply-To: <20201120150023.GH3040@hirez.programming.kicks-ass.net>

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().
> 
> 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.

I agree. The reason why clear_refs_write used the gather API was [1] and
seems like to overkill to me.

We could just do like [inc|dec]_tlb_flush_pending with flush_tlb_mm at
right before dec_tlb_flush_pending instead of gather.

thought?

[1] b3a81d0841a95, mm: fix KSM data corruption


  parent reply	other threads:[~2020-11-20 15:55 UTC|newest]

Thread overview: 85+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-20 14:35 [PATCH 0/6] tlb: Fix access and (soft-)dirty bit management Will Deacon
2020-11-20 14:35 ` Will Deacon
2020-11-20 14:35 ` [PATCH 1/6] arm64: pgtable: Fix pte_accessible() Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 16:03   ` Minchan Kim
2020-11-20 16:03     ` Minchan Kim
2020-11-20 19:53   ` Yu Zhao
2020-11-20 19:53     ` Yu Zhao
2020-11-23 13:27   ` Catalin Marinas
2020-11-23 13:27     ` Catalin Marinas
2020-11-24 10:02   ` Anshuman Khandual
2020-11-24 10:02     ` Anshuman Khandual
2020-11-20 14:35 ` [PATCH 2/6] arm64: pgtable: Ensure dirty bit is preserved across pte_wrprotect() Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 17:09   ` Minchan Kim
2020-11-20 17:09     ` Minchan Kim
2020-11-23 14:31     ` Catalin Marinas
2020-11-23 14:31       ` Catalin Marinas
2020-11-23 14:22   ` Catalin Marinas
2020-11-23 14:22     ` Catalin Marinas
2020-11-20 14:35 ` [PATCH 3/6] tlb: mmu_gather: Remove unused start/end arguments from tlb_finish_mmu() Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 17:20   ` Linus Torvalds
2020-11-20 17:20     ` Linus Torvalds
2020-11-23 16:48     ` Will Deacon
2020-11-23 16:48       ` Will Deacon
2020-11-20 14:35 ` [PATCH 4/6] mm: proc: Invalidate TLB after clearing soft-dirty page state Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 15:00   ` Peter Zijlstra
2020-11-20 15:00     ` Peter Zijlstra
2020-11-20 15:09     ` Peter Zijlstra
2020-11-20 15:09       ` Peter Zijlstra
2020-11-20 15:15     ` Will Deacon
2020-11-20 15:15       ` Will Deacon
2020-11-20 15:27       ` Peter Zijlstra
2020-11-20 15:27         ` Peter Zijlstra
2020-11-23 18:23         ` Will Deacon
2020-11-23 18:23           ` Will Deacon
2020-11-20 15:55     ` Minchan Kim [this message]
2020-11-20 15:55       ` Minchan Kim
2020-11-23 18:41       ` Will Deacon
2020-11-23 18:41         ` Will Deacon
2020-11-25 22:51         ` Minchan Kim
2020-11-25 22:51           ` Minchan Kim
2020-11-20 20:22   ` Yu Zhao
2020-11-20 20:22     ` Yu Zhao
2020-11-21  2:49     ` Yu Zhao
2020-11-21  2:49       ` Yu Zhao
2020-11-23 19:21       ` Yu Zhao
2020-11-23 19:21         ` Yu Zhao
2020-11-23 22:04       ` Will Deacon
2020-11-23 22:04         ` Will Deacon
2020-11-20 14:35 ` [PATCH 5/6] tlb: mmu_gather: Introduce tlb_gather_mmu_fullmm() Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 17:22   ` Linus Torvalds
2020-11-20 17:22     ` Linus Torvalds
2020-11-20 17:31     ` Linus Torvalds
2020-11-20 17:31       ` Linus Torvalds
2020-11-23 16:48       ` Will Deacon
2020-11-23 16:48         ` Will Deacon
2020-11-22 15:11   ` [tlb] e242a269fa: WARNING:at_mm/mmu_gather.c:#tlb_gather_mmu kernel test robot
2020-11-23 17:51     ` Will Deacon
2020-11-23 17:51       ` Will Deacon
2020-11-20 14:35 ` [PATCH 6/6] mm: proc: Avoid fullmm flush for young/dirty bit toggling Will Deacon
2020-11-20 14:35   ` Will Deacon
2020-11-20 17:41   ` Linus Torvalds
2020-11-20 17:41     ` Linus Torvalds
2020-11-20 17:45     ` Linus Torvalds
2020-11-20 17:45       ` Linus Torvalds
2020-11-20 20:40   ` Yu Zhao
2020-11-20 20:40     ` Yu Zhao
2020-11-23 18:35     ` Will Deacon
2020-11-23 18:35       ` Will Deacon
2020-11-23 20:04       ` Yu Zhao
2020-11-23 20:04         ` Yu Zhao
2020-11-23 21:17         ` Will Deacon
2020-11-23 21:17           ` Will Deacon
2020-11-24  1:13           ` Yu Zhao
2020-11-24  1:13             ` Yu Zhao
2020-11-24 14:31             ` Will Deacon
2020-11-24 14:31               ` Will Deacon
2020-11-25 22:01             ` Minchan Kim
2020-11-25 22:01               ` Minchan Kim
2020-11-24 14:46     ` Peter Zijlstra
2020-11-24 14:46       ` Peter Zijlstra

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