Implement arm64 specific huge_ptep_get()

Hi,

As Mike pointed out [1], the huge_ptep_get() will only return one specific pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD size hugetlb page for those functions that want to check the dirty or young flags of a hugetlb page. For example, the gather_hugetlb_stats() will get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring will also get inaccurate access statistics.

To fix this issue, this patch set introduces an ARM64 specific huge_ptep_get() implementation, which will take into account any subpages' dirty or young bits.

[1] https://lore.kernel.org/linux-mm/[email protected]/

Changes from v2:
- Fix some typos and update the commit message suggested by Anshuman.
- Improve some coding style issues.
- Add reviewed tag from Anshuman.

Changes from v1:
- Rebase on next-20220512.
- Update commit message suggested by Muchun.
- Add reviewed tag from Muchun.

Changes from RFC:
- Implement arm64 specific huge_ptep_get() instead of introducing a new interface.
- Add a new patch to convert huge_ptep_get() in hugetlbpage.c

Baolin Wang (2):
arm64/hugetlb: Use ptep_get() to get the pte value of a huge page arm64/hugetlb: Implement arm64 specific huge_ptep_get()

 arch/arm64/include/asm/hugetlb.h |  2 ++
 arch/arm64/mm/hugetlbpage.c      | 30 ++++++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

1.8.3.1

The original huge_ptep_get() on ARM64 is just a wrapper of ptep_get(), which will not take into account any contig-PTEs dirty and access bits. Meanwhile we will implement a new ARM64-specific huge_ptep_get() interface in following patch, which will take into account any contig-PTEs dirty and access bits. To keep the same efficient logic to get the pte value, change to use ptep_get() as a preparation.

---
 arch/arm64/mm/hugetlbpage.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index 30f5b76..9553851 100644
@@ -172,7 +172,7 @@ static pte_t get_clear_contig(struct mm_struct *mm,
 			     unsigned long pgsize,
 			     unsigned long ncontig)
 {
-	pte_t orig_pte = huge_ptep_get(ptep);
+	pte_t orig_pte = ptep_get(ptep);
 	unsigned long i;
 
 	for (i = 0; i < ncontig; i++, addr += pgsize, ptep++) {

@@ -379,7 +379,7 @@ pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
 {
 	int ncontig;
 	size_t pgsize;
-	pte_t orig_pte = huge_ptep_get(ptep);
+	pte_t orig_pte = ptep_get(ptep);
 
 	if (!pte_cont(orig_pte))
 		return ptep_get_and_clear(mm, addr, ptep);

@@ -402,11 +402,11 @@ static int __cont_access_flags_changed(pte_t *ptep, pte_t pte, int ncontig)
 {
 	int i;
 
-	if (pte_write(pte) != pte_write(huge_ptep_get(ptep)))
+	if (pte_write(pte) != pte_write(ptep_get(ptep)))
 		return 1;
 
 	for (i = 0; i < ncontig; i++) {
-		pte_t orig_pte = huge_ptep_get(ptep + i);
+		pte_t orig_pte = ptep_get(ptep + i);
 
 		if (pte_dirty(pte) != pte_dirty(orig_pte))
 			return 1;

1.8.3.1

Now we use huge_ptep_get() to get the pte value of a hugetlb page, however it will only return one specific pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which can contain several continuous pte or pmd entries with same page table attributes. And it will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD size hugetlb page.

So the huge_ptep_get() is inconsistent with huge_ptep_get_and_clear(), which already takes account the dirty or young bits for any subpages in this CONT-PTE/PMD size hugetlb [1]. Meanwhile we can miss dirty or young flags statistics for hugetlb pages with current huge_ptep_get(), such as the gather_hugetlb_stats() function, and CONT-PTE/PMD hugetlb monitoring with DAMON.

Thus define an ARM64 specific huge_ptep_get() implementation as well as enabling __HAVE_ARCH_HUGE_PTEP_GET, that will take into account any subpages' dirty or young bits for CONT-PTE/PMD size hugetlb page, for those functions that want to check the dirty and young flags of a hugetlb page.

[1] https://lore.kernel.org/linux-mm/[email protected]/

---
 arch/arm64/include/asm/hugetlb.h |  2 ++
 arch/arm64/mm/hugetlbpage.c      | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+)

index 616b2ca..1fd2846 100644
@@ -44,6 +44,8 @@ extern pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
 #define __HAVE_ARCH_HUGE_PTE_CLEAR
 extern void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
 			   pte_t *ptep, unsigned long sz);
+#define __HAVE_ARCH_HUGE_PTEP_GET
+extern pte_t huge_ptep_get(pte_t *ptep);
 extern void set_huge_swap_pte_at(struct mm_struct *mm, unsigned long addr,
 				 pte_t *ptep, pte_t pte, unsigned long sz);
 #define set_huge_swap_pte_at set_huge_swap_pte_at

index 9553851..2a7e143 100644
@@ -158,6 +158,28 @@ static inline int num_contig_ptes(unsigned long size, size_t *pgsize)
 	return contig_ptes;
 }
 
+pte_t huge_ptep_get(pte_t *ptep)
+{
+	int ncontig, i;
+	size_t pgsize;
+	pte_t orig_pte = ptep_get(ptep);
+
+	if (!pte_present(orig_pte) || !pte_cont(orig_pte))
+		return orig_pte;
+
+	ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize);
+	for (i = 0; i < ncontig; i++, ptep++) {
+		pte_t pte = ptep_get(ptep);
+
+		if (pte_dirty(pte))
+			orig_pte = pte_mkdirty(orig_pte);
+
+		if (pte_young(pte))
+			orig_pte = pte_mkyoung(orig_pte);
+	}
+	return orig_pte;
+}
+
 /*
  * Changing some bits of contiguous entries requires us to follow a
  * Break-Before-Make approach, breaking the whole contiguous set

1.8.3.1

Applied to arm64 (for-next/misc), thanks!

[1/2] arm64/hugetlb: Use ptep_get() to get the pte value of a huge page

      https://git.kernel.org/arm64/c/f0d9d79ec793
[2/2] arm64/hugetlb: Implement arm64 specific huge_ptep_get()
      https://git.kernel.org/arm64/c/bc5dfb4fd7bd

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