| Message ID | 20201120143557.6715-2-will@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | tlb: Fix access and (soft-)dirty bit management | expand |
On Fri, Nov 20, 2020 at 02:35:52PM +0000, Will Deacon wrote: > pte_accessible() is used by ptep_clear_flush() to figure out whether TLB > invalidation is necessary when unmapping pages for reclaim. Although our > implementation is correct according to the architecture, returning true > only for valid, young ptes in the absence of racing page-table > modifications, this is in fact flawed due to lazy invalidation of old > ptes in ptep_clear_flush_young() where we elide the expensive DSB > instruction for completing the TLB invalidation. > > Rather than penalise the aging path, adjust pte_accessible() to return > true for any valid pte, even if the access flag is cleared. > > Cc: <stable@vger.kernel.org> > Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()") > Reported-by: Yu Zhao <yuzhao@google.com> > Signed-off-by: Will Deacon <will@kernel.org> Reviewed-by: Minchan Kim <minchan@kernel.org>
On Fri, Nov 20, 2020 at 02:35:52PM +0000, Will Deacon wrote: > pte_accessible() is used by ptep_clear_flush() to figure out whether TLB > invalidation is necessary when unmapping pages for reclaim. Although our > implementation is correct according to the architecture, returning true > only for valid, young ptes in the absence of racing page-table > modifications, this is in fact flawed due to lazy invalidation of old > ptes in ptep_clear_flush_young() where we elide the expensive DSB > instruction for completing the TLB invalidation. > > Rather than penalise the aging path, adjust pte_accessible() to return > true for any valid pte, even if the access flag is cleared. The chance of a system hitting reclaim is proportional to how long it's been running, and that of having mapped but yet to be accessed PTEs is reciprocal, so to speak. I don't reboot my devices everyday, and therefore: Acked-by: Yu Zhao <yuzhao@google.com> > Cc: <stable@vger.kernel.org> > Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()") > Reported-by: Yu Zhao <yuzhao@google.com> > Signed-off-by: Will Deacon <will@kernel.org> > --- > arch/arm64/include/asm/pgtable.h | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 4ff12a7adcfd..1bdf51f01e73 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -115,8 +115,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; > #define pte_valid(pte) (!!(pte_val(pte) & PTE_VALID)) > #define pte_valid_not_user(pte) \ > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID) > -#define pte_valid_young(pte) \ > - ((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF)) > #define pte_valid_user(pte) \ > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) > > @@ -126,7 +124,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; > * remapped as PROT_NONE but are yet to be flushed from the TLB. > */ > #define pte_accessible(mm, pte) \ > - (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte)) > + (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte)) > > /* > * p??_access_permitted() is true for valid user mappings (subject to the > -- > 2.29.2.454.gaff20da3a2-goog >
On Fri, Nov 20, 2020 at 02:35:52PM +0000, Will Deacon wrote: > pte_accessible() is used by ptep_clear_flush() to figure out whether TLB > invalidation is necessary when unmapping pages for reclaim. Although our > implementation is correct according to the architecture, returning true > only for valid, young ptes in the absence of racing page-table > modifications, this is in fact flawed due to lazy invalidation of old > ptes in ptep_clear_flush_young() where we elide the expensive DSB > instruction for completing the TLB invalidation. > > Rather than penalise the aging path, adjust pte_accessible() to return > true for any valid pte, even if the access flag is cleared. > > Cc: <stable@vger.kernel.org> > Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()") > Reported-by: Yu Zhao <yuzhao@google.com> > Signed-off-by: Will Deacon <will@kernel.org> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
On 11/20/20 8:05 PM, Will Deacon wrote: > pte_accessible() is used by ptep_clear_flush() to figure out whether TLB > invalidation is necessary when unmapping pages for reclaim. Although our > implementation is correct according to the architecture, returning true > only for valid, young ptes in the absence of racing page-table Just curious, a PTE mapping would go into the TLB only if it is an young one with PTE_AF bit set per the architecture ? > modifications, this is in fact flawed due to lazy invalidation of old > ptes in ptep_clear_flush_young() where we elide the expensive DSB > instruction for completing the TLB invalidation. IOW, an old PTE might have missed the required TLB invalidation via ptep_clear_flush_young() because it's done in lazy mode. Hence just include old valid PTEs in pte_accessible() so that TLB invalidation could be done in ptep_clear_flush() path instead. May be TLB flush could be done for every PTE, irrespective of its PTE_AF bit in ptep_clear_flush_young(). > > Rather than penalise the aging path, adjust pte_accessible() to return > true for any valid pte, even if the access flag is cleared. But will not this cause more (possibly not required) TLB invalidation in normal unmapping paths ? The cover letter mentions that this patch fixes a real world crash. Should not the crash also be described here in the commit message as this patch is marked for stable and has a "Fixes: " tag. > > Cc: <stable@vger.kernel.org> > Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()") > Reported-by: Yu Zhao <yuzhao@google.com> > Signed-off-by: Will Deacon <will@kernel.org> > --- > arch/arm64/include/asm/pgtable.h | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 4ff12a7adcfd..1bdf51f01e73 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -115,8 +115,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; > #define pte_valid(pte) (!!(pte_val(pte) & PTE_VALID)) > #define pte_valid_not_user(pte) \ > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID) > -#define pte_valid_young(pte) \ > - ((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF)) > #define pte_valid_user(pte) \ > ((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) > > @@ -126,7 +124,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; > * remapped as PROT_NONE but are yet to be flushed from the TLB. > */ > #define pte_accessible(mm, pte) \ > - (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte)) > + (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte)) > > /* > * p??_access_permitted() is true for valid user mappings (subject to the >
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h index 4ff12a7adcfd..1bdf51f01e73 100644 --- a/arch/arm64/include/asm/pgtable.h +++ b/arch/arm64/include/asm/pgtable.h @@ -115,8 +115,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; #define pte_valid(pte) (!!(pte_val(pte) & PTE_VALID)) #define pte_valid_not_user(pte) \ ((pte_val(pte) & (PTE_VALID | PTE_USER)) == PTE_VALID) -#define pte_valid_young(pte) \ - ((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF)) #define pte_valid_user(pte) \ ((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER)) @@ -126,7 +124,7 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)]; * remapped as PROT_NONE but are yet to be flushed from the TLB. */ #define pte_accessible(mm, pte) \ - (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte)) + (mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte)) /* * p??_access_permitted() is true for valid user mappings (subject to the
pte_accessible() is used by ptep_clear_flush() to figure out whether TLB invalidation is necessary when unmapping pages for reclaim. Although our implementation is correct according to the architecture, returning true only for valid, young ptes in the absence of racing page-table modifications, this is in fact flawed due to lazy invalidation of old ptes in ptep_clear_flush_young() where we elide the expensive DSB instruction for completing the TLB invalidation. Rather than penalise the aging path, adjust pte_accessible() to return true for any valid pte, even if the access flag is cleared. Cc: <stable@vger.kernel.org> Fixes: 76c714be0e5e ("arm64: pgtable: implement pte_accessible()") Reported-by: Yu Zhao <yuzhao@google.com> Signed-off-by: Will Deacon <will@kernel.org> --- arch/arm64/include/asm/pgtable.h | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)