[PATCH mm-unstable v7 00/31] Split ptdesc from struct page

[Matthew Wilcox]

On Mon, Jul 24, 2023 at 09:41:36PM -0700, Hugh Dickins wrote:
> On Mon, 24 Jul 2023, Vishal Moola (Oracle) wrote:
> 
> > 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.