On Tue, Jun 20, 2023 at 01:01:39PM -0700, Vishal Moola wrote:

On Fri, Jun 16, 2023 at 5:38 AM Jason Gunthorpe [off-list ref] wrote:

quoted

On Mon, Jun 12, 2023 at 02:03:53PM -0700, Vishal Moola (Oracle) wrote:

quoted

Currently, page table information is stored within struct page. As part
of simplifying struct page, create struct ptdesc for page table
information.

Signed-off-by: Vishal Moola (Oracle) <redacted>
---
 include/linux/pgtable.h | 51 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index c5a51481bbb9..330de96ebfd6 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -975,6 +975,57 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
 #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
 #endif /* CONFIG_MMU */

+
+/**
+ * struct ptdesc - Memory descriptor for page tables.
+ * @__page_flags: Same as page flags. Unused for page tables.
+ * @pt_list: List of used page tables. Used for s390 and x86.
+ * @_pt_pad_1: Padding that aliases with page's compound head.
+ * @pmd_huge_pte: Protected by ptdesc->ptl, used for THPs.
+ * @_pt_s390_gaddr: Aliases with page's mapping. Used for s390 gmap only.
+ * @pt_mm: Used for x86 pgds.
+ * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
+ * @ptl: Lock for the page table.
+ *
+ * This struct overlays struct page for now. Do not modify without a good
+ * understanding of the issues.
+ */
+struct ptdesc {
+     unsigned long __page_flags;
+
+     union {
+             struct list_head pt_list;
+             struct {
+                     unsigned long _pt_pad_1;
+                     pgtable_t pmd_huge_pte;
+             };
+     };
+     unsigned long _pt_s390_gaddr;
+
+     union {
+             struct mm_struct *pt_mm;
+             atomic_t pt_frag_refcount;
+     };
+
+#if ALLOC_SPLIT_PTLOCKS
+     spinlock_t *ptl;
+#else
+     spinlock_t ptl;
+#endif
+};

I think you should include the memcg here too? It needs to be valid
for a ptdesc, even if we don't currently deref it through the ptdesc
type.

Yes, thanks for catching that! I'll add it to v5.

quoted

Also, do you see a way to someday put a 'struct rcu_head' into here?

Eventually, when they're being dynamically allocated independent of
struct page. Although at that point I'm not sure if we'll need one.

Sooner than dynamic struct page?

Probably it can overlap pt_list in alot of arches?

Jason

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