On Mon, Jul 24, 2023 at 09:41:36PM -0700, Hugh Dickins wrote:

On Mon, 24 Jul 2023, Vishal Moola (Oracle) wrote:

quoted

The MM subsystem is trying to shrink struct page. This patchset
introduces a memory descriptor for page table tracking - struct ptdesc.

This patchset introduces ptdesc, splits ptdesc from struct page, and
converts many callers of page table constructor/destructors to use ptdescs.

Ptdesc is a foundation to further standardize page tables, and eventually
allow for dynamic allocation of page tables independent of struct page.
However, the use of pages for page table tracking is quite deeply
ingrained and varied across archictectures, so there is still a lot of
work to be done before that can happen.

Others may differ, but it remains the case that I see no point to this
patchset, until the minimal descriptor that replaces struct page is
working, and struct page then becomes just overhead.  Until that time,
let architectures continue to use struct page as they do - whyever not?

Because it's easier for architecture maintainers to understand what they
should and shouldn't be using.  Look at the definition:

+struct ptdesc {
+	unsigned long __page_flags;
+
+	union {
+		struct rcu_head pt_rcu_head;
+		struct list_head pt_list;
+		struct {
+			unsigned long _pt_pad_1;
+			pgtable_t pmd_huge_pte;
+		};
+	};
+	unsigned long __page_mapping;
+
+	union {
+		struct mm_struct *pt_mm;
+		atomic_t pt_frag_refcount;
+	};
+
+	union {
+		unsigned long _pt_pad_2;
+#if ALLOC_SPLIT_PTLOCKS
+		spinlock_t *ptl;
+#else
+		spinlock_t ptl;
+#endif
+	};
+	unsigned int __page_type;
+	atomic_t _refcount;
+#ifdef CONFIG_MEMCG
+	unsigned long pt_memcg_data;
+#endif
+};

It's still a 31-line struct definition, I'll grant you.  But it's far
easier to comprehend than the definition of struct page (~140 lines).
An architecture maintainer can look at it and see what might be available,
and what is already used.  And hopefully we'll have less "Oh, I'll just
use page->private".  It's really not fair to expect arch maintainers to
learn so much about the mm.

It's still messier than I would like, but I don't think we can do better
for now.  I don't understand why you're so interested in delaying doing
this work.