On 2024/7/26 02:39, David Hildenbrand wrote:
pte_lockptr() is the only *_lockptr() function that doesn't consume
what would be expected: it consumes a pmd_t pointer instead of a pte_t
pointer.
Let's change that. The two callers in pgtable-generic.c are easily
adjusted. Adjust khugepaged.c:retract_page_tables() to simply do a
pte_offset_map_nolock() to obtain the lock, even though we won't actually
be traversing the page table.
This makes the code more similar to the other variants and avoids other
hacks to make the new pte_lockptr() version happy. pte_lockptr() users
reside now only in pgtable-generic.c.
Maybe, using pte_offset_map_nolock() is the right thing to do because
the PTE table could have been removed in the meantime? At least it sounds
more future proof if we ever have other means of page table reclaim.
Agree, this helps us recheck the pmd entry.
It's not quite clear if holding the PTE table lock is really required:
what if someone else obtains the lock just after we unlock it? But we'll
leave that as is for now, maybe there are good reasons.
This is a preparation for adapting hugetlb page table locking logic to
take the same locks as core-mm page table walkers would.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
include/linux/mm.h | 7 ++++---
mm/khugepaged.c | 21 +++++++++++++++------
mm/pgtable-generic.c | 4 ++--
3 files changed, 21 insertions(+), 11 deletions(-)
Since pte_lockptr() no longer has a pmd parameter, it is best to modify
the comments above __pte_offset_map_lock() as well:
```
This helps the caller to avoid a later pte_lockptr(mm, *pmd), which
might by that time act on a changed *pmd ...
```
Otherwise:
Reviewed-by: Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx>
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2c6ccf088c7be..0472a5090b180 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2873,9 +2873,10 @@ static inline spinlock_t *ptlock_ptr(struct ptdesc *ptdesc)
}
#endif /* ALLOC_SPLIT_PTLOCKS */
-static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
+static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pte_t *pte)
{
- return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
+ /* PTE page tables don't currently exceed a single page. */
+ return ptlock_ptr(virt_to_ptdesc(pte));
}
static inline bool ptlock_init(struct ptdesc *ptdesc)
@@ -2898,7 +2899,7 @@ static inline bool ptlock_init(struct ptdesc *ptdesc)
/*
* We use mm->page_table_lock to guard all pagetable pages of the mm.
*/
-static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
+static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pte_t *pte)
{
return &mm->page_table_lock;
}
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index cdd1d8655a76b..f3b3db1046155 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1697,12 +1697,13 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
i_mmap_lock_read(mapping);
vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
struct mmu_notifier_range range;
+ bool retracted = false;
struct mm_struct *mm;
unsigned long addr;
pmd_t *pmd, pgt_pmd;
spinlock_t *pml;
spinlock_t *ptl;
- bool skipped_uffd = false;
+ pte_t *pte;
/*
* Check vma->anon_vma to exclude MAP_PRIVATE mappings that
@@ -1739,9 +1740,17 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
mmu_notifier_invalidate_range_start(&range);
pml = pmd_lock(mm, pmd);
- ptl = pte_lockptr(mm, pmd);
+
+ /*
+ * No need to check the PTE table content, but we'll grab the
+ * PTE table lock while we zap it.
+ */
+ pte = pte_offset_map_nolock(mm, pmd, addr, &ptl);
+ if (!pte)
+ goto unlock_pmd;
if (ptl != pml)
spin_lock_nested(ptl, SINGLE_DEPTH_NESTING);
+ pte_unmap(pte);
/*
* Huge page lock is still held, so normally the page table
@@ -1752,20 +1761,20 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
* repeating the anon_vma check protects from one category,
* and repeating the userfaultfd_wp() check from another.
*/
- if (unlikely(vma->anon_vma || userfaultfd_wp(vma))) {
- skipped_uffd = true;
- } else {
+ if (likely(!vma->anon_vma && !userfaultfd_wp(vma))) {
pgt_pmd = pmdp_collapse_flush(vma, addr, pmd);
pmdp_get_lockless_sync();
+ retracted = true;
}
if (ptl != pml)
spin_unlock(ptl);
+unlock_pmd:
spin_unlock(pml);
mmu_notifier_invalidate_range_end(&range);
- if (!skipped_uffd) {
+ if (retracted) {
mm_dec_nr_ptes(mm);
page_table_check_pte_clear_range(mm, addr, pgt_pmd);
pte_free_defer(mm, pmd_pgtable(pgt_pmd));
diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
index a78a4adf711ac..13a7705df3f87 100644
--- a/mm/pgtable-generic.c
+++ b/mm/pgtable-generic.c
@@ -313,7 +313,7 @@ pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
pte = __pte_offset_map(pmd, addr, &pmdval);
if (likely(pte))
- *ptlp = pte_lockptr(mm, &pmdval);
+ *ptlp = pte_lockptr(mm, pte);
return pte;
}
@@ -371,7 +371,7 @@ pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
pte = __pte_offset_map(pmd, addr, &pmdval);
if (unlikely(!pte))
return pte;
- ptl = pte_lockptr(mm, &pmdval);
+ ptl = pte_lockptr(mm, pte);
spin_lock(ptl);
if (likely(pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
*ptlp = ptl;
