Messages in this thread | | | Date | Thu, 3 Nov 2022 11:51:26 +0100 | Subject | Re: [PATCH v1 4/6] mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite | From | David Hildenbrand <> |
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On 03.11.22 11:45, David Hildenbrand wrote: > On 02.11.22 22:22, Nadav Amit wrote: >> On Nov 2, 2022, at 12:12 PM, David Hildenbrand <david@redhat.com> wrote: >> >>> !! External Email >>> >>> commit b191f9b106ea ("mm: numa: preserve PTE write permissions across a >>> NUMA hinting fault") added remembering write permissions using ordinary >>> pte_write() for PROT_NONE mapped pages to avoid write faults when >>> remapping the page !PROT_NONE on NUMA hinting faults. >>> >> >> [ snip ] >> >> Here’s a very shallow reviewed with some minor points... > > Appreciated. > >> >>> --- >>> include/linux/mm.h | 2 ++ >>> mm/huge_memory.c | 28 +++++++++++++++++----------- >>> mm/ksm.c | 9 ++++----- >>> mm/memory.c | 19 ++++++++++++++++--- >>> mm/mprotect.c | 7 ++----- >>> 5 files changed, 41 insertions(+), 24 deletions(-) >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 25ff9a14a777..a0deeece5e87 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -1975,6 +1975,8 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma, >>> #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \ >>> MM_CP_UFFD_WP_RESOLVE) >>> >>> +bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr, >>> + pte_t pte); >> >> It might not be customary, but how about marking it as __pure? > > Right, there is no a single use of __pure in the mm domain. > >> >>> extern unsigned long change_protection(struct mmu_gather *tlb, >>> struct vm_area_struct *vma, unsigned long start, >>> unsigned long end, pgprot_t newprot, >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>> index 2ad68e91896a..45abd27d75a0 100644 >>> --- a/mm/huge_memory.c >>> +++ b/mm/huge_memory.c >>> @@ -1462,8 +1462,7 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf) >>> unsigned long haddr = vmf->address & HPAGE_PMD_MASK; >>> int page_nid = NUMA_NO_NODE; >>> int target_nid, last_cpupid = (-1 & LAST_CPUPID_MASK); >>> - bool migrated = false; >>> - bool was_writable = pmd_savedwrite(oldpmd); >>> + bool try_change_writable, migrated = false; >>> int flags = 0; >>> >>> vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd); >>> @@ -1472,13 +1471,22 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf) >>> goto out; >>> } >>> >>> + /* See mprotect_fixup(). */ >>> + if (vma->vm_flags & VM_SHARED) >>> + try_change_writable = vma_wants_writenotify(vma, vma->vm_page_prot); >>> + else >>> + try_change_writable = !!(vma->vm_flags & VM_WRITE); >> >> Do you find it better to copy the code instead of extracting it to a >> separate function? > > Yeah, you're right ;) usually the issue is coming up with a suitable name. Let me try. > > vma_wants_manual_writability_change() hmm ... > >> >>> + >>> pmd = pmd_modify(oldpmd, vma->vm_page_prot); >>> page = vm_normal_page_pmd(vma, haddr, pmd); >>> if (!page) >>> goto out_map; >>> >>> /* See similar comment in do_numa_page for explanation */ >>> - if (!was_writable) >>> + if (try_change_writable && !pmd_write(pmd) && >>> + can_change_pmd_writable(vma, vmf->address, pmd)) >>> + pmd = pmd_mkwrite(pmd); >>> + if (!pmd_write(pmd)) >>> flags |= TNF_NO_GROUP; >>> >>> page_nid = page_to_nid(page); >>> @@ -1523,8 +1531,12 @@ vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf) >>> /* Restore the PMD */ >>> pmd = pmd_modify(oldpmd, vma->vm_page_prot); >>> pmd = pmd_mkyoung(pmd); >>> - if (was_writable) >>> + >>> + /* Similar to mprotect() protection updates, avoid write faults. */ >>> + if (try_change_writable && !pmd_write(pmd) && >>> + can_change_pmd_writable(vma, vmf->address, pmd)) >> >> Why do I have a deja-vu? :) >> >> There must be a way to avoid the redundant code and specifically the call to >> can_change_pmd_writable(), no? > > The issue is that as soon as we drop the page table lock, that information is stale. > Especially, after we fail migration. > > So the following should work, however, if we fail migration we wouldn't map the > page writable and would have to re-calculate: > > diff --git a/mm/memory.c b/mm/memory.c > index c5599a9279b1..a997625641e4 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4674,10 +4674,10 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > struct vm_area_struct *vma = vmf->vma; > struct page *page = NULL; > int page_nid = NUMA_NO_NODE; > + bool writable = false; > int last_cpupid; > int target_nid; > pte_t pte, old_pte; > - bool was_writable = pte_savedwrite(vmf->orig_pte); > int flags = 0; > > /* > @@ -4696,6 +4696,17 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > old_pte = ptep_get(vmf->pte); > pte = pte_modify(old_pte, vma->vm_page_prot); > > + /* > + * Detect now whether the PTE is or can be writable. Note that this > + * information is valid as long as we're holding the PT lock, so also on > + * the remap path below. > + */ > + writable = pte_write(pte); > + if (!writable && vma_wants_manual_writability_change(vma) && > + can_change_pte_writable(vma, vmf->address, pte); > + writable = true; > + } > + > page = vm_normal_page(vma, vmf->address, pte); > if (!page || is_zone_device_page(page)) > goto out_map; > @@ -4712,7 +4723,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > * pte_dirty has unpredictable behaviour between PTE scan updates, > * background writeback, dirty balancing and application behaviour. > */ > - if (!was_writable) > + if (!writable) > flags |= TNF_NO_GROUP; > > /* > @@ -4738,6 +4749,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > put_page(page); > goto out_map; > } > + writable = false; > pte_unmap_unlock(vmf->pte, vmf->ptl); > > /* Migrate to the requested node */ > @@ -4767,7 +4779,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > old_pte = ptep_modify_prot_start(vma, vmf->address, vmf->pte); > pte = pte_modify(old_pte, vma->vm_page_prot); > pte = pte_mkyoung(pte); > - if (was_writable) > + if (writable) > pte = pte_mkwrite(pte); > ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte); > update_mmu_cache(vma, vmf->address, vmf->pte); > > > To me, the less error-prone approach is to re-calculate.
Hmm, thinking again, the "if (unlikely(!pte_same(*vmf->pte, vmf->orig_pte))) {" check might actually not require us to recalculate.
-- Thanks,
David / dhildenb
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