Sorry for taking so long to review ... getting there. Mostly nits.
@@ -497,6 +499,13 @@ static inline void split_huge_pmd_locked(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmd,
bool freeze, struct folio *folio) {}
+static inline bool unmap_huge_pmd_locked(struct vm_area_struct *vma,
+ unsigned long addr, pmd_t *pmdp,
+ struct folio *folio)
+{
+ return false;
+}
+
#define split_huge_pud(__vma, __pmd, __address) \
do { } while (0)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index e766d3f3a302..425374ae06ed 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2688,6 +2688,82 @@ static void unmap_folio(struct folio *folio)
try_to_unmap_flush();
}
+static bool __discard_anon_folio_pmd_locked(struct vm_area_struct *vma,
+ unsigned long addr, pmd_t *pmdp,
+ struct folio *folio)
+{
+ VM_WARN_ON_FOLIO(folio_test_swapbacked(folio), folio);
+ VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
I would drop these (that's exactly what the single caller checks). In
any case don't place them above the variable declaration ;)
+
+ struct mm_struct *mm = vma->vm_mm;
+ int ref_count, map_count;
+ pmd_t orig_pmd = *pmdp;
+ struct page *page;
+
+ if (unlikely(!pmd_present(orig_pmd) || !pmd_trans_huge(orig_pmd)))
+ return false;
+
+ page = pmd_page(orig_pmd);
+ if (unlikely(page_folio(page) != folio))
+ return false;
I'm curious, how could that happen? And how could it happen that we have
!pmd_trans_huge() ? Didn't rmap walking code make sure that this PMD
maps the folio already, and we are holding the PTL?
+
+ if (folio_test_dirty(folio) || pmd_dirty(orig_pmd))
+ return false;
+
+ orig_pmd = pmdp_huge_clear_flush(vma, addr, pmdp);
+
+ /*
+ * Syncing against concurrent GUP-fast:
+ * - clear PMD; barrier; read refcount
+ * - inc refcount; barrier; read PMD
+ */
+ smp_mb();
+
+ ref_count = folio_ref_count(folio);
+ map_count = folio_mapcount(folio);
+
+ /*
+ * Order reads for folio refcount and dirty flag
+ * (see comments in __remove_mapping()).
+ */
+ smp_rmb();
+
+ /*
+ * If the folio or its PMD is redirtied at this point, or if there
+ * are unexpected references, we will give up to discard this folio
+ * and remap it.
+ *
+ * The only folio refs must be one from isolation plus the rmap(s).
+ */
+ if (folio_test_dirty(folio) || pmd_dirty(orig_pmd) ||
+ ref_count != map_count + 1) {
+ set_pmd_at(mm, addr, pmdp, orig_pmd);
+ return false;
+ }
+
+ folio_remove_rmap_pmd(folio, page, vma);
+ zap_deposited_table(mm, pmdp);
+ add_mm_counter(mm, MM_ANONPAGES, -HPAGE_PMD_NR);
+ if (vma->vm_flags & VM_LOCKED)
+ mlock_drain_local();
+ folio_put(folio);
+
+ return true;
+}
+
+bool unmap_huge_pmd_locked(struct vm_area_struct *vma, unsigned long addr,
+ pmd_t *pmdp, struct folio *folio)
+{
+ VM_WARN_ON_FOLIO(!folio_test_pmd_mappable(folio), folio);
+ VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
+ VM_WARN_ON_ONCE(!IS_ALIGNED(addr, HPAGE_PMD_SIZE));
+
+ if (folio_test_anon(folio) && !folio_test_swapbacked(folio))
+ return __discard_anon_folio_pmd_locked(vma, addr, pmdp, folio);
+
+ return false;
+}
+
static void remap_page(struct folio *folio, unsigned long nr)
{
int i = 0;
diff --git a/mm/rmap.c b/mm/rmap.c
index dacf24bc82f0..7d97806f74cd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1678,16 +1678,23 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
goto walk_abort;
}
- if (!pvmw.pte && (flags & TTU_SPLIT_HUGE_PMD)) {
- /*
- * We temporarily have to drop the PTL and start once
- * again from that now-PTE-mapped page table.
- */
- split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
- false, folio);
- flags &= ~TTU_SPLIT_HUGE_PMD;
- page_vma_mapped_walk_restart(&pvmw);
- continue;
+ if (!pvmw.pte) {
+ if (unmap_huge_pmd_locked(vma, pvmw.address, pvmw.pmd,
+ folio))
+ goto walk_done;
+
+ if (flags & TTU_SPLIT_HUGE_PMD) {
+ /*
+ * We temporarily have to drop the PTL and start
+ * once again from that now-PTE-mapped page
+ * table.
Nit: it's not a PTE-mapped page table.
Maybe
"... restart so we can process the PTE-mapped THP."
}
/* Unexpected PMD-mapped THP? */
Nothing else jumped at me :)
--
Cheers,
David / dhildenb
