| Message ID | 20240228034151.459370-1-jhubbard@nvidia.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/memory.c: do_numa_page(): remove a redundant page table read | expand |
On 28.02.24 04:41, John Hubbard wrote: > do_numa_page() is reading from the same page table entry, twice, while > holding the page table lock: once while checking that the pte hasn't > changed, and again in order to modify the pte. > > Instead, just read the pte once, and save it in the same old_pte > variable that already exists. This has no effect on behavior, other than > to provide a tiny potential improvement to performance, by avoiding the > redundant memory read (which the compiler cannot elide, due to > READ_ONCE()). > > Also improve the associated comments nearby. > > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: David Hildenbrand <david@redhat.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> > --- > mm/memory.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 0bfc8b007c01..df0711982901 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4928,18 +4928,18 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > int flags = 0; > > /* > - * The "pte" at this point cannot be used safely without > - * validation through pte_unmap_same(). It's of NUMA type but > - * the pfn may be screwed if the read is non atomic. > + * The pte cannot be used safely until we verify, while holding the page > + * table lock, that its contents have not changed during fault handling. > */ > spin_lock(vmf->ptl); > - if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) { > + /* Read the live PTE from the page tables: */ > + old_pte = ptep_get(vmf->pte); > + > + if (unlikely(!pte_same(old_pte, vmf->orig_pte))) { > pte_unmap_unlock(vmf->pte, vmf->ptl); > goto out; > } > > - /* Get the normal PTE */ > - old_pte = ptep_get(vmf->pte); > pte = pte_modify(old_pte, vma->vm_page_prot); > > /* Reviewed-by: David Hildenbrand <david@redhat.com>
On 28/02/2024 03:41, John Hubbard wrote: > do_numa_page() is reading from the same page table entry, twice, while > holding the page table lock: once while checking that the pte hasn't > changed, and again in order to modify the pte. > > Instead, just read the pte once, and save it in the same old_pte > variable that already exists. This has no effect on behavior, other than > to provide a tiny potential improvement to performance, by avoiding the > redundant memory read (which the compiler cannot elide, due to > READ_ONCE()). > > Also improve the associated comments nearby. > > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: David Hildenbrand <david@redhat.com> > Signed-off-by: John Hubbard <jhubbard@nvidia.com> Reviewed-by: Ryan Roberts <ryan.roberts@arm.com> > --- > mm/memory.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index 0bfc8b007c01..df0711982901 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4928,18 +4928,18 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) > int flags = 0; > > /* > - * The "pte" at this point cannot be used safely without > - * validation through pte_unmap_same(). It's of NUMA type but > - * the pfn may be screwed if the read is non atomic. > + * The pte cannot be used safely until we verify, while holding the page > + * table lock, that its contents have not changed during fault handling. > */ > spin_lock(vmf->ptl); > - if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) { > + /* Read the live PTE from the page tables: */ > + old_pte = ptep_get(vmf->pte); > + > + if (unlikely(!pte_same(old_pte, vmf->orig_pte))) { > pte_unmap_unlock(vmf->pte, vmf->ptl); > goto out; > } > > - /* Get the normal PTE */ > - old_pte = ptep_get(vmf->pte); > pte = pte_modify(old_pte, vma->vm_page_prot); > > /*
diff --git a/mm/memory.c b/mm/memory.c index 0bfc8b007c01..df0711982901 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4928,18 +4928,18 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) int flags = 0; /* - * The "pte" at this point cannot be used safely without - * validation through pte_unmap_same(). It's of NUMA type but - * the pfn may be screwed if the read is non atomic. + * The pte cannot be used safely until we verify, while holding the page + * table lock, that its contents have not changed during fault handling. */ spin_lock(vmf->ptl); - if (unlikely(!pte_same(ptep_get(vmf->pte), vmf->orig_pte))) { + /* Read the live PTE from the page tables: */ + old_pte = ptep_get(vmf->pte); + + if (unlikely(!pte_same(old_pte, vmf->orig_pte))) { pte_unmap_unlock(vmf->pte, vmf->ptl); goto out; } - /* Get the normal PTE */ - old_pte = ptep_get(vmf->pte); pte = pte_modify(old_pte, vma->vm_page_prot); /*
do_numa_page() is reading from the same page table entry, twice, while holding the page table lock: once while checking that the pte hasn't changed, and again in order to modify the pte. Instead, just read the pte once, and save it in the same old_pte variable that already exists. This has no effect on behavior, other than to provide a tiny potential improvement to performance, by avoiding the redundant memory read (which the compiler cannot elide, due to READ_ONCE()). Also improve the associated comments nearby. Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: David Hildenbrand <david@redhat.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- mm/memory.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-)