On Tue, Apr 18, 2023 at 8:45 AM David Hildenbrand [off-list ref] wrote:

On 17.04.23 22:50, Vishal Moola (Oracle) wrote:

quoted

s390 uses page->index to keep track of page tables for the guest address
space. In an attempt to consolidate the usage of page fields in s390,
replace _pt_pad_2 with _pt_s390_gaddr to replace page->index in gmap.

This will help with the splitting of struct ptdesc from struct page, as
well as allow s390 to use _pt_frag_refcount for fragmented page table
tracking.

Since page->_pt_s390_gaddr aliases with mapping, ensure its set to NULL
before freeing the pages as well.

Signed-off-by: Vishal Moola (Oracle) <redacted>
---

[...]

quoted

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3fc9e680f174..2616d64c0e8c 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h

@@ -144,7 +144,7 @@ struct page {
              struct {        /* Page table pages */
                      unsigned long _pt_pad_1;        /* compound_head */
                      pgtable_t pmd_huge_pte; /* protected by page->ptl */
-                     unsigned long _pt_pad_2;        /* mapping */
+                     unsigned long _pt_s390_gaddr;   /* mapping */
                      union {
                              struct mm_struct *pt_mm; /* x86 pgds only */
                              atomic_t pt_frag_refcount; /* powerpc */

The confusing part is, that these gmap page tables are not ordinary
process page tables that we would ordinarily place into this section
here. That's why they are also not allocated/freed using the typical
page table constructor/destructor ...

I initially thought the same, so I was quite confused when I saw
__gmap_segment_gaddr was using pmd_pgtable_page().

Although they are not ordinary process page tables, since we
eventually want to move them out of struct page, I think shifting them
to be in ptdescs, being a memory descriptor for page tables, makes
the most sense.

Another option is to leave pmd_pgtable_page() as is just for this case.
Or we can revert commit 7e25de77bc5ea which uses the function here
then figure out where these gmap pages table pages will go later.