[PATCH v2] hugetlb: simplify hugetlb handling in follow_page_mask

Mike Kravetz
2022-09-02T19:03:57Z

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.

[1] https://lore.kernel.org/linux-mm/cover.1661240170.git.baolin.wang@linux.alibaba.com/

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
v2 - Added WARN_ON_ONCE() and updated comment as suggested by David
Fixed build issue found by kernel test robot
Added vma (pmd sharing) locking to hugetlb_follow_page_mask
ReBased on Baolin's patch to fix issues with CONT_* entries

arch/ia64/mm/hugetlbpage.c | 15 ---
arch/powerpc/mm/hugetlbpage.c | 37 -------
include/linux/hugetlb.h | 51 ++--------
mm/gup.c | 80 +++------------
mm/hugetlb.c | 182 ++++++++++++----------------------
5 files changed, 87 insertions(+), 278 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 fe4944f89d34..275e554dd365 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_pte(struct vm_area_struct *vma, unsigned long address,
- 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 inline struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
+ unsigned long address, unsigned int flags)
+{
+ WARN_ON_ONCE(1); /* should never be called if !CONFIG_HUGETLB_PAGE*/
+ 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_pte(struct vm_area_struct *vma,
- unsigned long address, 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 7691c65233c3..1515892a9d98 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -530,18 +530,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) =
(FOLL_PIN | FOLL_GET)))
return ERR_PTR(-EINVAL);
-
- /*
- * Considering PTE level hugetlb, like continuous-PTE hugetlb on
- * ARM64 architecture.
- */
- if (is_vm_hugetlb_page(vma)) {
- page = follow_huge_pmd_pte(vma, address, flags);
- if (page)
- return page;
- return no_page_table(vma, flags);
- }
-
retry:
if (unlikely(pmd_bad(*pmd)))
return no_page_table(vma, flags);
@@ -673,20 +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_pte(vma, address, 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)) {
/*
@@ -776,20 +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);
@@ -809,7 +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))
@@ -818,14 +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);
}

@@ -863,10 +814,18 @@ 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.
+ *
+ * hugetlb_follow_page_mask is only for follow_page() handling here.
+ * Ordinary GUP uses follow_hugetlb_page for hugetlb processing.
+ */
+ if (is_vm_hugetlb_page(vma)) {
+ page = hugetlb_follow_page_mask(vma, address, flags);
+ if (!page)
+ page = no_page_table(vma, flags);
return page;
}

@@ -875,21 +834,6 @@ 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 c613d3c0c06a..5b4123b0be83 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6190,6 +6190,72 @@ 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;
+
+retry:
+ /*
+ * vma lock prevents racing with another thread doing a pmd unshare.
+ * This keeps pte as returned by huge_pte_offset valid.
+ */
+ hugetlb_vma_lock_read(vma);
+
+ pte = huge_pte_offset(mm, haddr, huge_page_size(h));
+ if (!pte) {
+ hugetlb_vma_unlock_read(vma);
+ return NULL;
+ }
+
+ 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);
+ hugetlb_vma_unlock_read(vma);
+ __migration_entry_wait_huge(pte, ptl);
+ goto retry;
+ }
+ /*
+ * hwpoisoned entry is treated as no_page_table in
+ * follow_page_mask().
+ */
+ }
+out:
+ spin_unlock(ptl);
+ hugetlb_vma_unlock_read(vma);
+ 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,
@@ -7140,122 +7206,6 @@ __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_pte(struct vm_area_struct *vma, unsigned long address, int flags)
-{
- struct hstate *h = hstate_vma(vma);
- struct mm_struct *mm = vma->vm_mm;
- struct page *page = NULL;
- spinlock_t *ptl;
- pte_t *ptep, 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:
- ptep = huge_pte_offset(mm, address, huge_page_size(h));
- if (!ptep)
- return NULL;
-
- ptl = huge_pte_lock(h, mm, ptep);
- pte = huge_ptep_get(ptep);
- if (pte_present(pte)) {
- page = pte_page(pte) +
- ((address & ~huge_page_mask(h)) >> 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(ptep, 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;
--
2.37.1