Re: [PATCH] mm: export folio_pte_batch as a couple of modules might need it
From: David Hildenbrand
Date: Tue Feb 27 2024 - 04:15:00 EST
On 27.02.24 10:07, Ryan Roberts wrote:
On 27/02/2024 02:40, Barry Song wrote:
From: Barry Song <v-songbaohua@xxxxxxxx>
madvise and some others might need folio_pte_batch to check if a range
of PTEs are completely mapped to a large folio with contiguous physcial
addresses. Let's export it for others to use.
Cc: Lance Yang <ioworker0@xxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
-v1:
at least two jobs madv_free and madv_pageout depend on it. To avoid
conflicts and dependencies, after discussing with Lance, we prefer
this one can land earlier.
I think this will also ultimately be useful for mprotect too, though I haven't
looked at it properly yet.
Yes, I think we briefly discussed that.
mm/internal.h | 13 +++++++++++++
mm/memory.c | 11 +----------
2 files changed, 14 insertions(+), 10 deletions(-)
diff --git a/mm/internal.h b/mm/internal.h
index 13b59d384845..8e2bc304f671 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -83,6 +83,19 @@ static inline void *folio_raw_mapping(struct folio *folio)
return (void *)(mapping & ~PAGE_MAPPING_FLAGS);
}
+/* Flags for folio_pte_batch(). */
+typedef int __bitwise fpb_t;
+
+/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
+#define FPB_IGNORE_DIRTY ((__force fpb_t)BIT(0))
+
+/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
+#define FPB_IGNORE_SOFT_DIRTY ((__force fpb_t)BIT(1))
+
+extern int folio_pte_batch(struct folio *folio, unsigned long addr,
+ pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
+ bool *any_writable);
+
void __acct_reclaim_writeback(pg_data_t *pgdat, struct folio *folio,
int nr_throttled);
static inline void acct_reclaim_writeback(struct folio *folio)
diff --git a/mm/memory.c b/mm/memory.c
index 1c45b6a42a1b..319b3be05e75 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -953,15 +953,6 @@ static __always_inline void __copy_present_ptes(struct vm_area_struct *dst_vma,
set_ptes(dst_vma->vm_mm, addr, dst_pte, pte, nr);
}
-/* Flags for folio_pte_batch(). */
-typedef int __bitwise fpb_t;
-
-/* Compare PTEs after pte_mkclean(), ignoring the dirty bit. */
-#define FPB_IGNORE_DIRTY ((__force fpb_t)BIT(0))
-
-/* Compare PTEs after pte_clear_soft_dirty(), ignoring the soft-dirty bit. */
-#define FPB_IGNORE_SOFT_DIRTY ((__force fpb_t)BIT(1))
-
static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
{
if (flags & FPB_IGNORE_DIRTY)
@@ -982,7 +973,7 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
* If "any_writable" is set, it will indicate if any other PTE besides the
* first (given) PTE is writable.
*/
David was talking in Lance's patch thread, about improving the docs for this
function now that its exported. Might be worth syncing on that.
Here is my take:
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
mm/memory.c | 22 ++++++++++++++++++----
1 file changed, 18 insertions(+), 4 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index d0b855a1837a8..098356b8805ae 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -971,16 +971,28 @@ static inline pte_t __pte_batch_clear_ignored(pte_t pte, fpb_t flags)
return pte_wrprotect(pte_mkold(pte));
}
-/*
+/**
+ * folio_pte_batch - detect a PTE batch for a large folio
+ * @folio: The large folio to detect a PTE batch for.
+ * @addr: The user virtual address the first page is mapped at.
+ * @start_ptep: Page table pointer for the first entry.
+ * @pte: Page table entry for the first page.
+ * @max_nr: The maximum number of table entries to consider.
+ * @flags: Flags to modify the PTE batch semantics.
+ * @any_writable: Optional pointer to indicate whether any entry except the
+ * first one is writable.
+ *
* Detect a PTE batch: consecutive (present) PTEs that map consecutive
- * pages of the same folio.
+ * pages of the same large folio.
*
* All PTEs inside a PTE batch have the same PTE bits set, excluding the PFN,
* the accessed bit, writable bit, dirty bit (with FPB_IGNORE_DIRTY) and
* soft-dirty bit (with FPB_IGNORE_SOFT_DIRTY).
*
- * If "any_writable" is set, it will indicate if any other PTE besides the
- * first (given) PTE is writable.
+ * start_ptep must map any page of the folio. max_nr must be at least one and
+ * must be limited by the caller so scanning cannot exceed a single page table.
+ *
+ * Return: the number of table entries in the batch.
*/
static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
@@ -996,6 +1008,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
*any_writable = false;
VM_WARN_ON_FOLIO(!pte_present(pte), folio);
+ VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
+ VM_WARN_ON_FOLIO(page_folio(pfn_to_page(pte_pfn(pte))) != folio, folio);
nr = pte_batch_hint(start_ptep, pte);
expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);
--
2.43.2
-static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
+int folio_pte_batch(struct folio *folio, unsigned long addr,
fork() is very performance sensitive. Is there a risk we are regressing
performance by making this out-of-line? Although its in the same compilation
unit so the compiler may well inline it anyway?
Easy to verify by looking at the generated asm I guess?
Either way, perhaps we are better off making it inline in the header? That would
avoid needing to rerun David's micro-benchmarks for fork() and munmap().
That way, the compiler can most certainly better optimize it also outside of mm/memory.c
--
Cheers,
David / dhildenb