[PATCH v2 30/32] mm/pgtable: delete pmd_trans_unstable() and friends

Hugh Dickins
2023-06-09T01:51:00Z

Delete pmd_trans_unstable, pmd_none_or_trans_huge_or_clear_bad() and
pmd_devmap_trans_unstable(), all now unused.

With mixed feelings, delete all the comments on pmd_trans_unstable().
That was very good documentation of a subtle state, and this series does
not even eliminate that state: but rather, normalizes and extends it,
asking pte_offset_map[_lock]() callers to anticipate failure, without
regard for whether mmap_read_lock() or mmap_write_lock() is held.

Retain pud_trans_unstable(), which has one use in __handle_mm_fault(),
but delete its equivalent pud_none_or_trans_huge_or_dev_or_clear_bad().
While there, move the default arch_needs_pgtable_deposit() definition
up near where pgtable_trans_huge_deposit() and withdraw() are declared.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
include/linux/pgtable.h | 103 +++-------------------------------------
mm/khugepaged.c | 4 --
2 files changed, 7 insertions(+), 100 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 3fabbb018557..a1326e61d7ee 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -599,6 +599,10 @@ extern void pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp,
extern pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp);
#endif

+#ifndef arch_needs_pgtable_deposit
+#define arch_needs_pgtable_deposit() (false)
+#endif
+
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
/*
* This is an implementation of pmdp_establish() that is only suitable for an
@@ -1300,9 +1304,10 @@ static inline int pud_trans_huge(pud_t pud)
}
#endif

-/* See pmd_none_or_trans_huge_or_clear_bad for discussion. */
-static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud)
+static inline int pud_trans_unstable(pud_t *pud)
{
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
+ defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
pud_t pudval = READ_ONCE(*pud);

if (pud_none(pudval) || pud_trans_huge(pudval) || pud_devmap(pudval))
@@ -1311,104 +1316,10 @@ static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud)
pud_clear_bad(pud);
return 1;
}
- return 0;
-}
-
-/* See pmd_trans_unstable for discussion. */
-static inline int pud_trans_unstable(pud_t *pud)
-{
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \
- defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
- return pud_none_or_trans_huge_or_dev_or_clear_bad(pud);
-#else
- return 0;
#endif
-}
-
-#ifndef arch_needs_pgtable_deposit
-#define arch_needs_pgtable_deposit() (false)
-#endif
-/*
- * This function is meant to be used by sites walking pagetables with
- * the mmap_lock held in read mode to protect against MADV_DONTNEED and
- * transhuge page faults. MADV_DONTNEED can convert a transhuge pmd
- * into a null pmd and the transhuge page fault can convert a null pmd
- * into an hugepmd or into a regular pmd (if the hugepage allocation
- * fails). While holding the mmap_lock in read mode the pmd becomes
- * stable and stops changing under us only if it's not null and not a
- * transhuge pmd. When those races occurs and this function makes a
- * difference vs the standard pmd_none_or_clear_bad, the result is
- * undefined so behaving like if the pmd was none is safe (because it
- * can return none anyway). The compiler level barrier() is critically
- * important to compute the two checks atomically on the same pmdval.
- *
- * For 32bit kernels with a 64bit large pmd_t this automatically takes
- * care of reading the pmd atomically to avoid SMP race conditions
- * against pmd_populate() when the mmap_lock is hold for reading by the
- * caller (a special atomic read not done by "gcc" as in the generic
- * version above, is also needed when THP is disabled because the page
- * fault can populate the pmd from under us).
- */
-static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
-{
- pmd_t pmdval = pmdp_get_lockless(pmd);
- /*
- * !pmd_present() checks for pmd migration entries
- *
- * The complete check uses is_pmd_migration_entry() in linux/swapops.h
- * But using that requires moving current function and pmd_trans_unstable()
- * to linux/swapops.h to resolve dependency, which is too much code move.
- *
- * !pmd_present() is equivalent to is_pmd_migration_entry() currently,
- * because !pmd_present() pages can only be under migration not swapped
- * out.
- *
- * pmd_none() is preserved for future condition checks on pmd migration
- * entries and not confusing with this function name, although it is
- * redundant with !pmd_present().
- */
- if (pmd_none(pmdval) || pmd_trans_huge(pmdval) ||
- (IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION) && !pmd_present(pmdval)))
- return 1;
- if (unlikely(pmd_bad(pmdval))) {
- pmd_clear_bad(pmd);
- return 1;
- }
return 0;
}

-/*
- * This is a noop if Transparent Hugepage Support is not built into
- * the kernel. Otherwise it is equivalent to
- * pmd_none_or_trans_huge_or_clear_bad(), and shall only be called in
- * places that already verified the pmd is not none and they want to
- * walk ptes while holding the mmap sem in read mode (write mode don't
- * need this). If THP is not enabled, the pmd can't go away under the
- * code even if MADV_DONTNEED runs, but if THP is enabled we need to
- * run a pmd_trans_unstable before walking the ptes after
- * split_huge_pmd returns (because it may have run when the pmd become
- * null, but then a page fault can map in a THP and not a regular page).
- */
-static inline int pmd_trans_unstable(pmd_t *pmd)
-{
-#ifdef CONFIG_TRANSPARENT_HUGEPAGE
- return pmd_none_or_trans_huge_or_clear_bad(pmd);
-#else
- return 0;
-#endif
-}
-
-/*
- * the ordering of these checks is important for pmds with _page_devmap set.
- * if we check pmd_trans_unstable() first we will trip the bad_pmd() check
- * inside of pmd_none_or_trans_huge_or_clear_bad(). this will end up correctly
- * returning 1 but not before it spams dmesg with the pmd_clear_bad() output.
- */
-static inline int pmd_devmap_trans_unstable(pmd_t *pmd)
-{
- return pmd_devmap(*pmd) || pmd_trans_unstable(pmd);
-}
-
#ifndef CONFIG_NUMA_BALANCING
/*
* Technically a PTE can be PROTNONE even when not doing NUMA balancing but
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index c11db2e78e95..1083f0e38a07 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -946,10 +946,6 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
return SCAN_SUCCEED;
}

-/*
- * See pmd_trans_unstable() for how the result may change out from
- * underneath us, even if we hold mmap_lock in read.
- */
static int find_pmd_or_thp_or_none(struct mm_struct *mm,
unsigned long address,
pmd_t **pmd)
--
2.35.3