[PATCH v2 26/32] mm/huge_memory: split huge pmd under one pte_offset_map()

Hugh Dickins
2023-06-09T01:41:42Z

__split_huge_zero_page_pmd() use a single pte_offset_map() to sweep the
extent: it's already under pmd_lock(), so this is no worse for latency;
and since it's supposed to have full control of the just-withdrawn page
table, here choose to VM_BUG_ON if it were to fail. And please don't
increment haddr by PAGE_SIZE, that should remain huge aligned: declare
a separate addr (not a bugfix, but it was deceptive).

__split_huge_pmd_locked() likewise (but it had declared a separate addr);
and change its BUG_ON(!pte_none) to VM_BUG_ON, for consistency with zero
(those deposited page tables are sometimes victims of random corruption).

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
---
mm/huge_memory.c | 28 ++++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index d4bd5fa7c823..839c13fa0bbe 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2037,6 +2037,8 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
struct mm_struct *mm = vma->vm_mm;
pgtable_t pgtable;
pmd_t _pmd, old_pmd;
+ unsigned long addr;
+ pte_t *pte;
int i;

/*
@@ -2052,17 +2054,20 @@ static void __split_huge_zero_page_pmd(struct vm_area_struct *vma,
pgtable = pgtable_trans_huge_withdraw(mm, pmd);
pmd_populate(mm, &_pmd, pgtable);

- for (i = 0; i < HPAGE_PMD_NR; i++, haddr += PAGE_SIZE) {
- pte_t *pte, entry;
- entry = pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
+ pte = pte_offset_map(&_pmd, haddr);
+ VM_BUG_ON(!pte);
+ for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
+ pte_t entry;
+
+ entry = pfn_pte(my_zero_pfn(addr), vma->vm_page_prot);
entry = pte_mkspecial(entry);
if (pmd_uffd_wp(old_pmd))
entry = pte_mkuffd_wp(entry);
- pte = pte_offset_map(&_pmd, haddr);
VM_BUG_ON(!pte_none(*pte));
- set_pte_at(mm, haddr, pte, entry);
- pte_unmap(pte);
+ set_pte_at(mm, addr, pte, entry);
+ pte++;
}
+ pte_unmap(pte - 1);
smp_wmb(); /* make pte visible before pmd */
pmd_populate(mm, pmd, pgtable);
}
@@ -2077,6 +2082,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
bool young, write, soft_dirty, pmd_migration = false, uffd_wp = false;
bool anon_exclusive = false, dirty = false;
unsigned long addr;
+ pte_t *pte;
int i;

VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
@@ -2205,8 +2211,10 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
pgtable = pgtable_trans_huge_withdraw(mm, pmd);
pmd_populate(mm, &_pmd, pgtable);

+ pte = pte_offset_map(&_pmd, haddr);
+ VM_BUG_ON(!pte);
for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
- pte_t entry, *pte;
+ pte_t entry;
/*
* Note that NUMA hinting access restrictions are not
* transferred to avoid any possibility of altering
@@ -2249,11 +2257,11 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
entry = pte_mkuffd_wp(entry);
page_add_anon_rmap(page + i, vma, addr, false);
}
- pte = pte_offset_map(&_pmd, addr);
- BUG_ON(!pte_none(*pte));
+ VM_BUG_ON(!pte_none(*pte));
set_pte_at(mm, addr, pte, entry);
- pte_unmap(pte);
+ pte++;
}
+ pte_unmap(pte - 1);

if (!pmd_migration)
page_remove_rmap(page, vma, true);
--
2.35.3