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Date: Tue, 07 Feb 2023 13:58:55 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>,
Hyeonggon Yoo <42.hyeyoo@...il.com>,
Yang Shi <shy828301@...il.com>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Oscar Salvador" <osalvador@...e.de>,
Matthew Wilcox <willy@...radead.org>,
"Bharata B Rao" <bharata@....com>,
Alistair Popple <apopple@...dia.com>,
haoxin <xhao@...ux.alibaba.com>,
Minchan Kim <minchan@...nel.org>,
Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH -v4 5/9] migrate_pages: batch _unmap and _move
Zi Yan <ziy@...dia.com> writes:
> On 6 Feb 2023, at 1:33, 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()
>>
>> 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@...el.com>
>> Tested-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
>> Cc: Zi Yan <ziy@...dia.com>
>> Cc: Yang Shi <shy828301@...il.com>
>> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Matthew Wilcox <willy@...radead.org>
>> Cc: Bharata B Rao <bharata@....com>
>> Cc: Alistair Popple <apopple@...dia.com>
>> Cc: haoxin <xhao@...ux.alibaba.com>
>> Cc: Minchan Kim <minchan@...nel.org>
>> Cc: Mike Kravetz <mike.kravetz@...cle.com>
>> ---
>> mm/migrate.c | 208 +++++++++++++++++++++++++++++++++++++++++++++------
>> 1 file changed, 184 insertions(+), 24 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 0428449149f4..fa7212330cb6 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1033,6 +1033,33 @@ static void __migrate_folio_extract(struct folio *dst,
>> dst->private = NULL;
>> }
>>
>> +/* Restore the source folio to the original state upon failure */
>> +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);
>> + /* Drop an anon_vma reference if we took one */
>> + if (anon_vma)
>> + put_anon_vma(anon_vma);
>> + folio_unlock(src);
>> + list_move_tail(&src->lru, ret);
>> +}
>> +
>> +/* Restore the destination folio to the original state upon failure */
>> +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);
>> +}
>> +
>> /* Cleanup src folio upon migration success */
>> static void migrate_folio_done(struct folio *src,
>> enum migrate_reason reason)
>> @@ -1052,7 +1079,7 @@ static void migrate_folio_done(struct folio *src,
>> }
>>
>> 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;
>> @@ -1079,6 +1106,17 @@ static int __migrate_folio_unmap(struct folio *src, struct folio *dst,
>> if (current->flags & PF_MEMALLOC)
>> goto out;
>>
>> + /*
>> + * We have locked some folios, to avoid deadlock, we cannot
>> + * lock the folio synchronously. Go out to process (and
>> + * unlock) all the locked folios. Then we can lock the folio
>> + * synchronously.
>> + */
> The comment alone is quite confusing and the variable might be better
> renamed to avoid_force_lock, since there is a force variable to force
> lock folio already. And the variable intends to discourage force lock
> on a folio to avoid potential deadlock.
OK. Will rename "force_lock" to "avoid_force_lock" in the next version.
> How about? Since "lock synchronously" might not be as straightforward
> as wait to lock.
>
> /*
> * We have locked some folios and are going to wait to lock this folio.
> * To avoid a potential deadlock, let's bail out and not do that. The
> * locked folios will be moved and unlocked, then we can wait to lock
> * this folio
> */
Thanks! It looks better. Will use it in the next version.
Best Regards,
Huang, Ying
>> + if (!force_lock) {
>> + rc = -EDEADLOCK;
>> + goto out;
>> + }
>> +
>> folio_lock(src);
>> }
>>
>> @@ -1187,10 +1225,20 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
>> int page_was_mapped = 0;
>> struct anon_vma *anon_vma = NULL;
>> bool is_lru = !__PageMovable(&src->page);
>> + struct list_head *prev;
>>
>> __migrate_folio_extract(dst, &page_was_mapped, &anon_vma);
>> + prev = dst->lru.prev;
>> + list_del(&dst->lru);
>>
>> rc = move_to_new_folio(dst, src, mode);
>> +
>> + if (rc == -EAGAIN) {
>> + list_add(&dst->lru, prev);
>> + __migrate_folio_record(dst, page_was_mapped, anon_vma);
>> + return rc;
>> + }
>> +
>> if (unlikely(!is_lru))
>> goto out_unlock_both;
>>
>> @@ -1233,7 +1281,7 @@ static int __migrate_folio_move(struct folio *src, struct folio *dst,
>> /* 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)
>> {
>> @@ -1261,7 +1309,7 @@ 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;
>>
>> @@ -1270,7 +1318,7 @@ 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)
>> @@ -1309,9 +1357,8 @@ 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);
>> @@ -1591,7 +1638,7 @@ 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)
>
> Like I said in my last comment to this patch, migrate_pages_batch() function
> deserves a detailed comment about its working flow including the error handling.
> Now you only put some in the git log, which is hard to access after several code
> changes later.
>
> How about?
>
> /*
> * migrate_pages_batch() first unmaps pages in the from as many as possible,
> * then migrates the unmapped pages. During unmap process, different situations
> * are handled differently:
> * 1. ENOSYS, unsupported large folio migration: move to ret_folios list
> * 2. ENOMEM, lower memory at the destination: migrate existing unmapped folios
> * and stop, since existing unmapped folios have new pages allocated and can
> * be migrated
> * 3. EDEADLOCK, to be unmapped page is locked by someone else, to avoid deadlock,
> * we migrate existing unmapped pages and try to lock again
> * 4. MIGRATEPAGE_SUCCESS, the folios was freed under us: no action
> * 5. MIGRATEPAGE_UNMAP, unmap succeeded: set avoid_force_lock to true to avoid
> * wait to lock a folio in the future to avoid deadlock.
> *
> * For folios unmapped but cannot be migrated, we will restore their original
> * states during cleanup stage at the end.
> */
>
>> {
>> - int retry = 1;
>> + int retry;
>> int large_retry = 1;
>> int thp_retry = 1;
>> int nr_failed = 0;
>> @@ -1600,13 +1647,19 @@ 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 < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
>> pass++) {
>> @@ -1628,16 +1681,15 @@ 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
>> @@ -1672,7 +1724,7 @@ 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++;
>> @@ -1711,7 +1763,19 @@ 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:
>> + /*
>> + * The folio cannot be locked for potential deadlock.
>> + * Go move (and unlock) all locked folios. Then we can
>> + * try again.
>> + */
>> + rc_saved = rc;
>> + goto move;
>> case -EAGAIN:
>> if (is_large) {
>> large_retry++;
>> @@ -1725,6 +1789,15 @@ 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 folios, 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.):
>> @@ -1748,12 +1821,95 @@ 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 < NR_MAX_MIGRATE_PAGES_RETRY && (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 NR_MAX_MIGRATE_PAGES_RETRY
>> * retries) to ret_folios to avoid migrating them again.
>> @@ -1761,12 +1917,16 @@ 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 folios, so we can force lock now, let's
>> + * try again.
>> + */
>> + if (rc == -EDEADLOCK)
>> + goto retry;
>> +
>> return rc;
>> }
>>
>> --
>> 2.35.1
>
> After rename the variable (or give it a better name) and add the comments,
> you can add Reviewed-by: Zi Yan <ziy@...dia.com>
>
> Thanks.
>
> --
> Best Regards,
> Yan, Zi
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