On Tue, Feb 07, 2017 at 05:33:47PM +0300, Kirill A. Shutemov wrote:
> lock_pte_protection() uses pmd_lock() to make sure that we have stable
> PTE page table before walking pte range.
>
> That's not necessary. We only need to make sure that PTE page table is
> established. It cannot vanish under us as long as we hold mmap_sem at
> least for read.
>
> And we already have helper for that -- pmd_trans_unstable().
>
> Signed-off-by: Kirill A. Shutemov <[email protected]>
> ---
> mm/mprotect.c | 43 ++++++++++++-------------------------------
> 1 file changed, 12 insertions(+), 31 deletions(-)
>
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index f9c07f54dd62..e919e4613eab 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -33,34 +33,6 @@
>
> #include "internal.h"
>
> -/*
> - * For a prot_numa update we only hold mmap_sem for read so there is a
> - * potential race with faulting where a pmd was temporarily none. This
> - * function checks for a transhuge pmd under the appropriate lock. It
> - * returns a pte if it was successfully locked or NULL if it raced with
> - * a transhuge insertion.
> - */
> -static pte_t *lock_pte_protection(struct vm_area_struct *vma, pmd_t *pmd,
> - unsigned long addr, int prot_numa, spinlock_t **ptl)
> -{
> - pte_t *pte;
> - spinlock_t *pmdl;
> -
> - /* !prot_numa is protected by mmap_sem held for write */
> - if (!prot_numa)
> - return pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl);
> -
> - pmdl = pmd_lock(vma->vm_mm, pmd);
> - if (unlikely(pmd_trans_huge(*pmd) || pmd_none(*pmd))) {
> - spin_unlock(pmdl);
> - return NULL;
> - }
> -
> - pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, ptl);
> - spin_unlock(pmdl);
> - return pte;
> -}
> -
> static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> unsigned long addr, unsigned long end, pgprot_t newprot,
> int dirty_accountable, int prot_numa)
> @@ -71,7 +43,7 @@ static unsigned long change_pte_range(struct vm_area_struct
> *vma, pmd_t *pmd,
> unsigned long pages = 0;
> int target_node = NUMA_NO_NODE;
>
> - pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl);
> + pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> if (!pte)
I cleaned it up too but I moved the pmd_trans_unstable in the caller
above instead of the callee, otherwise it's the same.
>
> @@ -177,8 +149,6 @@ static inline unsigned long change_pmd_range(struct
> vm_area_struct *vma,
> if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
> if (next - addr != HPAGE_PMD_SIZE) {
> __split_huge_pmd(vma, pmd, addr, false, NULL);
> - if (pmd_trans_unstable(pmd))
> - continue;
Agree it can be removed too, but I only removed lock_pte_protection in
my version.
If you prefer this version to be merged so we don't have to cleanup
the above superfluous check in a incremental patch that's fine of
course, otherwise at runtime they're equivalent as far as I can
tell. The version in -mm is here.
https://git.kernel.org/cgit/linux/kernel/git/mhocko/mm.git/commit/?h=auto-latest&id=d84ff4e4985f397ca4ecfe7ec029c45c6f2b9906
Thanks,
Andrea
