[PATCH v2 11/17] mm/rmap: handle device-exclusive entries correctly in try_to_migrate_one()

[David Hildenbrand <david@redhat.com>]

Ever since commit b756a3b5e7ea ("mm: device exclusive memory access")
we can return with a device-exclusive entry from page_vma_mapped_walk().

try_to_migrate_one() is not prepared for that, so teach it about these
PFN swap PTEs. We already handle device-private entries by specializing
on the folio, so we can reshuffle that code to make it work on the
PFN swap PTEs instead.

Get rid of the folio_is_device_private() handling. Note that we never
currently expect device-private folios with HWPoison flag set at that
point, so add a warning in case that ever changes and we can figure out
what the right thing to do is.

Note that we could currently only run into this case with
device-exclusive entries on THPs. We still adjust the mapcount on
conversion to device-exclusive; this makes the rmap walk
abort early for small folios, because we'll always have
!folio_mapped() with a single device-exclusive entry. We'll adjust the
mapcount logic once all page_vma_mapped_walk() users can properly
handle device-exclusive entries.

Further note that try_to_migrate() calls MMU notifiers and holds the
folio lock, so any device-exclusive users should be properly prepared
for a device-exclusive PTE to "vanish".

Fixes: b756a3b5e7ea ("mm: device exclusive memory access")
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/rmap.c | 124 ++++++++++++++++++++++--------------------------------
 1 file changed, 51 insertions(+), 73 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index 47142a656ae51..7c471c3ea64c4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -2039,9 +2039,9 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 {
 	struct mm_struct *mm = vma->vm_mm;
 	DEFINE_FOLIO_VMA_WALK(pvmw, folio, vma, address, 0);
+	bool anon_exclusive, writable, ret = true;
 	pte_t pteval;
 	struct page *subpage;
-	bool anon_exclusive, ret = true;
 	struct mmu_notifier_range range;
 	enum ttu_flags flags = (enum ttu_flags)(long)arg;
 	unsigned long pfn;
@@ -2108,24 +2108,19 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 		/* Unexpected PMD-mapped THP? */
 		VM_BUG_ON_FOLIO(!pvmw.pte, folio);
 
-		pfn = pte_pfn(ptep_get(pvmw.pte));
-
-		if (folio_is_zone_device(folio)) {
-			/*
-			 * Our PTE is a non-present device exclusive entry and
-			 * calculating the subpage as for the common case would
-			 * result in an invalid pointer.
-			 *
-			 * Since only PAGE_SIZE pages can currently be
-			 * migrated, just set it to page. This will need to be
-			 * changed when hugepage migrations to device private
-			 * memory are supported.
-			 */
-			VM_BUG_ON_FOLIO(folio_nr_pages(folio) > 1, folio);
-			subpage = &folio->page;
+		/*
+		 * Handle PFN swap PTEs, such as device-exclusive ones, that
+		 * actually map pages.
+		 */
+		pteval = ptep_get(pvmw.pte);
+		if (likely(pte_present(pteval))) {
+			pfn = pte_pfn(pteval);
 		} else {
-			subpage = folio_page(folio, pfn - folio_pfn(folio));
+			pfn = swp_offset_pfn(pte_to_swp_entry(pteval));
+			VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
 		}
+
+		subpage = folio_page(folio, pfn - folio_pfn(folio));
 		address = pvmw.address;
 		anon_exclusive = folio_test_anon(folio) &&
 				 PageAnonExclusive(subpage);
@@ -2181,7 +2176,10 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			}
 			/* Nuke the hugetlb page table entry */
 			pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
-		} else {
+			if (pte_dirty(pteval))
+				folio_mark_dirty(folio);
+			writable = pte_write(pteval);
+		} else if (likely(pte_present(pteval))) {
 			flush_cache_page(vma, address, pfn);
 			/* Nuke the page table entry. */
 			if (should_defer_flush(mm, flags)) {
@@ -2199,54 +2197,23 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			} else {
 				pteval = ptep_clear_flush(vma, address, pvmw.pte);
 			}
+			if (pte_dirty(pteval))
+				folio_mark_dirty(folio);
+			writable = pte_write(pteval);
+		} else {
+			pte_clear(mm, address, pvmw.pte);
+			writable = is_writable_device_private_entry(pte_to_swp_entry(pteval));
 		}
 
-		/* Set the dirty flag on the folio now the pte is gone. */
-		if (pte_dirty(pteval))
-			folio_mark_dirty(folio);
+		VM_WARN_ON_FOLIO(writable && folio_test_anon(folio) &&
+				!anon_exclusive, folio);
 
 		/* Update high watermark before we lower rss */
 		update_hiwater_rss(mm);
 
-		if (folio_is_device_private(folio)) {
-			unsigned long pfn = folio_pfn(folio);
-			swp_entry_t entry;
-			pte_t swp_pte;
-
-			if (anon_exclusive)
-				WARN_ON_ONCE(folio_try_share_anon_rmap_pte(folio,
-									   subpage));
+		if (PageHWPoison(subpage)) {
+			VM_WARN_ON_FOLIO(folio_is_device_private(folio), folio);
 
-			/*
-			 * Store the pfn of the page in a special migration
-			 * pte. do_swap_page() will wait until the migration
-			 * pte is removed and then restart fault handling.
-			 */
-			entry = pte_to_swp_entry(pteval);
-			if (is_writable_device_private_entry(entry))
-				entry = make_writable_migration_entry(pfn);
-			else if (anon_exclusive)
-				entry = make_readable_exclusive_migration_entry(pfn);
-			else
-				entry = make_readable_migration_entry(pfn);
-			swp_pte = swp_entry_to_pte(entry);
-
-			/*
-			 * pteval maps a zone device page and is therefore
-			 * a swap pte.
-			 */
-			if (pte_swp_soft_dirty(pteval))
-				swp_pte = pte_swp_mksoft_dirty(swp_pte);
-			if (pte_swp_uffd_wp(pteval))
-				swp_pte = pte_swp_mkuffd_wp(swp_pte);
-			set_pte_at(mm, pvmw.address, pvmw.pte, swp_pte);
-			trace_set_migration_pte(pvmw.address, pte_val(swp_pte),
-						folio_order(folio));
-			/*
-			 * No need to invalidate here it will synchronize on
-			 * against the special swap migration pte.
-			 */
-		} else if (PageHWPoison(subpage)) {
 			pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
 			if (folio_test_hugetlb(folio)) {
 				hugetlb_count_sub(folio_nr_pages(folio), mm);
@@ -2256,8 +2223,8 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				dec_mm_counter(mm, mm_counter(folio));
 				set_pte_at(mm, address, pvmw.pte, pteval);
 			}
-
-		} else if (pte_unused(pteval) && !userfaultfd_armed(vma)) {
+		} else if (likely(pte_present(pteval)) && pte_unused(pteval) &&
+			   !userfaultfd_armed(vma)) {
 			/*
 			 * The guest indicated that the page content is of no
 			 * interest anymore. Simply discard the pte, vmscan
@@ -2273,6 +2240,11 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			swp_entry_t entry;
 			pte_t swp_pte;
 
+			/*
+			 * arch_unmap_one() is expected to be a NOP on
+			 * architectures where we could have PFN swap PTEs,
+			 * so we'll not check/care.
+			 */
 			if (arch_unmap_one(mm, vma, address, pteval) < 0) {
 				if (folio_test_hugetlb(folio))
 					set_huge_pte_at(mm, address, pvmw.pte,
@@ -2283,8 +2255,6 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 				page_vma_mapped_walk_done(&pvmw);
 				break;
 			}
-			VM_BUG_ON_PAGE(pte_write(pteval) && folio_test_anon(folio) &&
-				       !anon_exclusive, subpage);
 
 			/* See folio_try_share_anon_rmap_pte(): clear PTE first. */
 			if (folio_test_hugetlb(folio)) {
@@ -2309,7 +2279,7 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			 * pte. do_swap_page() will wait until the migration
 			 * pte is removed and then restart fault handling.
 			 */
-			if (pte_write(pteval))
+			if (writable)
 				entry = make_writable_migration_entry(
 							page_to_pfn(subpage));
 			else if (anon_exclusive)
@@ -2318,15 +2288,23 @@ static bool try_to_migrate_one(struct folio *folio, struct vm_area_struct *vma,
 			else
 				entry = make_readable_migration_entry(
 							page_to_pfn(subpage));
-			if (pte_young(pteval))
-				entry = make_migration_entry_young(entry);
-			if (pte_dirty(pteval))
-				entry = make_migration_entry_dirty(entry);
-			swp_pte = swp_entry_to_pte(entry);
-			if (pte_soft_dirty(pteval))
-				swp_pte = pte_swp_mksoft_dirty(swp_pte);
-			if (pte_uffd_wp(pteval))
-				swp_pte = pte_swp_mkuffd_wp(swp_pte);
+			if (likely(pte_present(pteval))) {
+				if (pte_young(pteval))
+					entry = make_migration_entry_young(entry);
+				if (pte_dirty(pteval))
+					entry = make_migration_entry_dirty(entry);
+				swp_pte = swp_entry_to_pte(entry);
+				if (pte_soft_dirty(pteval))
+					swp_pte = pte_swp_mksoft_dirty(swp_pte);
+				if (pte_uffd_wp(pteval))
+					swp_pte = pte_swp_mkuffd_wp(swp_pte);
+			} else {
+				swp_pte = swp_entry_to_pte(entry);
+				if (pte_swp_soft_dirty(pteval))
+					swp_pte = pte_swp_mksoft_dirty(swp_pte);
+				if (pte_swp_uffd_wp(pteval))
+					swp_pte = pte_swp_mkuffd_wp(swp_pte);
+			}
 			if (folio_test_hugetlb(folio))
 				set_huge_pte_at(mm, address, pvmw.pte, swp_pte,
 						hsz);
-- 
2.48.1