Messages in this thread Patch in this message | | | Date | Thu, 11 Apr 2024 14:17:48 +0100 | Subject | Re: [PATCH v5 2/2] mm/arm64: override mkold_clean_ptes() batch helper | From | Ryan Roberts <> |
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On 08/04/2024 05:24, Lance Yang wrote: > The per-pte get_and_clear/modify/set approach would result in > unfolding/refolding for contpte mappings on arm64. So we need > to override mkold_clean_ptes() for arm64 to avoid it.
IIRC, in the last version, I suggested copying the wrprotect_ptes() pattern to correctly iterate over contpte blocks. I meant for you to take it as inspiration but looks like you have done a carbon copy, including lots of things that are unneeded here. That's my fault for not being clear - sorry!
> > Suggested-by: David Hildenbrand <david@redhat.com> > Suggested-by: Barry Song <21cnbao@gmail.com> > Suggested-by: Ryan Roberts <ryan.roberts@arm.com> > Signed-off-by: Lance Yang <ioworker0@gmail.com> > --- > arch/arm64/include/asm/pgtable.h | 55 ++++++++++++++++++++++++++++++++ > arch/arm64/mm/contpte.c | 15 +++++++++ > 2 files changed, 70 insertions(+) > > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h > index 9fd8613b2db2..395754638a9a 100644 > --- a/arch/arm64/include/asm/pgtable.h > +++ b/arch/arm64/include/asm/pgtable.h > @@ -1223,6 +1223,34 @@ static inline void __wrprotect_ptes(struct mm_struct *mm, unsigned long address, > __ptep_set_wrprotect(mm, address, ptep); > } > > +static inline void ___ptep_mkold_clean(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, pte_t pte) > +{ > + pte_t old_pte; > + > + do { > + old_pte = pte; > + pte = pte_mkclean(pte_mkold(pte)); > + pte_val(pte) = cmpxchg_relaxed(&pte_val(*ptep), > + pte_val(old_pte), pte_val(pte)); > + } while (pte_val(pte) != pte_val(old_pte)); > +}
Given you are clearing old and dirty, you have nothing to race against, so you shouldn't need the cmpxchg loop here; just a get/modify/set should do? Of course if you are setting one or the other, then you need the loop.
> + > +static inline void __ptep_mkold_clean(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep) > +{ > + ___ptep_mkold_clean(mm, addr, ptep, __ptep_get(ptep)); > +}
I don't see a need for this intermediate function.
> + > +static inline void __mkold_clean_ptes(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, unsigned int nr) > +{ > + unsigned int i; > + > + for (i = 0; i < nr; i++, addr += PAGE_SIZE, ptep++)
It would probably be good to use the for() loop pattern used by the generic impls here too.
> + __ptep_mkold_clean(mm, addr, ptep); > +} > + > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > #define __HAVE_ARCH_PMDP_SET_WRPROTECT > static inline void pmdp_set_wrprotect(struct mm_struct *mm, > @@ -1379,6 +1407,8 @@ extern void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr, > extern int contpte_ptep_set_access_flags(struct vm_area_struct *vma, > unsigned long addr, pte_t *ptep, > pte_t entry, int dirty); > +extern void contpte_mkold_clean_ptes(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, unsigned int nr); > > static __always_inline void contpte_try_fold(struct mm_struct *mm, > unsigned long addr, pte_t *ptep, pte_t pte) > @@ -1603,6 +1633,30 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma, > return contpte_ptep_set_access_flags(vma, addr, ptep, entry, dirty); > } > > +#define mkold_clean_ptes mkold_clean_ptes > +static inline void mkold_clean_ptes(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, unsigned int nr) > +{ > + if (likely(nr == 1)) { > + /* > + * Optimization: mkold_clean_ptes() can only be called for present > + * ptes so we only need to check contig bit as condition for unfold, > + * and we can remove the contig bit from the pte we read to avoid > + * re-reading. This speeds up madvise(MADV_FREE) which is sensitive > + * for order-0 folios. Equivalent to contpte_try_unfold(). > + */
Is this true? Do you have data that shows the cost? If not, I'd prefer to avoid the optimization and do it the more standard way:
contpte_try_unfold(mm, addr, ptep, __ptep_get(ptep));
> + pte_t orig_pte = __ptep_get(ptep); > + > + if (unlikely(pte_cont(orig_pte))) { > + __contpte_try_unfold(mm, addr, ptep, orig_pte); > + orig_pte = pte_mknoncont(orig_pte); > + } > + ___ptep_mkold_clean(mm, addr, ptep, orig_pte); > + } else { > + contpte_mkold_clean_ptes(mm, addr, ptep, nr); > + }
..but I don't think you should ever need to unfold in the first place. Even if it's folded and you are trying to clear access/dirty for a single pte, you can just clear the whole block. See existing comment in contpte_ptep_test_and_clear_young().
So this ends up as something like:
static inline void clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr, pte_t *ptep, unsigned int nr, bool clear_young, bool clear_dirty) { if (likely(nr == 1 && !pte_cont(__ptep_get(ptep)))) clear_young_dirty_ptes(mm, addr, ptep, nr, clear_young, clear_dirty); else contpte_clear_young_dirty_ptes(mm, addr, ptep, nr, clear_young, clear_dirty); }
> +} > + > #else /* CONFIG_ARM64_CONTPTE */ > > #define ptep_get __ptep_get > @@ -1622,6 +1676,7 @@ static inline int ptep_set_access_flags(struct vm_area_struct *vma, > #define wrprotect_ptes __wrprotect_ptes > #define __HAVE_ARCH_PTEP_SET_ACCESS_FLAGS > #define ptep_set_access_flags __ptep_set_access_flags > +#define mkold_clean_ptes __mkold_clean_ptes > > #endif /* CONFIG_ARM64_CONTPTE */ > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index 1b64b4c3f8bf..dbff9c5e9eff 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -361,6 +361,21 @@ void contpte_wrprotect_ptes(struct mm_struct *mm, unsigned long addr, > } > EXPORT_SYMBOL_GPL(contpte_wrprotect_ptes); > > +void contpte_mkold_clean_ptes(struct mm_struct *mm, unsigned long addr, > + pte_t *ptep, unsigned int nr) > +{ > + /* > + * If clearing the young and dirty bits for an entire contig range, we can > + * avoid unfolding. Just set old/clean and wait for the later mmu_gather > + * flush to invalidate the tlb. If it's a partial range though, we need to > + * unfold. > + */
nit: Please reflow comments like this to 80 cols.
We can avoid unfolding in all cases. See existing comment in contpte_ptep_test_and_clear_young(). Suggest something like this (untested):
void clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr, pte_t *ptep, unsigned int nr, bool clear_young, bool clear_dirty) { /* * We can safely clear access/dirty without needing to unfold from the * architectures perspective, even when contpte is set. If the range * starts or ends midway through a contpte block, we can just expand to * include the full contpte block. While this is not exactly what the * core-mm asked for, it tracks access/dirty per folio, not per page. * And since we only create a contpte block when it is covered by a * single folio, we can get away with clearing access/dirty for the * whole block. */
unsigned int start = addr; unsigned int end = start + nr;
if (pte_cont(__ptep_get(ptep + nr - 1))) end = ALIGN(end, CONT_PTE_SIZE);
if (pte_cont(__ptep_get(ptep))) { start = ALIGN_DOWN(start, CONT_PTE_SIZE); ptep = contpte_align_down(ptep); } __clear_young_dirty_ptes(mm, start, ptep, end - start, clear_young, clear_dirty); } Thanks, Ryan
> + > + contpte_try_unfold_partial(mm, addr, ptep, nr); > + __mkold_clean_ptes(mm, addr, ptep, nr); > +} > +EXPORT_SYMBOL_GPL(contpte_mkold_clean_ptes); > + > int contpte_ptep_set_access_flags(struct vm_area_struct *vma, > unsigned long addr, pte_t *ptep, > pte_t entry, int dirty)
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