From: Andrea Arcangeli <aarcange@xxxxxxxxxx> Subject: mm: mprotect: use pmd_trans_unstable instead of taking the pmd_lock pmd_trans_unstable does an atomic read on the pmd so it doesn't require the pmd_lock for the same check. This also removes the special assumption that the mmap_sem is hold for writing if prot_numa is not set. userfaultfd will hold the mmap_sem only for reading in change_pte_range like prot_numa, but it will not set prot_numa. This is always a valid micro-optimization regardless of userfaultfd. [kirill@xxxxxxxxxxxxx: drop unneeded pmd_trans_unstable(pmd) check after __split_huge_pmd()] Link: http://lkml.kernel.org/r/20170208120421.GE5578@xxxxxxxxxxxxxxxxxx Link: http://lkml.kernel.org/r/20161216144821.5183-43-aarcange@xxxxxxxxxx Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> Cc: Hillf Danton <hillf.zj@xxxxxxxxxxxxxxx> Cc: Michael Rapoport <RAPOPORT@xxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx> Cc: "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/mprotect.c | 46 +++++++++++++++------------------------------- 1 file changed, 15 insertions(+), 31 deletions(-) diff -puN mm/mprotect.c~mm-mprotect-use-pmd_trans_unstable-instead-of-taking-the-pmd_lock mm/mprotect.c --- a/mm/mprotect.c~mm-mprotect-use-pmd_trans_unstable-instead-of-taking-the-pmd_lock +++ a/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 +43,21 @@ static unsigned long change_pte_range(st unsigned long pages = 0; int target_node = NUMA_NO_NODE; - pte = lock_pte_protection(vma, pmd, addr, prot_numa, &ptl); + /* + * Can be called with only the mmap_sem for reading by + * prot_numa so we must check the pmd isn't constantly + * changing from under us from pmd_none to pmd_trans_huge + * and/or the other way around. + */ + if (pmd_trans_unstable(pmd)) + return 0; + + /* + * The pmd points to a regular pte so the pmd can't change + * from under us even if the mmap_sem is only hold for + * reading. + */ + pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl); if (!pte) return 0; @@ -177,8 +163,6 @@ static inline unsigned long change_pmd_r 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; } else { int nr_ptes = change_huge_pmd(vma, pmd, addr, newprot, prot_numa); _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
