This is a preparatory patch that enables batch copying for folios
undergoing migration. By enabling batch copying the folio content, we can
efficiently utilize the capabilities of DMA hardware or multi-threaded
folio copy. It also adds MIGRATE_NO_COPY back to migrate_mode, so that
folio copy will be skipped during metadata copy process and performed
in a batch later.
Currently, the folio move operation is performed individually for each
folio in sequential manner:
for_each_folio() {
Copy folio metadata like flags and mappings
Copy the folio content from src to dst
Update page tables with dst folio
}
With this patch, we transition to a batch processing approach as shown
below:
for_each_folio() {
Copy folio metadata like flags and mappings
}
Batch copy all src folios to dst
for_each_folio() {
Update page tables with dst folios
}
dst->private is used to store page states and possible anon_vma value,
thus needs to be cleared during metadata copy process. To avoid additional
memory allocation to store the data during batch copy process, src->private
is used to store the data after metadata copy process, since src is no
longer used.
Originally-by: Shivank Garg <shivankg@xxxxxxx>
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
---
include/linux/migrate_mode.h | 2 +
mm/migrate.c | 207 +++++++++++++++++++++++++++++++++--
2 files changed, 201 insertions(+), 8 deletions(-)
diff --git a/include/linux/migrate_mode.h b/include/linux/migrate_mode.h
index 265c4328b36a..9af6c949a057 100644
--- a/include/linux/migrate_mode.h
+++ b/include/linux/migrate_mode.h
@@ -7,11 +7,13 @@
* on most operations but not ->writepage as the potential stall time
* is too significant
* MIGRATE_SYNC will block when migrating pages
+ * MIGRATE_NO_COPY will not copy page content
*/
enum migrate_mode {
MIGRATE_ASYNC,
MIGRATE_SYNC_LIGHT,
MIGRATE_SYNC,
+ MIGRATE_NO_COPY,
};
enum migrate_reason {
diff --git a/mm/migrate.c b/mm/migrate.c
index a83508f94c57..95c4cc4a7823 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -51,6 +51,7 @@
#include "internal.h"
+
bool isolate_movable_page(struct page *page, isolate_mode_t mode)
{
struct folio *folio = folio_get_nontail_page(page);
@@ -752,14 +753,19 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
enum migrate_mode mode)
{
int rc, expected_count = folio_expected_refs(mapping, src);
+ unsigned long dst_private = (unsigned long)dst->private;
/* Check whether src does not have extra refs before we do more work */
if (folio_ref_count(src) != expected_count)
return -EAGAIN;
- rc = folio_mc_copy(dst, src);
- if (unlikely(rc))
- return rc;
+ if (mode == MIGRATE_NO_COPY)
+ dst->private = NULL;
+ else {
+ rc = folio_mc_copy(dst, src);
+ if (unlikely(rc))
+ return rc;
+ }
rc = __folio_migrate_mapping(mapping, dst, src, expected_count);
if (rc != MIGRATEPAGE_SUCCESS)
@@ -769,6 +775,10 @@ static int __migrate_folio(struct address_space *mapping, struct folio *dst,
folio_attach_private(dst, folio_detach_private(src));
folio_migrate_flags(dst, src);
+
+ if (mode == MIGRATE_NO_COPY)
+ src->private = (void *)dst_private;
+
return MIGRATEPAGE_SUCCESS;
}
@@ -1042,7 +1052,7 @@ static int _move_to_new_folio_prep(struct folio *dst, struct folio *src,
mode);
else
rc = fallback_migrate_folio(mapping, dst, src, mode);
- } else {
+ } else if (mode != MIGRATE_NO_COPY) {
const struct movable_operations *mops;
/*
@@ -1060,7 +1070,8 @@ static int _move_to_new_folio_prep(struct folio *dst, struct folio *src,
rc = mops->migrate_page(&dst->page, &src->page, mode);
WARN_ON_ONCE(rc == MIGRATEPAGE_SUCCESS &&
!folio_test_isolated(src));
- }
+ } else
+ rc = -EAGAIN;
out:
return rc;
}
@@ -1138,7 +1149,7 @@ static void __migrate_folio_record(struct folio *dst,
dst->private = (void *)anon_vma + old_page_state;
}
-static void __migrate_folio_extract(struct folio *dst,
+static void __migrate_folio_read(struct folio *dst,
int *old_page_state,
struct anon_vma **anon_vmap)
{
@@ -1146,6 +1157,13 @@ static void __migrate_folio_extract(struct folio *dst,
*anon_vmap = (struct anon_vma *)(private & ~PAGE_OLD_STATES);
*old_page_state = private & PAGE_OLD_STATES;
+}
+
+static void __migrate_folio_extract(struct folio *dst,
+ int *old_page_state,
+ struct anon_vma **anon_vmap)
+{
+ __migrate_folio_read(dst, old_page_state, anon_vmap);
dst->private = NULL;
}
@@ -1771,6 +1789,174 @@ static void migrate_folios_move(struct list_head *src_folios,
}
}
+static void migrate_folios_batch_move(struct list_head *src_folios,
+ struct list_head *dst_folios,
+ free_folio_t put_new_folio, unsigned long private,
+ enum migrate_mode mode, int reason,
+ struct list_head *ret_folios,
+ struct migrate_pages_stats *stats,
+ int *retry, int *thp_retry, int *nr_failed,
+ int *nr_retry_pages)
+{
+ struct folio *folio, *folio2, *dst, *dst2;
+ int rc, nr_pages = 0, nr_mig_folios = 0;
+ int old_page_state = 0;
+ struct anon_vma *anon_vma = NULL;
+ bool is_lru;
+ int is_thp = 0;
+ LIST_HEAD(err_src);
+ LIST_HEAD(err_dst);
+
+ if (mode != MIGRATE_ASYNC) {
+ *retry += 1;
+ return;
+ }
+
+ /*
+ * Iterate over the list of locked src/dst folios to copy the metadata
+ */
+ dst = list_first_entry(dst_folios, struct folio, lru);
+ dst2 = list_next_entry(dst, lru);
+ list_for_each_entry_safe(folio, folio2, src_folios, lru) {
+ is_thp = folio_test_large(folio) && folio_test_pmd_mappable(folio);
+ nr_pages = folio_nr_pages(folio);
+ is_lru = !__folio_test_movable(folio);
+
+ /*
+ * dst->private is not cleared here. It is cleared and moved to
+ * src->private in __migrate_folio().
+ */
+ __migrate_folio_read(dst, &old_page_state, &anon_vma);
+
+ /*
+ * Use MIGRATE_NO_COPY mode in migrate_folio family functions
+ * to copy the flags, mapping and some other ancillary information.
+ * This does everything except the page copy. The actual page copy
+ * is handled later in a batch manner.
+ */
+ rc = _move_to_new_folio_prep(dst, folio, MIGRATE_NO_COPY);
+
+ /*
+ * -EAGAIN: Move src/dst folios to tmp lists for retry
+ * Other Errno: Put src folio on ret_folios list, remove the dst folio
+ * Success: Copy the folio bytes, restoring working pte, unlock and
+ * decrement refcounter
+ */
+ if (rc == -EAGAIN) {
+ *retry += 1;
+ *thp_retry += is_thp;
+ *nr_retry_pages += nr_pages;
+
+ list_move_tail(&folio->lru, &err_src);
+ list_move_tail(&dst->lru, &err_dst);
+ __migrate_folio_record(dst, old_page_state, anon_vma);
+ } else if (rc != MIGRATEPAGE_SUCCESS) {
+ *nr_failed += 1;
+ stats->nr_thp_failed += is_thp;
+ stats->nr_failed_pages += nr_pages;
+
+ list_del(&dst->lru);
+ migrate_folio_undo_src(folio, old_page_state & PAGE_WAS_MAPPED,
+ anon_vma, true, ret_folios);
+ migrate_folio_undo_dst(dst, true, put_new_folio, private);
+ } else /* MIGRATEPAGE_SUCCESS */
+ nr_mig_folios++;
+
+ dst = dst2;
+ dst2 = list_next_entry(dst, lru);
+ }
+
+ /* Exit if folio list for batch migration is empty */
+ if (!nr_mig_folios)
+ goto out;
+
+ /* Batch copy the folios */
+ {
+ dst = list_first_entry(dst_folios, struct folio, lru);
+ dst2 = list_next_entry(dst, lru);
+ list_for_each_entry_safe(folio, folio2, src_folios, lru) {
+ is_thp = folio_test_large(folio) &&
+ folio_test_pmd_mappable(folio);
+ nr_pages = folio_nr_pages(folio);
+ rc = folio_mc_copy(dst, folio);
+
+ if (rc) {
+ int old_page_state = 0;
+ struct anon_vma *anon_vma = NULL;
+
+ /*
+ * dst->private is moved to src->private in
+ * __migrate_folio(), so page state and anon_vma
+ * values can be extracted from (src) folio.
+ */
+ __migrate_folio_extract(folio, &old_page_state,
+ &anon_vma);
+ migrate_folio_undo_src(folio,
+ old_page_state & PAGE_WAS_MAPPED,
+ anon_vma, true, ret_folios);
+ list_del(&dst->lru);
+ migrate_folio_undo_dst(dst, true, put_new_folio,
+ private);
+ }
+
+ switch (rc) {
+ case MIGRATEPAGE_SUCCESS:
+ stats->nr_succeeded += nr_pages;
+ stats->nr_thp_succeeded += is_thp;
+ break;
+ default:
+ *nr_failed += 1;
+ stats->nr_thp_failed += is_thp;
+ stats->nr_failed_pages += nr_pages;
+ break;
+ }
+
+ dst = dst2;
+ dst2 = list_next_entry(dst, lru);
+ }
+ }
+
+ /*
+ * Iterate the folio lists to remove migration pte and restore them
+ * as working pte. Unlock the folios, add/remove them to LRU lists (if
+ * applicable) and release the src folios.
+ */
+ dst = list_first_entry(dst_folios, struct folio, lru);
+ dst2 = list_next_entry(dst, lru);
+ list_for_each_entry_safe(folio, folio2, src_folios, lru) {
+ is_thp = folio_test_large(folio) && folio_test_pmd_mappable(folio);
+ nr_pages = folio_nr_pages(folio);
+ /*
+ * dst->private is moved to src->private in __migrate_folio(),
+ * so page state and anon_vma values can be extracted from
+ * (src) folio.
+ */
+ __migrate_folio_extract(folio, &old_page_state, &anon_vma);
+ list_del(&dst->lru);
+
+ _move_to_new_folio_finalize(dst, folio, MIGRATEPAGE_SUCCESS);
+
+ /*
+ * Below few steps are only applicable for lru pages which is
+ * ensured as we have removed the non-lru pages from our list.
+ */
+ _migrate_folio_move_finalize1(folio, dst, old_page_state);
+
+ _migrate_folio_move_finalize2(folio, dst, reason, anon_vma);
+
+ /* Page migration successful, increase stat counter */
+ stats->nr_succeeded += nr_pages;
+ stats->nr_thp_succeeded += is_thp;
+
+ dst = dst2;
+ dst2 = list_next_entry(dst, lru);
+ }
+out:
+ /* Add tmp folios back to the list to let CPU re-attempt migration. */
+ list_splice(&err_src, src_folios);
+ list_splice(&err_dst, dst_folios);
+}
+
static void migrate_folios_undo(struct list_head *src_folios,
struct list_head *dst_folios,
free_folio_t put_new_folio, unsigned long private,
@@ -1981,13 +2167,18 @@ static int migrate_pages_batch(struct list_head *from,
/* Flush TLBs for all unmapped folios */
try_to_unmap_flush();
- retry = 1;
+ retry = 0;
+ /* Batch move the unmapped folios */
+ migrate_folios_batch_move(&unmap_folios, &dst_folios, put_new_folio,
+ private, mode, reason, ret_folios, stats, &retry,
+ &thp_retry, &nr_failed, &nr_retry_pages);
+
for (pass = 0; pass < nr_pass && retry; pass++) {
retry = 0;
thp_retry = 0;
nr_retry_pages = 0;
- /* Move the unmapped folios */
+ /* Move the remaining unmapped folios */
migrate_folios_move(&unmap_folios, &dst_folios,
put_new_folio, private, mode, reason,
ret_folios, stats, &retry, &thp_retry,
--
2.45.2
