Re: [PATCH 5/8] migrate_pages: batch _unmap and _move

Zi Yan
2023-01-03T19:01:59Z

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On 26 Dec 2022, at 19:28, Huang Ying wrote:

> In this patch the _unmap and _move stage of the folio migration is
> batched. That for, previously, it is,
>
> for each folio
> _unmap()
> _move()
>
> Now, it is,
>
> for each folio
> _unmap()
> for each folio
> _move()

Also worth adding some notes here, we need extra code to undo the _unmap()
if _move() fails. Andrew has asked for comments on *_undo_src/dst(),
but I think it would be better to provide a high level new workflow,
in the form of pseudo code, in git log and the comment for migrate_pages().
The extra cleanup code for a failed _move() with a previously successful
_unmap() might not be obvious to everyone.

>
> Based on this, we can batch the TLB flushing and use some hardware
> accelerator to copy folios between batched _unmap and batched _move
> stages.
>
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Bharata B Rao <bharata@amd.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: haoxin <xhao@linux.alibaba.com>
> ---
> mm/migrate.c | 189 ++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 165 insertions(+), 24 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e2383b430932..dd68c3de3da8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1027,8 +1027,32 @@ static void __migrate_folio_extract(struct folio *dst,
> dst->private = NULL;
> }
>
> +static void migrate_folio_undo_src(struct folio *src,
> + int page_was_mapped,
> + struct anon_vma *anon_vma,
> + struct list_head *ret)
> +{
> + if (page_was_mapped)
> + remove_migration_ptes(src, src, false);
> + if (anon_vma)
> + put_anon_vma(anon_vma);
> + folio_unlock(src);
> + list_move_tail(&src->lru, ret);
> +}
> +
> +static void migrate_folio_undo_dst(struct folio *dst,
> + free_page_t put_new_page,
> + unsigned long private)
> +{
> + folio_unlock(dst);
> + if (put_new_page)
> + put_new_page(&dst->page, private);
> + else
> + folio_put(dst);
> +}
> +
> static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
> - int force, enum migrate_mode mode)
> + int force, bool force_lock, enum migrate_mode mode)
> {
> int rc = -EAGAIN;
> int page_was_mapped = 0;
> @@ -1055,6 +1079,11 @@ static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
> if (current->flags & PF_MEMALLOC)
> goto out;
>
> + if (!force_lock) {
> + rc = -EDEADLOCK;
> + goto out;
> + }
> +
> folio_lock(src);
> }
>
> @@ -1168,6 +1197,8 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
>
> rc = move_to_new_folio(dst, src, mode);
>
> + if (rc != -EAGAIN)
> + list_del(&dst->lru);
> /*
> * When successful, push dst to LRU immediately: so that if it
> * turns out to be an mlocked page, remove_migration_ptes() will
> @@ -1183,6 +1214,11 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
> lru_add_drain();
> }
>
> + if (rc == -EAGAIN) {
> + __migrate_folio_record(dst, page_was_mapped, anon_vma);
> + return rc;
> + }
> +
> if (page_was_mapped)
> remove_migration_ptes(src,
> rc == MIGRATEPAGE_SUCCESS ? dst : src, false);
> @@ -1223,7 +1259,7 @@ static void migrate_folio_done(struct folio *src,
> /* Obtain the lock on page, remove all ptes. */
> static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page,
> unsigned long private, struct folio *src,
> - struct folio **dstp, int force,
> + struct folio **dstp, int force, bool force_lock,
> enum migrate_mode mode, enum migrate_reason reason,
> struct list_head *ret)
> {
> @@ -1251,7 +1287,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
> *dstp = dst;
>
> dst->private = NULL;
> - rc = __migrate_folio_unmap(src, dst, force, mode);
> + rc = __migrate_folio_unmap(src, dst, force, force_lock, mode);
> if (rc == MIGRATEPAGE_UNMAP)
> return rc;
>
> @@ -1260,7 +1296,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
> * references and be restored.
> */
> /* restore the folio to right list. */
> - if (rc != -EAGAIN)
> + if (rc != -EAGAIN && rc != -EDEADLOCK)
> list_move_tail(&src->lru, ret);
>
> if (put_new_page)
> @@ -1299,9 +1335,8 @@ static int migrate_folio_move(free_page_t put_new_page, unsigned long private,
> */
> if (rc == MIGRATEPAGE_SUCCESS) {
> migrate_folio_done(src, reason);
> - } else {
> - if (rc != -EAGAIN)
> - list_add_tail(&src->lru, ret);
> + } else if (rc != -EAGAIN) {
> + list_add_tail(&src->lru, ret);
>
> if (put_new_page)
> put_new_page(&dst->page, private);
> @@ -1564,7 +1599,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> enum migrate_mode mode, int reason, struct list_head *ret_folios,
> struct migrate_pages_stats *stats)
> {
> - int retry = 1;
> + int retry;
> int large_retry = 1;
> int thp_retry = 1;
> int nr_failed = 0;
> @@ -1573,13 +1608,19 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> int pass = 0;
> bool is_large = false;
> bool is_thp = false;
> - struct folio *folio, *folio2, *dst = NULL;
> - int rc, nr_pages;
> + struct folio *folio, *folio2, *dst = NULL, *dst2;
> + int rc, rc_saved, nr_pages;
> LIST_HEAD(split_folios);
> + LIST_HEAD(unmap_folios);
> + LIST_HEAD(dst_folios);
> bool nosplit = (reason == MR_NUMA_MISPLACED);
> bool no_split_folio_counting = false;
> + bool force_lock;
>
> -split_folio_migration:
> +retry:
> + rc_saved = 0;
> + force_lock = true;
> + retry = 1;
> for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
> retry = 0;
> large_retry = 0;
> @@ -1599,16 +1640,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> cond_resched();
>
> rc = migrate_folio_unmap(get_new_page, put_new_page, private,
> - folio, &dst, pass > 2, mode,
> - reason, ret_folios);
> - if (rc == MIGRATEPAGE_UNMAP)
> - rc = migrate_folio_move(put_new_page, private,
> - folio, dst, mode,
> - reason, ret_folios);
> + folio, &dst, pass > 2, force_lock,
> + mode, reason, ret_folios);
> /*
> * The rules are:
> * Success: folio will be freed
> + * Unmap: folio will be put on unmap_folios list,
> + * dst folio put on dst_folios list
> * -EAGAIN: stay on the from list
> + * -EDEADLOCK: stay on the from list
> * -ENOMEM: stay on the from list
> * -ENOSYS: stay on the from list
> * Other errno: put on ret_folios list
> @@ -1643,7 +1683,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> case -ENOMEM:
> /*
> * When memory is low, don't bother to try to migrate
> - * other folios, just exit.
> + * other folios, move unmapped folios, then exit.
> */
> if (is_large) {
> nr_large_failed++;
> @@ -1682,7 +1722,14 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> /* nr_failed isn't updated for not used */
> nr_large_failed += large_retry;
> stats->nr_thp_failed += thp_retry;
> - goto out;
> + rc_saved = rc;
> + if (list_empty(&unmap_folios))
> + goto out;
> + else
> + goto move;
> + case -EDEADLOCK:
> + rc_saved = rc;
> + goto move;
> case -EAGAIN:
> if (is_large) {
> large_retry++;
> @@ -1696,6 +1743,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> stats->nr_succeeded += nr_pages;
> stats->nr_thp_succeeded += is_thp;
> break;
> + case MIGRATEPAGE_UNMAP:
> + /*
> + * We have locked some pages, don't force lock
> + * to avoid deadlock.
> + */
> + force_lock = false;
> + list_move_tail(&folio->lru, &unmap_folios);
> + list_add_tail(&dst->lru, &dst_folios);
> + break;
> default:
> /*
> * Permanent failure (-EBUSY, etc.):
> @@ -1719,12 +1775,93 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> nr_large_failed += large_retry;
> stats->nr_thp_failed += thp_retry;
> stats->nr_failed_pages += nr_retry_pages;
> +move:
> + retry = 1;
> + for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
> + retry = 0;
> + large_retry = 0;
> + thp_retry = 0;
> + nr_retry_pages = 0;
> +
> + dst = list_first_entry(&dst_folios, struct folio, lru);
> + dst2 = list_next_entry(dst, lru);
> + list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
> + is_large = folio_test_large(folio);
> + is_thp = is_large && folio_test_pmd_mappable(folio);
> + nr_pages = folio_nr_pages(folio);
> +
> + cond_resched();
> +
> + rc = migrate_folio_move(put_new_page, private,
> + folio, dst, mode,
> + reason, ret_folios);
> + /*
> + * The rules are:
> + * Success: folio will be freed
> + * -EAGAIN: stay on the unmap_folios list
> + * Other errno: put on ret_folios list
> + */
> + switch(rc) {
> + case -EAGAIN:
> + if (is_large) {
> + large_retry++;
> + thp_retry += is_thp;
> + } else if (!no_split_folio_counting) {
> + retry++;
> + }
> + nr_retry_pages += nr_pages;
> + break;
> + case MIGRATEPAGE_SUCCESS:
> + stats->nr_succeeded += nr_pages;
> + stats->nr_thp_succeeded += is_thp;
> + break;
> + default:
> + if (is_large) {
> + nr_large_failed++;
> + stats->nr_thp_failed += is_thp;
> + } else if (!no_split_folio_counting) {
> + nr_failed++;
> + }
> +
> + stats->nr_failed_pages += nr_pages;
> + break;
> + }
> + dst = dst2;
> + dst2 = list_next_entry(dst, lru);
> + }
> + }
> + nr_failed += retry;
> + nr_large_failed += large_retry;
> + stats->nr_thp_failed += thp_retry;
> + stats->nr_failed_pages += nr_retry_pages;
> +
> + if (rc_saved)
> + rc = rc_saved;
> + else
> + rc = nr_failed + nr_large_failed;
> +out:
> + /* Cleanup remaining folios */
> + dst = list_first_entry(&dst_folios, struct folio, lru);
> + dst2 = list_next_entry(dst, lru);
> + list_for_each_entry_safe(folio, folio2, &unmap_folios, lru) {
> + int page_was_mapped = 0;
> + struct anon_vma *anon_vma = NULL;
> +
> + __migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
> + migrate_folio_undo_src(folio, page_was_mapped, anon_vma,
> + ret_folios);
> + list_del(&dst->lru);
> + migrate_folio_undo_dst(dst, put_new_page, private);
> + dst = dst2;
> + dst2 = list_next_entry(dst, lru);
> + }
> +
> /*
> * Try to migrate split folios of fail-to-migrate large folios, no
> * nr_failed counting in this round, since all split folios of a
> * large folio is counted as 1 failure in the first round.
> */
> - if (!list_empty(&split_folios)) {
> + if (rc >= 0 && !list_empty(&split_folios)) {
> /*
> * Move non-migrated folios (after 10 retries) to ret_folios
> * to avoid migrating them again.
> @@ -1732,12 +1869,16 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
> list_splice_init(from, ret_folios);
> list_splice_init(&split_folios, from);
> no_split_folio_counting = true;
> - retry = 1;
> - goto split_folio_migration;
> + goto retry;
> }
>
> - rc = nr_failed + nr_large_failed;
> -out:
> + /*
> + * We have unlocked all locked pages, so we can force lock now, let's
> + * try again.
> + */
> + if (rc == -EDEADLOCK)
> + goto retry;
> +
> return rc;
> }
>
> --
> 2.35.1

--
Best Regards,
Yan, Zi

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