Most callers of is_huge_zero_pmd() supply a pmd already verified present;
but a few (notably zap_huge_pmd()) do not - it might be a pmd migration
entry, in which the pfn is encoded differently from a present pmd: which
might pass the is_huge_zero_pmd() test (though not on x86, since L1TF
forced us to protect against that); or perhaps even crash in pmd_page()
applied to a swap-like entry.
Make it safe by adding pmd_present() check into is_huge_zero_pmd() itself;
and make it quicker by saving huge_zero_pfn, so that is_huge_zero_pmd()
will not need to do that pmd_page() lookup each time.
__split_huge_pmd_locked() checked pmd_trans_huge() before: that worked,
but is unnecessary now that is_huge_zero_pmd() checks present.
Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
---
Patch added (replacing part of first) since the v1 series was posted.
include/linux/huge_mm.h | 8 +++++++-
mm/huge_memory.c | 5 ++++-
2 files changed, 11 insertions(+), 2 deletions(-)
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 9626fda5efce..2a8ebe6c222e 100644
--- a/include/linux/huge_mm.h
+++ b/include/linux/huge_mm.h
@@ -286,6 +286,7 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
vm_fault_t do_huge_pmd_numa_page(struct vm_fault *vmf, pmd_t orig_pmd);
extern struct page *huge_zero_page;
+extern unsigned long huge_zero_pfn;
static inline bool is_huge_zero_page(struct page *page)
{
@@ -294,7 +295,7 @@ static inline bool is_huge_zero_page(struct page *page)
static inline bool is_huge_zero_pmd(pmd_t pmd)
{
- return is_huge_zero_page(pmd_page(pmd));
+ return READ_ONCE(huge_zero_pfn) == pmd_pfn(pmd) && pmd_present(pmd);
}
static inline bool is_huge_zero_pud(pud_t pud)
@@ -440,6 +441,11 @@ static inline bool is_huge_zero_page(struct page *page)
return false;
}
+static inline bool is_huge_zero_pmd(pmd_t pmd)
+{
+ return false;
+}
+
static inline bool is_huge_zero_pud(pud_t pud)
{
return false;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 42cfefc6e66e..5885c5f5836f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -62,6 +62,7 @@ static struct shrinker deferred_split_shrinker;
static atomic_t huge_zero_refcount;
struct page *huge_zero_page __read_mostly;
+unsigned long huge_zero_pfn __read_mostly = ~0UL;
bool transparent_hugepage_enabled(struct vm_area_struct *vma)
{
@@ -98,6 +99,7 @@ static bool get_huge_zero_page(void)
__free_pages(zero_page, compound_order(zero_page));
goto retry;
}
+ WRITE_ONCE(huge_zero_pfn, page_to_pfn(zero_page));
/* We take additional reference here. It will be put back by shrinker */
atomic_set(&huge_zero_refcount, 2);
@@ -147,6 +149,7 @@ static unsigned long shrink_huge_zero_page_scan(struct shrinker *shrink,
if (atomic_cmpxchg(&huge_zero_refcount, 1, 0) == 1) {
struct page *zero_page = xchg(&huge_zero_page, NULL);
BUG_ON(zero_page == NULL);
+ WRITE_ONCE(huge_zero_pfn, ~0UL);
__free_pages(zero_page, compound_order(zero_page));
return HPAGE_PMD_NR;
}
@@ -2071,7 +2074,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
return;
}
- if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
+ if (is_huge_zero_pmd(*pmd)) {
/*
* FIXME: Do we want to invalidate secondary mmu by calling
* mmu_notifier_invalidate_range() see comments below inside
--
2.26.2
