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SubjectRe: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask
From
Hi Mike,

On 8/30/2022 7:40 AM, Mike Kravetz wrote:
> During discussions of this series [1], it was suggested that hugetlb
> handling code in follow_page_mask could be simplified. At the beginning
> of follow_page_mask, there currently is a call to follow_huge_addr which
> 'may' handle hugetlb pages. ia64 is the only architecture which provides
> a follow_huge_addr routine that does not return error. Instead, at each
> level of the page table a check is made for a hugetlb entry. If a hugetlb
> entry is found, a call to a routine associated with that entry is made.
>
> Currently, there are two checks for hugetlb entries at each page table
> level. The first check is of the form:
> if (p?d_huge())
> page = follow_huge_p?d();
> the second check is of the form:
> if (is_hugepd())
> page = follow_huge_pd().
>
> We can replace these checks, as well as the special handling routines
> such as follow_huge_p?d() and follow_huge_pd() with a single routine to
> handle hugetlb vmas.
>
> A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
> beginning of follow_page_mask. hugetlb_follow_page_mask will use the
> existing routine huge_pte_offset to walk page tables looking for hugetlb
> entries. huge_pte_offset can be overwritten by architectures, and already
> handles special cases such as hugepd entries.

Could you also mention that this patch will fix the lock issue for
CONT-PTE/PMD hugetlb by changing to use huge_pte_lock()? which will help
people to understand the issue.

Otherwise the changes look good to me.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

>
> [1] https://lore.kernel.org/linux-mm/cover.1661240170.git.baolin.wang@linux.alibaba.com/
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> arch/ia64/mm/hugetlbpage.c | 15 ---
> arch/powerpc/mm/hugetlbpage.c | 37 --------
> include/linux/hugetlb.h | 51 ++--------
> mm/gup.c | 65 ++-----------
> mm/hugetlb.c | 173 +++++++++++-----------------------
> 5 files changed, 74 insertions(+), 267 deletions(-)
>
> diff --git a/arch/ia64/mm/hugetlbpage.c b/arch/ia64/mm/hugetlbpage.c
> index f993cb36c062..380d2f3966c9 100644
> --- a/arch/ia64/mm/hugetlbpage.c
> +++ b/arch/ia64/mm/hugetlbpage.c
> @@ -91,21 +91,6 @@ int prepare_hugepage_range(struct file *file,
> return 0;
> }
>
> -struct page *follow_huge_addr(struct mm_struct *mm, unsigned long addr, int write)
> -{
> - struct page *page;
> - pte_t *ptep;
> -
> - if (REGION_NUMBER(addr) != RGN_HPAGE)
> - return ERR_PTR(-EINVAL);
> -
> - ptep = huge_pte_offset(mm, addr, HPAGE_SIZE);
> - if (!ptep || pte_none(*ptep))
> - return NULL;
> - page = pte_page(*ptep);
> - page += ((addr & ~HPAGE_MASK) >> PAGE_SHIFT);
> - return page;
> -}
> int pmd_huge(pmd_t pmd)
> {
> return 0;
> diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
> index bc84a594ca62..b0e037c75c12 100644
> --- a/arch/powerpc/mm/hugetlbpage.c
> +++ b/arch/powerpc/mm/hugetlbpage.c
> @@ -506,43 +506,6 @@ void hugetlb_free_pgd_range(struct mmu_gather *tlb,
> } while (addr = next, addr != end);
> }
>
> -struct page *follow_huge_pd(struct vm_area_struct *vma,
> - unsigned long address, hugepd_t hpd,
> - int flags, int pdshift)
> -{
> - pte_t *ptep;
> - spinlock_t *ptl;
> - struct page *page = NULL;
> - unsigned long mask;
> - int shift = hugepd_shift(hpd);
> - struct mm_struct *mm = vma->vm_mm;
> -
> -retry:
> - /*
> - * hugepage directory entries are protected by mm->page_table_lock
> - * Use this instead of huge_pte_lockptr
> - */
> - ptl = &mm->page_table_lock;
> - spin_lock(ptl);
> -
> - ptep = hugepte_offset(hpd, address, pdshift);
> - if (pte_present(*ptep)) {
> - mask = (1UL << shift) - 1;
> - page = pte_page(*ptep);
> - page += ((address & mask) >> PAGE_SHIFT);
> - if (flags & FOLL_GET)
> - get_page(page);
> - } else {
> - if (is_hugetlb_entry_migration(*ptep)) {
> - spin_unlock(ptl);
> - __migration_entry_wait(mm, ptep, ptl);
> - goto retry;
> - }
> - }
> - spin_unlock(ptl);
> - return page;
> -}
> -
> bool __init arch_hugetlb_valid_size(unsigned long size)
> {
> int shift = __ffs(size);
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 852f911d676e..8ea3e5e726e4 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -142,6 +142,8 @@ int move_hugetlb_page_tables(struct vm_area_struct *vma,
> unsigned long len);
> int copy_hugetlb_page_range(struct mm_struct *, struct mm_struct *,
> struct vm_area_struct *, struct vm_area_struct *);
> +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags);
> long follow_hugetlb_page(struct mm_struct *, struct vm_area_struct *,
> struct page **, struct vm_area_struct **,
> unsigned long *, unsigned long *, long, unsigned int,
> @@ -202,17 +204,6 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long addr, pte_t *ptep);
> void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> unsigned long *start, unsigned long *end);
> -struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
> - int write);
> -struct page *follow_huge_pd(struct vm_area_struct *vma,
> - unsigned long address, hugepd_t hpd,
> - int flags, int pdshift);
> -struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> - pmd_t *pmd, int flags);
> -struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
> - pud_t *pud, int flags);
> -struct page *follow_huge_pgd(struct mm_struct *mm, unsigned long address,
> - pgd_t *pgd, int flags);
>
> void hugetlb_vma_lock_read(struct vm_area_struct *vma);
> void hugetlb_vma_unlock_read(struct vm_area_struct *vma);
> @@ -264,6 +255,13 @@ static inline void adjust_range_if_pmd_sharing_possible(
> {
> }
>
> +static struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags)
> +{
> + /* should never happen, but do not want to BUG */
> + return ERR_PTR(-EINVAL);
> +}
> +
> static inline long follow_hugetlb_page(struct mm_struct *mm,
> struct vm_area_struct *vma, struct page **pages,
> struct vm_area_struct **vmas, unsigned long *position,
> @@ -274,12 +272,6 @@ static inline long follow_hugetlb_page(struct mm_struct *mm,
> return 0;
> }
>
> -static inline struct page *follow_huge_addr(struct mm_struct *mm,
> - unsigned long address, int write)
> -{
> - return ERR_PTR(-EINVAL);
> -}
> -
> static inline int copy_hugetlb_page_range(struct mm_struct *dst,
> struct mm_struct *src,
> struct vm_area_struct *dst_vma,
> @@ -312,31 +304,6 @@ static inline void hugetlb_show_meminfo_node(int nid)
> {
> }
>
> -static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
> - unsigned long address, hugepd_t hpd, int flags,
> - int pdshift)
> -{
> - return NULL;
> -}
> -
> -static inline struct page *follow_huge_pmd(struct mm_struct *mm,
> - unsigned long address, pmd_t *pmd, int flags)
> -{
> - return NULL;
> -}
> -
> -static inline struct page *follow_huge_pud(struct mm_struct *mm,
> - unsigned long address, pud_t *pud, int flags)
> -{
> - return NULL;
> -}
> -
> -static inline struct page *follow_huge_pgd(struct mm_struct *mm,
> - unsigned long address, pgd_t *pgd, int flags)
> -{
> - return NULL;
> -}
> -
> static inline int prepare_hugepage_range(struct file *file,
> unsigned long addr, unsigned long len)
> {
> diff --git a/mm/gup.c b/mm/gup.c
> index 66d8619e02ad..80ce04a5bae5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -661,20 +661,6 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> pmdval = READ_ONCE(*pmd);
> if (pmd_none(pmdval))
> return no_page_table(vma, flags);
> - if (pmd_huge(pmdval) && is_vm_hugetlb_page(vma)) {
> - page = follow_huge_pmd(mm, address, pmd, flags);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> - if (is_hugepd(__hugepd(pmd_val(pmdval)))) {
> - page = follow_huge_pd(vma, address,
> - __hugepd(pmd_val(pmdval)), flags,
> - PMD_SHIFT);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> retry:
> if (!pmd_present(pmdval)) {
> /*
> @@ -764,20 +750,6 @@ static struct page *follow_pud_mask(struct vm_area_struct *vma,
> pud = pud_offset(p4dp, address);
> if (pud_none(*pud))
> return no_page_table(vma, flags);
> - if (pud_huge(*pud) && is_vm_hugetlb_page(vma)) {
> - page = follow_huge_pud(mm, address, pud, flags);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> - if (is_hugepd(__hugepd(pud_val(*pud)))) {
> - page = follow_huge_pd(vma, address,
> - __hugepd(pud_val(*pud)), flags,
> - PUD_SHIFT);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> if (pud_devmap(*pud)) {
> ptl = pud_lock(mm, pud);
> page = follow_devmap_pud(vma, address, pud, flags, &ctx->pgmap);
> @@ -797,7 +769,6 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
> struct follow_page_context *ctx)
> {
> p4d_t *p4d;
> - struct page *page;
>
> p4d = p4d_offset(pgdp, address);
> if (p4d_none(*p4d))
> @@ -806,14 +777,6 @@ static struct page *follow_p4d_mask(struct vm_area_struct *vma,
> if (unlikely(p4d_bad(*p4d)))
> return no_page_table(vma, flags);
>
> - if (is_hugepd(__hugepd(p4d_val(*p4d)))) {
> - page = follow_huge_pd(vma, address,
> - __hugepd(p4d_val(*p4d)), flags,
> - P4D_SHIFT);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> return follow_pud_mask(vma, address, p4d, flags, ctx);
> }
>
> @@ -851,10 +814,15 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
>
> ctx->page_mask = 0;
>
> - /* make this handle hugepd */
> - page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> - if (!IS_ERR(page)) {
> - WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
> + /*
> + * Call hugetlb_follow_page_mask for hugetlb vmas as it will use
> + * special hugetlb page table walking code. This eliminates the
> + * need to check for hugetlb entries in the general walking code.
> + */
> + if (is_vm_hugetlb_page(vma)) {
> + page = hugetlb_follow_page_mask(vma, address, flags);
> + if (!page)
> + page = no_page_table(vma, flags);
> return page;
> }
>
> @@ -863,21 +831,6 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
> if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> return no_page_table(vma, flags);
>
> - if (pgd_huge(*pgd)) {
> - page = follow_huge_pgd(mm, address, pgd, flags);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> - if (is_hugepd(__hugepd(pgd_val(*pgd)))) {
> - page = follow_huge_pd(vma, address,
> - __hugepd(pgd_val(*pgd)), flags,
> - PGDIR_SHIFT);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> -
> return follow_p4d_mask(vma, address, pgd, flags, ctx);
> }
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d0617d64d718..b3da421ba5be 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6190,6 +6190,62 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
> return false;
> }
>
> +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags)
> +{
> + struct hstate *h = hstate_vma(vma);
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long haddr = address & huge_page_mask(h);
> + struct page *page = NULL;
> + spinlock_t *ptl;
> + pte_t *pte, entry;
> +
> + /*
> + * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> + * follow_hugetlb_page().
> + */
> + if (WARN_ON_ONCE(flags & FOLL_PIN))
> + return NULL;
> +
> + pte = huge_pte_offset(mm, haddr, huge_page_size(h));
> + if (!pte)
> + return NULL;
> +
> +retry:
> + ptl = huge_pte_lock(h, mm, pte);
> + entry = huge_ptep_get(pte);
> + if (pte_present(entry)) {
> + page = pte_page(entry) +
> + ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> + /*
> + * Note that page may be a sub-page, and with vmemmap
> + * optimizations the page struct may be read only.
> + * try_grab_page() will increase the ref count on the
> + * head page, so this will be OK.
> + *
> + * try_grab_page() should always succeed here, because we hold
> + * the ptl lock and have verified pte_present().
> + */
> + if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> + page = NULL;
> + goto out;
> + }
> + } else {
> + if (is_hugetlb_entry_migration(entry)) {
> + spin_unlock(ptl);
> + __migration_entry_wait_huge(pte, ptl);
> + goto retry;
> + }
> + /*
> + * hwpoisoned entry is treated as no_page_table in
> + * follow_page_mask().
> + */
> + }
> +out:
> + spin_unlock(ptl);
> + return page;
> +}
> +
> long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> struct page **pages, struct vm_area_struct **vmas,
> unsigned long *position, unsigned long *nr_pages,
> @@ -7140,123 +7196,6 @@ __weak unsigned long hugetlb_mask_last_page(struct hstate *h)
> * These functions are overwritable if your architecture needs its own
> * behavior.
> */
> -struct page * __weak
> -follow_huge_addr(struct mm_struct *mm, unsigned long address,
> - int write)
> -{
> - return ERR_PTR(-EINVAL);
> -}
> -
> -struct page * __weak
> -follow_huge_pd(struct vm_area_struct *vma,
> - unsigned long address, hugepd_t hpd, int flags, int pdshift)
> -{
> - WARN(1, "hugepd follow called with no support for hugepage directory format\n");
> - return NULL;
> -}
> -
> -struct page * __weak
> -follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> - pmd_t *pmd, int flags)
> -{
> - struct page *page = NULL;
> - spinlock_t *ptl;
> - pte_t pte;
> -
> - /*
> - * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> - * follow_hugetlb_page().
> - */
> - if (WARN_ON_ONCE(flags & FOLL_PIN))
> - return NULL;
> -
> -retry:
> - ptl = pmd_lockptr(mm, pmd);
> - spin_lock(ptl);
> - /*
> - * make sure that the address range covered by this pmd is not
> - * unmapped from other threads.
> - */
> - if (!pmd_huge(*pmd))
> - goto out;
> - pte = huge_ptep_get((pte_t *)pmd);
> - if (pte_present(pte)) {
> - page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
> - /*
> - * try_grab_page() should always succeed here, because: a) we
> - * hold the pmd (ptl) lock, and b) we've just checked that the
> - * huge pmd (head) page is present in the page tables. The ptl
> - * prevents the head page and tail pages from being rearranged
> - * in any way. So this page must be available at this point,
> - * unless the page refcount overflowed:
> - */
> - if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> - page = NULL;
> - goto out;
> - }
> - } else {
> - if (is_hugetlb_entry_migration(pte)) {
> - spin_unlock(ptl);
> - __migration_entry_wait_huge((pte_t *)pmd, ptl);
> - goto retry;
> - }
> - /*
> - * hwpoisoned entry is treated as no_page_table in
> - * follow_page_mask().
> - */
> - }
> -out:
> - spin_unlock(ptl);
> - return page;
> -}
> -
> -struct page * __weak
> -follow_huge_pud(struct mm_struct *mm, unsigned long address,
> - pud_t *pud, int flags)
> -{
> - struct page *page = NULL;
> - spinlock_t *ptl;
> - pte_t pte;
> -
> - if (WARN_ON_ONCE(flags & FOLL_PIN))
> - return NULL;
> -
> -retry:
> - ptl = huge_pte_lock(hstate_sizelog(PUD_SHIFT), mm, (pte_t *)pud);
> - if (!pud_huge(*pud))
> - goto out;
> - pte = huge_ptep_get((pte_t *)pud);
> - if (pte_present(pte)) {
> - page = pud_page(*pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
> - if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> - page = NULL;
> - goto out;
> - }
> - } else {
> - if (is_hugetlb_entry_migration(pte)) {
> - spin_unlock(ptl);
> - __migration_entry_wait(mm, (pte_t *)pud, ptl);
> - goto retry;
> - }
> - /*
> - * hwpoisoned entry is treated as no_page_table in
> - * follow_page_mask().
> - */
> - }
> -out:
> - spin_unlock(ptl);
> - return page;
> -}
> -
> -struct page * __weak
> -follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int flags)
> -{
> - if (flags & (FOLL_GET | FOLL_PIN))
> - return NULL;
> -
> - return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);
> -}
> -
> int isolate_hugetlb(struct page *page, struct list_head *list)
> {
> int ret = 0;

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