Re: [PATCH v2 07/12] mm: thp: check pmd migration entry in common path

From: Kirill A. Shutemov
Date: Thu Nov 17 2016 - 18:56:35 EST

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On Tue, Nov 08, 2016 at 08:31:52AM +0900, Naoya Horiguchi wrote:
> If one of callers of page migration starts to handle thp, memory management code
> start to see pmd migration entry, so we need to prepare for it before enabling.
> This patch changes various code point which checks the status of given pmds in
> order to prevent race between thp migration and the pmd-related works.
>
> Signed-off-by: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> ---
> ChangeLog v1 -> v2:
> - introduce pmd_related() (I know the naming is not good, but can't think up
> no better name. Any suggesntion is welcomed.)
> ---
> arch/x86/mm/gup.c | 4 +--
> fs/proc/task_mmu.c | 23 +++++++------
> include/linux/huge_mm.h | 9 ++++-
> mm/gup.c | 10 ++++--
> mm/huge_memory.c | 88 ++++++++++++++++++++++++++++++++++++++++---------
> mm/madvise.c | 2 +-
> mm/memcontrol.c | 2 ++
> mm/memory.c | 6 +++-
> mm/mprotect.c | 2 ++
> mm/mremap.c | 2 +-
> 10 files changed, 114 insertions(+), 34 deletions(-)
>
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/arch/x86/mm/gup.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/arch/x86/mm/gup.c
> index 0d4fb3e..78a153d 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/arch/x86/mm/gup.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/arch/x86/mm/gup.c
> @@ -222,9 +222,9 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
> pmd_t pmd = *pmdp;
>
> next = pmd_addr_end(addr, end);
> - if (pmd_none(pmd))
> + if (!pmd_present(pmd))
> return 0;
> - if (unlikely(pmd_large(pmd) || !pmd_present(pmd))) {
> + if (unlikely(pmd_large(pmd))) {
> /*
> * NUMA hinting faults need to be handled in the GUP
> * slowpath for accounting purposes and so that they
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/fs/proc/task_mmu.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/fs/proc/task_mmu.c
> index 35b92d8..c1f9cf4 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/fs/proc/task_mmu.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/fs/proc/task_mmu.c
> @@ -596,7 +596,8 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
>
> ptl = pmd_trans_huge_lock(pmd, vma);
> if (ptl) {
> - smaps_pmd_entry(pmd, addr, walk);
> + if (pmd_present(*pmd))
> + smaps_pmd_entry(pmd, addr, walk);
> spin_unlock(ptl);
> return 0;
> }
> @@ -929,6 +930,9 @@ static int clear_refs_pte_range(pmd_t *pmd, unsigned long addr,
> goto out;
> }
>
> + if (!pmd_present(*pmd))
> + goto out;
> +

Hm. Looks like clear_soft_dirty_pmd() should handle !present. It doesn't.

Ah.. Found it in 08/12.

> page = pmd_page(*pmd);
>
> /* Clear accessed and referenced bits. */
> @@ -1208,19 +1212,18 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
> if (ptl) {
> u64 flags = 0, frame = 0;
> pmd_t pmd = *pmdp;
> + struct page *page;
>
> if ((vma->vm_flags & VM_SOFTDIRTY) || pmd_soft_dirty(pmd))
> flags |= PM_SOFT_DIRTY;
>
> - /*
> - * Currently pmd for thp is always present because thp
> - * can not be swapped-out, migrated, or HWPOISONed
> - * (split in such cases instead.)
> - * This if-check is just to prepare for future implementation.
> - */
> - if (pmd_present(pmd)) {
> - struct page *page = pmd_page(pmd);
> -
> + if (is_pmd_migration_entry(pmd)) {
> + swp_entry_t entry = pmd_to_swp_entry(pmd);
> + frame = swp_type(entry) |
> + (swp_offset(entry) << MAX_SWAPFILES_SHIFT);
> + page = migration_entry_to_page(entry);
> + } else if (pmd_present(pmd)) {
> + page = pmd_page(pmd);
> if (page_mapcount(page) == 1)
> flags |= PM_MMAP_EXCLUSIVE;
>
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/include/linux/huge_mm.h v4.9-rc2-mmotm-2016-10-27-18-27_patched/include/linux/huge_mm.h
> index fcbca51..3c252cd 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/include/linux/huge_mm.h
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/include/linux/huge_mm.h
> @@ -125,12 +125,19 @@ extern void vma_adjust_trans_huge(struct vm_area_struct *vma,
> long adjust_next);
> extern spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd,
> struct vm_area_struct *vma);
> +
> +static inline int pmd_related(pmd_t pmd)
> +{
> + return !pmd_none(pmd) &&
> + (!pmd_present(pmd) || pmd_trans_huge(pmd) || pmd_devmap(pmd));
> +}
> +

I would rather create is_swap_pmd() -- (!none && !present) and leave the
reset open-codded.

> /* mmap_sem must be held on entry */
> static inline spinlock_t *pmd_trans_huge_lock(pmd_t *pmd,
> struct vm_area_struct *vma)
> {
> VM_BUG_ON_VMA(!rwsem_is_locked(&vma->vm_mm->mmap_sem), vma);
> - if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd))
> + if (pmd_related(*pmd))
> return __pmd_trans_huge_lock(pmd, vma);
> else
> return NULL;
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/gup.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/gup.c
> index e50178c..2dc4978 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/gup.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/gup.c
> @@ -267,6 +267,8 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
> }
> if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
> return no_page_table(vma, flags);
> + if (!pmd_present(*pmd))
> + return no_page_table(vma, flags);

Don't we need FOLL_MIGRATION, like on pte side?

> if (pmd_devmap(*pmd)) {
> ptl = pmd_lock(mm, pmd);
> page = follow_devmap_pmd(vma, address, pmd, flags);
> @@ -278,6 +280,10 @@ struct page *follow_page_mask(struct vm_area_struct *vma,
> return follow_page_pte(vma, address, pmd, flags);
>
> ptl = pmd_lock(mm, pmd);
> + if (unlikely(!pmd_present(*pmd))) {
> + spin_unlock(ptl);
> + return no_page_table(vma, flags);
> + }

Ditto.

> if (unlikely(!pmd_trans_huge(*pmd))) {
> spin_unlock(ptl);
> return follow_page_pte(vma, address, pmd, flags);
> @@ -333,7 +339,7 @@ static int get_gate_page(struct mm_struct *mm, unsigned long address,
> pud = pud_offset(pgd, address);
> BUG_ON(pud_none(*pud));
> pmd = pmd_offset(pud, address);
> - if (pmd_none(*pmd))
> + if (!pmd_present(*pmd))
> return -EFAULT;
> VM_BUG_ON(pmd_trans_huge(*pmd));
> pte = pte_offset_map(pmd, address);
> @@ -1357,7 +1363,7 @@ static int gup_pmd_range(pud_t pud, unsigned long addr, unsigned long end,
> pmd_t pmd = READ_ONCE(*pmdp);
>
> next = pmd_addr_end(addr, end);
> - if (pmd_none(pmd))
> + if (!pmd_present(pmd))
> return 0;
>
> if (unlikely(pmd_trans_huge(pmd) || pmd_huge(pmd))) {
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/huge_memory.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/huge_memory.c
> index b3022b3..4e9090c 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/huge_memory.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/huge_memory.c
> @@ -825,6 +825,20 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>
> ret = -EAGAIN;
> pmd = *src_pmd;
> +
> + if (unlikely(is_pmd_migration_entry(pmd))) {
> + swp_entry_t entry = pmd_to_swp_entry(pmd);
> +
> + if (is_write_migration_entry(entry)) {
> + make_migration_entry_read(&entry);
> + pmd = swp_entry_to_pmd(entry);
> + set_pmd_at(src_mm, addr, src_pmd, pmd);
> + }

I think we should put at least WARN_ONCE() in 'else' here. We don't want
to miss such places when swap will be supported (or other swap entry type).

> + set_pmd_at(dst_mm, addr, dst_pmd, pmd);
> + ret = 0;
> + goto out_unlock;
> + }
> +
> if (unlikely(!pmd_trans_huge(pmd))) {
> pte_free(dst_mm, pgtable);
> goto out_unlock;
> @@ -1013,6 +1027,9 @@ int do_huge_pmd_wp_page(struct fault_env *fe, pmd_t orig_pmd)
> if (unlikely(!pmd_same(*fe->pmd, orig_pmd)))
> goto out_unlock;
>
> + if (unlikely(!pmd_present(orig_pmd)))
> + goto out_unlock;
> +
> page = pmd_page(orig_pmd);
> VM_BUG_ON_PAGE(!PageCompound(page) || !PageHead(page), page);
> /*
> @@ -1137,7 +1154,14 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
> if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
> goto out;
>
> - page = pmd_page(*pmd);
> + if (is_pmd_migration_entry(*pmd)) {
> + swp_entry_t entry;
> + entry = pmd_to_swp_entry(*pmd);
> + page = pfn_to_page(swp_offset(entry));
> + if (!is_migration_entry(entry))
> + goto out;

follow_page_pte() does different thing: wait for page to be migrated and
retry. Any reason you don't do the same?

> + } else
> + page = pmd_page(*pmd);
> VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
> if (flags & FOLL_TOUCH)
> touch_pmd(vma, addr, pmd);
> @@ -1332,6 +1356,9 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> if (is_huge_zero_pmd(orig_pmd))
> goto out;
>
> + if (unlikely(!pmd_present(orig_pmd)))
> + goto out;
> +
> page = pmd_page(orig_pmd);
> /*
> * If other processes are mapping this page, we couldn't discard
> @@ -1410,20 +1437,35 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> tlb_remove_page_size(tlb, pmd_page(orig_pmd), HPAGE_PMD_SIZE);
> } else {
> struct page *page = pmd_page(orig_pmd);
> - page_remove_rmap(page, true);
> - VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
> - VM_BUG_ON_PAGE(!PageHead(page), page);
> - if (PageAnon(page)) {
> - pgtable_t pgtable;
> - pgtable = pgtable_trans_huge_withdraw(tlb->mm, pmd);
> - pte_free(tlb->mm, pgtable);
> - atomic_long_dec(&tlb->mm->nr_ptes);
> - add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> + int migration = 0;
> +
> + if (!is_pmd_migration_entry(orig_pmd)) {
> + page_remove_rmap(page, true);
> + VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
> + VM_BUG_ON_PAGE(!PageHead(page), page);
> + if (PageAnon(page)) {
> + pgtable_t pgtable;
> + pgtable = pgtable_trans_huge_withdraw(tlb->mm,
> + pmd);
> + pte_free(tlb->mm, pgtable);
> + atomic_long_dec(&tlb->mm->nr_ptes);
> + add_mm_counter(tlb->mm, MM_ANONPAGES,
> + -HPAGE_PMD_NR);
> + } else {
> + add_mm_counter(tlb->mm, MM_FILEPAGES,
> + -HPAGE_PMD_NR);
> + }
> } else {
> - add_mm_counter(tlb->mm, MM_FILEPAGES, -HPAGE_PMD_NR);
> + swp_entry_t entry;
> +
> + entry = pmd_to_swp_entry(orig_pmd);
> + free_swap_and_cache(entry); /* waring in failure? */
> + add_mm_counter(tlb->mm, MM_ANONPAGES, -HPAGE_PMD_NR);
> + migration = 1;
> }
> spin_unlock(ptl);
> - tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
> + if (!migration)
> + tlb_remove_page_size(tlb, page, HPAGE_PMD_SIZE);
> }
> return 1;
> }
> @@ -1496,14 +1538,27 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> bool preserve_write = prot_numa && pmd_write(*pmd);
> ret = 1;
>
> + if (!pmd_present(*pmd))
> + goto unlock;
> /*
> * Avoid trapping faults against the zero page. The read-only
> * data is likely to be read-cached on the local CPU and
> * local/remote hits to the zero page are not interesting.
> */
> - if (prot_numa && is_huge_zero_pmd(*pmd)) {
> - spin_unlock(ptl);
> - return ret;
> + if (prot_numa && is_huge_zero_pmd(*pmd))
> + goto unlock;
> +
> + if (is_pmd_migration_entry(*pmd)) {

Hm? But we filtered out !present above?

> + swp_entry_t entry = pmd_to_swp_entry(*pmd);
> +
> + if (is_write_migration_entry(entry)) {
> + pmd_t newpmd;
> +
> + make_migration_entry_read(&entry);
> + newpmd = swp_entry_to_pmd(entry);
> + set_pmd_at(mm, addr, pmd, newpmd);
> + }
> + goto unlock;
> }
>
> if (!prot_numa || !pmd_protnone(*pmd)) {
> @@ -1516,6 +1571,7 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> BUG_ON(vma_is_anonymous(vma) && !preserve_write &&
> pmd_write(entry));
> }
> +unlock:
> spin_unlock(ptl);
> }
>
> @@ -1532,7 +1588,7 @@ spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma)
> {
> spinlock_t *ptl;
> ptl = pmd_lock(vma->vm_mm, pmd);
> - if (likely(pmd_trans_huge(*pmd) || pmd_devmap(*pmd)))
> + if (likely(pmd_related(*pmd)))
> return ptl;
> spin_unlock(ptl);
> return NULL;
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/madvise.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/madvise.c
> index 0e3828e..eaa2b02 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/madvise.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/madvise.c
> @@ -274,7 +274,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> unsigned long next;
>
> next = pmd_addr_end(addr, end);
> - if (pmd_trans_huge(*pmd))
> + if (pmd_related(*pmd))

I don't see a point going for madvise_free_huge_pmd(), just to fall off on
!present inside.

And is it safe for devmap?

> if (madvise_free_huge_pmd(tlb, vma, pmd, addr, next))
> goto next;
>
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/memcontrol.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/memcontrol.c
> index 91dfc7c..ebc2c42 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/memcontrol.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/memcontrol.c
> @@ -4635,6 +4635,8 @@ static enum mc_target_type get_mctgt_type_thp(struct vm_area_struct *vma,
> struct page *page = NULL;
> enum mc_target_type ret = MC_TARGET_NONE;
>
> + if (unlikely(!pmd_present(pmd)))
> + return ret;
> page = pmd_page(pmd);
> VM_BUG_ON_PAGE(!page || !PageHead(page), page);
> if (!(mc.flags & MOVE_ANON))
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/memory.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/memory.c
> index 94b5e2c..33fa439 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/memory.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/memory.c
> @@ -999,7 +999,7 @@ static inline int copy_pmd_range(struct mm_struct *dst_mm, struct mm_struct *src
> src_pmd = pmd_offset(src_pud, addr);
> do {
> next = pmd_addr_end(addr, end);
> - if (pmd_trans_huge(*src_pmd) || pmd_devmap(*src_pmd)) {
> + if (pmd_related(*src_pmd)) {
> int err;
> VM_BUG_ON(next-addr != HPAGE_PMD_SIZE);
> err = copy_huge_pmd(dst_mm, src_mm,
> @@ -3591,6 +3591,10 @@ static int __handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> int ret;
>
> barrier();
> + if (unlikely(is_pmd_migration_entry(orig_pmd))) {
> + pmd_migration_entry_wait(mm, fe.pmd);
> + return 0;
> + }
> if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) {
> if (pmd_protnone(orig_pmd) && vma_is_accessible(vma))
> return do_huge_pmd_numa_page(&fe, orig_pmd);
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/mprotect.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/mprotect.c
> index c5ba2aa..81186e3 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/mprotect.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/mprotect.c
> @@ -164,6 +164,8 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> unsigned long this_pages;
>
> next = pmd_addr_end(addr, end);
> + if (!pmd_present(*pmd))
> + continue;
> if (!pmd_trans_huge(*pmd) && !pmd_devmap(*pmd)
> && pmd_none_or_clear_bad(pmd))
> continue;
> diff --git v4.9-rc2-mmotm-2016-10-27-18-27/mm/mremap.c v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/mremap.c
> index da22ad2..a94a698 100644
> --- v4.9-rc2-mmotm-2016-10-27-18-27/mm/mremap.c
> +++ v4.9-rc2-mmotm-2016-10-27-18-27_patched/mm/mremap.c
> @@ -194,7 +194,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
> if (!new_pmd)
> break;
> - if (pmd_trans_huge(*old_pmd)) {
> + if (pmd_related(*old_pmd)) {
> if (extent == HPAGE_PMD_SIZE) {
> bool moved;
> /* See comment in move_ptes() */
> --
> 2.7.0
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
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> see: http://www.linux-mm.org/ .
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--
Kirill A. Shutemov

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