mm: remember young/dirty bit for page migrations

When page migration happens, we always ignore the young/dirty bit settings
in the old pgtable, and marking the page as old in the new page table
using either pte_mkold() or pmd_mkold(), and keeping the pte clean.

That's fine from functional-wise, but that's not friendly to page reclaim
because the moving page can be actively accessed within the procedure. 
Not to mention hardware setting the young bit can bring quite some
overhead on some systems, e.g.  x86_64 needs a few hundreds nanoseconds to
set the bit.  The same slowdown problem to dirty bits when the memory is
first written after page migration happened.

Actually we can easily remember the A/D bit configuration and recover the
information after the page is migrated.  To achieve it, define a new set
of bits in the migration swap offset field to cache the A/D bits for old
pte.  Then when removing/recovering the migration entry, we can recover
the A/D bits even if the page changed.

One thing to mention is that here we used max_swapfile_size() to detect
how many swp offset bits we have, and we'll only enable this feature if we
know the swp offset is big enough to store both the PFN value and the A/D
bits.  Otherwise the A/D bits are dropped like before.

Link: https://lkml.kernel.org/r/20220811161331.37055-6-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Dave Hansen <dave.hansen@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index b4666774abf0..f4a656b279b1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2121,7 +2121,8 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 		write = is_writable_migration_entry(entry);
 		if (PageAnon(page))
 			anon_exclusive = is_readable_exclusive_migration_entry(entry);
-		young = false;
+		young = is_migration_entry_young(entry);
+		dirty = is_migration_entry_dirty(entry);
 		soft_dirty = pmd_swp_soft_dirty(old_pmd);
 		uffd_wp = pmd_swp_uffd_wp(old_pmd);
 	} else {
@@ -2183,6 +2184,10 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
 			else
 				swp_entry = make_readable_migration_entry(
 							page_to_pfn(page + i));
+			if (young)
+				swp_entry = make_migration_entry_young(swp_entry);
+			if (dirty)
+				swp_entry = make_migration_entry_dirty(swp_entry);
 			entry = swp_entry_to_pte(swp_entry);
 			if (soft_dirty)
 				entry = pte_swp_mksoft_dirty(entry);
@@ -3201,6 +3206,10 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw,
 		entry = make_readable_exclusive_migration_entry(page_to_pfn(page));
 	else
 		entry = make_readable_migration_entry(page_to_pfn(page));
+	if (pmd_young(pmdval))
+		entry = make_migration_entry_young(entry);
+	if (pmd_dirty(pmdval))
+		entry = make_migration_entry_dirty(entry);
 	pmdswp = swp_entry_to_pmd(entry);
 	if (pmd_soft_dirty(pmdval))
 		pmdswp = pmd_swp_mksoft_dirty(pmdswp);
@@ -3226,13 +3235,18 @@ void remove_migration_pmd(struct page_vma_mapped_walk *pvmw, struct page *new)
 
 	entry = pmd_to_swp_entry(*pvmw->pmd);
 	get_page(new);
-	pmde = pmd_mkold(mk_huge_pmd(new, READ_ONCE(vma->vm_page_prot)));
+	pmde = mk_huge_pmd(new, READ_ONCE(vma->vm_page_prot));
 	if (pmd_swp_soft_dirty(*pvmw->pmd))
 		pmde = pmd_mksoft_dirty(pmde);
 	if (is_writable_migration_entry(entry))
 		pmde = maybe_pmd_mkwrite(pmde, vma);
 	if (pmd_swp_uffd_wp(*pvmw->pmd))
 		pmde = pmd_wrprotect(pmd_mkuffd_wp(pmde));
+	if (!is_migration_entry_young(entry))
+		pmde = pmd_mkold(pmde);
+	/* NOTE: this may contain setting soft-dirty on some archs */
+	if (PageDirty(new) && is_migration_entry_dirty(entry))
+		pmde = pmd_mkdirty(pmde);
 
 	if (PageAnon(new)) {
 		rmap_t rmap_flags = RMAP_COMPOUND;