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

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.

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.

Thanks,
Jason