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SubjectRe: [PATCH -mm v2 04/11] pagewalk: move pmd_trans_huge_lock() from callbacks to common code
On 06/12/2014 11:48 PM, Naoya Horiguchi wrote:
> Now all of current users of page table walker are canonicalized, i.e.
> pmd_entry() handles only trans_pmd entry, and pte_entry() handles pte entry.
> So we can factorize common code more.
> This patch moves pmd_trans_huge_lock() in each pmd_entry() to pagewalk core.
>
> ChangeLog v2:
> - add null check walk->vma in walk_pmd_range()

An older version of this patch already made it to linux-next (commit
b0e08c5) and I've actually hit the NULL pointer dereference.

Moreover, that patch (or maybe another recent pagewalk patch) breaks
/proc/<pid>/smaps. All fields that should have been filled by
smaps_pte() are almost always zero (and when it isn't, it's always a
multiple of 2MB). It seems to me that the page walk never goes below
pmd level.

Jerome

> - move comment update into a separate patch
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> ---
> arch/powerpc/mm/subpage-prot.c | 2 ++
> fs/proc/task_mmu.c | 66 ++++++++++++++----------------------------
> mm/memcontrol.c | 55 ++++++++++-------------------------
> mm/pagewalk.c | 18 ++++++++++--
> 4 files changed, 55 insertions(+), 86 deletions(-)
>
> diff --git mmotm-2014-05-21-16-57.orig/arch/powerpc/mm/subpage-prot.c mmotm-2014-05-21-16-57/arch/powerpc/mm/subpage-prot.c
> index fa9fb5b4c66c..d0d94ac606f3 100644
> --- mmotm-2014-05-21-16-57.orig/arch/powerpc/mm/subpage-prot.c
> +++ mmotm-2014-05-21-16-57/arch/powerpc/mm/subpage-prot.c
> @@ -135,7 +135,9 @@ static int subpage_walk_pmd_entry(pmd_t *pmd, unsigned long addr,
> unsigned long end, struct mm_walk *walk)
> {
> struct vm_area_struct *vma = walk->vma;
> + spin_unlock(walk->ptl);
> split_huge_page_pmd(vma, addr, pmd);
> + spin_lock(walk->ptl);
> return 0;
> }
>
> diff --git mmotm-2014-05-21-16-57.orig/fs/proc/task_mmu.c mmotm-2014-05-21-16-57/fs/proc/task_mmu.c
> index fa6d6a4e85b3..059206ea3c6b 100644
> --- mmotm-2014-05-21-16-57.orig/fs/proc/task_mmu.c
> +++ mmotm-2014-05-21-16-57/fs/proc/task_mmu.c
> @@ -496,15 +496,8 @@ static int smaps_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
> struct mm_walk *walk)
> {
> struct mem_size_stats *mss = walk->private;
> - spinlock_t *ptl;
> -
> - if (pmd_trans_huge_lock(pmd, walk->vma, &ptl) == 1) {
> - smaps_pte((pte_t *)pmd, addr, addr + HPAGE_PMD_SIZE, walk);
> - spin_unlock(ptl);
> - mss->anonymous_thp += HPAGE_PMD_SIZE;
> - /* don't call smaps_pte() */
> - walk->skip = 1;
> - }
> + smaps_pte((pte_t *)pmd, addr, addr + HPAGE_PMD_SIZE, walk);
> + mss->anonymous_thp += HPAGE_PMD_SIZE;
> return 0;
> }
>
> @@ -983,31 +976,21 @@ static int pagemap_pmd(pmd_t *pmd, unsigned long addr, unsigned long end,
> struct vm_area_struct *vma = walk->vma;
> struct pagemapread *pm = walk->private;
> pagemap_entry_t pme = make_pme(PM_NOT_PRESENT(pm->v2));
> - spinlock_t *ptl;
> -
> - if (!vma)
> - return err;
> - if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
> - int pmd_flags2;
> + int pmd_flags2;
>
> - if ((vma->vm_flags & VM_SOFTDIRTY) || pmd_soft_dirty(*pmd))
> - pmd_flags2 = __PM_SOFT_DIRTY;
> - else
> - pmd_flags2 = 0;
> + if ((vma->vm_flags & VM_SOFTDIRTY) || pmd_soft_dirty(*pmd))
> + pmd_flags2 = __PM_SOFT_DIRTY;
> + else
> + pmd_flags2 = 0;
>
> - for (; addr != end; addr += PAGE_SIZE) {
> - unsigned long offset;
> + for (; addr != end; addr += PAGE_SIZE) {
> + unsigned long offset;
>
> - offset = (addr & ~PAGEMAP_WALK_MASK) >>
> - PAGE_SHIFT;
> - thp_pmd_to_pagemap_entry(&pme, pm, *pmd, offset, pmd_flags2);
> - err = add_to_pagemap(addr, &pme, pm);
> - if (err)
> - break;
> - }
> - spin_unlock(ptl);
> - /* don't call pagemap_pte() */
> - walk->skip = 1;
> + offset = (addr & ~PAGEMAP_WALK_MASK) >> PAGE_SHIFT;
> + thp_pmd_to_pagemap_entry(&pme, pm, *pmd, offset, pmd_flags2);
> + err = add_to_pagemap(addr, &pme, pm);
> + if (err)
> + break;
> }
> return err;
> }
> @@ -1277,20 +1260,13 @@ static int gather_pmd_stats(pmd_t *pmd, unsigned long addr,
> {
> struct numa_maps *md = walk->private;
> struct vm_area_struct *vma = walk->vma;
> - spinlock_t *ptl;
> -
> - if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
> - pte_t huge_pte = *(pte_t *)pmd;
> - struct page *page;
> -
> - page = can_gather_numa_stats(huge_pte, vma, addr);
> - if (page)
> - gather_stats(page, md, pte_dirty(huge_pte),
> - HPAGE_PMD_SIZE/PAGE_SIZE);
> - spin_unlock(ptl);
> - /* don't call gather_pte_stats() */
> - walk->skip = 1;
> - }
> + pte_t huge_pte = *(pte_t *)pmd;
> + struct page *page;
> +
> + page = can_gather_numa_stats(huge_pte, vma, addr);
> + if (page)
> + gather_stats(page, md, pte_dirty(huge_pte),
> + HPAGE_PMD_SIZE/PAGE_SIZE);
> return 0;
> }
> #ifdef CONFIG_HUGETLB_PAGE
> diff --git mmotm-2014-05-21-16-57.orig/mm/memcontrol.c mmotm-2014-05-21-16-57/mm/memcontrol.c
> index 01a66a208769..bb987cb9e043 100644
> --- mmotm-2014-05-21-16-57.orig/mm/memcontrol.c
> +++ mmotm-2014-05-21-16-57/mm/memcontrol.c
> @@ -6723,15 +6723,9 @@ static int mem_cgroup_count_precharge_pmd(pmd_t *pmd,
> struct mm_walk *walk)
> {
> struct vm_area_struct *vma = walk->vma;
> - spinlock_t *ptl;
> -
> - if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
> - if (get_mctgt_type_thp(vma, addr, *pmd, NULL) == MC_TARGET_PAGE)
> - mc.precharge += HPAGE_PMD_NR;
> - spin_unlock(ptl);
> - /* don't call mem_cgroup_count_precharge_pte() */
> - walk->skip = 1;
> - }
> +
> + if (get_mctgt_type_thp(vma, addr, *pmd, NULL) == MC_TARGET_PAGE)
> + mc.precharge += HPAGE_PMD_NR;
> return 0;
> }
>
> @@ -6952,38 +6946,21 @@ static int mem_cgroup_move_charge_pmd(pmd_t *pmd,
> struct page *page;
> struct page_cgroup *pc;
>
> - /*
> - * We don't take compound_lock() here but no race with splitting thp
> - * happens because:
> - * - if pmd_trans_huge_lock() returns 1, the relevant thp is not
> - * under splitting, which means there's no concurrent thp split,
> - * - if another thread runs into split_huge_page() just after we
> - * entered this if-block, the thread must wait for page table lock
> - * to be unlocked in __split_huge_page_splitting(), where the main
> - * part of thp split is not executed yet.
> - */
> - if (pmd_trans_huge_lock(pmd, vma, &ptl) == 1) {
> - if (mc.precharge < HPAGE_PMD_NR) {
> - spin_unlock(ptl);
> - return 0;
> - }
> - target_type = get_mctgt_type_thp(vma, addr, *pmd, &target);
> - if (target_type == MC_TARGET_PAGE) {
> - page = target.page;
> - if (!isolate_lru_page(page)) {
> - pc = lookup_page_cgroup(page);
> - if (!mem_cgroup_move_account(page, HPAGE_PMD_NR,
> - pc, mc.from, mc.to)) {
> - mc.precharge -= HPAGE_PMD_NR;
> - mc.moved_charge += HPAGE_PMD_NR;
> - }
> - putback_lru_page(page);
> + if (mc.precharge < HPAGE_PMD_NR)
> + return 0;
> + target_type = get_mctgt_type_thp(vma, addr, *pmd, &target);
> + if (target_type == MC_TARGET_PAGE) {
> + page = target.page;
> + if (!isolate_lru_page(page)) {
> + pc = lookup_page_cgroup(page);
> + if (!mem_cgroup_move_account(page, HPAGE_PMD_NR,
> + pc, mc.from, mc.to)) {
> + mc.precharge -= HPAGE_PMD_NR;
> + mc.moved_charge += HPAGE_PMD_NR;
> }
> - put_page(page);
> + putback_lru_page(page);
> }
> - spin_unlock(ptl);
> - /* don't call mem_cgroup_move_charge_pte() */
> - walk->skip = 1;
> + put_page(page);
> }
> return 0;
> }
> diff --git mmotm-2014-05-21-16-57.orig/mm/pagewalk.c mmotm-2014-05-21-16-57/mm/pagewalk.c
> index 24311d6f5c20..f1a3417d0b51 100644
> --- mmotm-2014-05-21-16-57.orig/mm/pagewalk.c
> +++ mmotm-2014-05-21-16-57/mm/pagewalk.c
> @@ -73,8 +73,22 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr,
> continue;
> }
>
> - if (walk->pmd_entry) {
> - err = walk->pmd_entry(pmd, addr, next, walk);
> + /*
> + * We don't take compound_lock() here but no race with splitting
> + * thp happens because:
> + * - if pmd_trans_huge_lock() returns 1, the relevant thp is
> + * not under splitting, which means there's no concurrent
> + * thp split,
> + * - if another thread runs into split_huge_page() just after
> + * we entered this if-block, the thread must wait for page
> + * table lock to be unlocked in __split_huge_page_splitting(),
> + * where the main part of thp split is not executed yet.
> + */
> + if (walk->pmd_entry && walk->vma) {
> + if (pmd_trans_huge_lock(pmd, walk->vma, &walk->ptl) == 1) {
> + err = walk->pmd_entry(pmd, addr, next, walk);
> + spin_unlock(walk->ptl);
> + }
> if (skip_lower_level_walking(walk))
> continue;
> if (err)
>



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