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Message-ID: <87ilhd4yic.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Wed, 11 Jan 2023 10:04:59 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
Minchan Kim <minchan.kim@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, Zi Yan <ziy@...dia.com>,
Yang Shi <shy828301@...il.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>
Subject: Re: [PATCH -v2 2/9] migrate_pages: separate hugetlb folios migration
Baolin Wang <baolin.wang@...ux.alibaba.com> writes:
> On 1/10/2023 3:53 PM, Huang Ying wrote:
>> This is a preparation patch to batch the folio unmapping and moving
>> for the non-hugetlb folios. Based on that we can batch the TLB
>> shootdown during the folio migration and make it possible to use some
>> hardware accelerator for the folio copying.
>> In this patch the hugetlb folios and non-hugetlb folios migration is
>> separated in migrate_pages() to make it easy to change the non-hugetlb
>> folios migration implementation.
>> Signed-off-by: "Huang, Ying" <ying.huang@...el.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>
>> ---
>> mm/migrate.c | 141 +++++++++++++++++++++++++++++++++++++++++++--------
>> 1 file changed, 119 insertions(+), 22 deletions(-)
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index d21de40861a0..04e6802c236c 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1396,6 +1396,8 @@ static inline int try_split_folio(struct folio *folio, struct list_head *split_f
>> return rc;
>> }
>> +#define NR_MAX_MIGRATE_PAGES_RETRY 10
>> +
>> struct migrate_pages_stats {
>> int nr_succeeded; /* Normal pages and THP migrated successfully, in units
>> of base pages */
>> @@ -1406,6 +1408,95 @@ struct migrate_pages_stats {
>> int nr_thp_split; /* THP split before migrating */
>> };
>> +/*
>> + * Returns the number of hugetlb folios that were not migrated, or an error code
>> + * after NR_MAX_MIGRATE_PAGES_RETRY attempts or if no hugetlb folios are movable
>> + * any more because the list has become empty or no retryable hugetlb folios
>> + * exist any more. It is caller's responsibility to call putback_movable_pages()
>> + * to return hugetlb folios to the LRU or free list only if ret != 0.
>
> This can be confused, since we did not put the hugetlb back into any
> LRU by putback_movable_pages(), right?
Yes. Thanks for pointing this out.
And I'm confused about when the returned folios (hugetlb and
non-hugetlb) will be put into free list (that is, freed). Checked all
callers and found that the returned folios will not be freed. So
checked the code history with `git blame`. It turns out the words were
added to comments in commit aaa994b300a1 ("[PATCH] page migration:
handle freeing of pages in migrate_pages()"). Where the failed
migrated pages will be put to LRU or free list by migrate_pages()
itself. Then in commit cf608ac19c95 ("mm: compaction: fix
COMPACTPAGEFAILED counting"), putback_lru_pages() are moved out of
migrate_pages(), while freeing pages are still there. But the comments
were not updated.
So, it appears that we need to update the comments in migrate_pages()
too.
> The code look good to me.
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
Thanks!
Best Regards,
Huang, Ying
>> + */
>> +static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
>> + free_page_t put_new_page, unsigned long private,
>> + enum migrate_mode mode, int reason,
>> + struct migrate_pages_stats *stats,
>> + struct list_head *ret_folios)
>> +{
>> + int retry = 1;
>> + int nr_failed = 0;
>> + int nr_retry_pages = 0;
>> + int pass = 0;
>> + struct folio *folio, *folio2;
>> + int rc, nr_pages;
>> +
>> + for (pass = 0; pass < NR_MAX_MIGRATE_PAGES_RETRY && retry; pass++) {
>> + retry = 0;
>> + nr_retry_pages = 0;
>> +
>> + list_for_each_entry_safe(folio, folio2, from, lru) {
>> + if (!folio_test_hugetlb(folio))
>> + continue;
>> +
>> + nr_pages = folio_nr_pages(folio);
>> +
>> + cond_resched();
>> +
>> + rc = unmap_and_move_huge_page(get_new_page,
>> + put_new_page, private,
>> + &folio->page, pass > 2, mode,
>> + reason, ret_folios);
>> + /*
>> + * The rules are:
>> + * Success: hugetlb folio will be put back
>> + * -EAGAIN: stay on the from list
>> + * -ENOMEM: stay on the from list
>> + * -ENOSYS: stay on the from list
>> + * Other errno: put on ret_folios list
>> + */
>> + switch(rc) {
>> + case -ENOSYS:
>> + /* Hugetlb migration is unsupported */
>> + nr_failed++;
>> + stats->nr_failed_pages += nr_pages;
>> + list_move_tail(&folio->lru, ret_folios);
>> + break;
>> + case -ENOMEM:
>> + /*
>> + * When memory is low, don't bother to try to migrate
>> + * other folios, just exit.
>> + */
>> + stats->nr_failed_pages += nr_pages + nr_retry_pages;
>> + return -ENOMEM;
>> + case -EAGAIN:
>> + retry++;
>> + nr_retry_pages += nr_pages;
>> + break;
>> + case MIGRATEPAGE_SUCCESS:
>> + stats->nr_succeeded += nr_pages;
>> + break;
>> + default:
>> + /*
>> + * Permanent failure (-EBUSY, etc.):
>> + * unlike -EAGAIN case, the failed folio is
>> + * removed from migration folio list and not
>> + * retried in the next outer loop.
>> + */
>> + nr_failed++;
>> + stats->nr_failed_pages += nr_pages;
>> + break;
>> + }
>> + }
>> + }
>> + /*
>> + * nr_failed is number of hugetlb folios failed to be migrated. After
>> + * NR_MAX_MIGRATE_PAGES_RETRY attempts, give up and count retried hugetlb
>> + * folios as failed.
>> + */
>> + nr_failed += retry;
>> + stats->nr_failed_pages += nr_retry_pages;
>> +
>> + return nr_failed;
>> +}
>> +
>> /*
>> * migrate_pages - migrate the folios specified in a list, to the free folios
>> * supplied as the target for the page migration
>> @@ -1422,10 +1513,10 @@ struct migrate_pages_stats {
>> * @ret_succeeded: Set to the number of folios migrated successfully if
>> * the caller passes a non-NULL pointer.
>> *
>> - * The function returns after 10 attempts or if no folios are movable any more
>> - * because the list has become empty or no retryable folios exist any more.
>> - * It is caller's responsibility to call putback_movable_pages() to return folios
>> - * to the LRU or free list only if ret != 0.
>> + * The function returns after NR_MAX_MIGRATE_PAGES_RETRY attempts or if no folios
>> + * are movable any more because the list has become empty or no retryable folios
>> + * exist any more. It is caller's responsibility to call putback_movable_pages()
>> + * to return folios to the LRU or free list only if ret != 0.
>> *
>> * Returns the number of {normal folio, large folio, hugetlb} that were not
>> * migrated, or an error code. The number of large folio splits will be
>> @@ -1439,7 +1530,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> int retry = 1;
>> int large_retry = 1;
>> int thp_retry = 1;
>> - int nr_failed = 0;
>> + int nr_failed;
>> int nr_retry_pages = 0;
>> int nr_large_failed = 0;
>> int pass = 0;
>> @@ -1456,38 +1547,45 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> trace_mm_migrate_pages_start(mode, reason);
>> memset(&stats, 0, sizeof(stats));
>> + rc = migrate_hugetlbs(from, get_new_page, put_new_page, private, mode, reason,
>> + &stats, &ret_folios);
>> + if (rc < 0)
>> + goto out;
>> + nr_failed = rc;
>> +
>> split_folio_migration:
>> - for (pass = 0; pass < 10 && (retry || large_retry); pass++) {
>> + 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;
>> list_for_each_entry_safe(folio, folio2, from, lru) {
>> + /* Retried hugetlb folios will be kept in list */
>> + if (folio_test_hugetlb(folio)) {
>> + list_move_tail(&folio->lru, &ret_folios);
>> + continue;
>> + }
>> +
>> /*
>> * Large folio statistics is based on the source large
>> * folio. Capture required information that might get
>> * lost during migration.
>> */
>> - is_large = folio_test_large(folio) && !folio_test_hugetlb(folio);
>> + is_large = folio_test_large(folio);
>> is_thp = is_large && folio_test_pmd_mappable(folio);
>> nr_pages = folio_nr_pages(folio);
>> +
>> cond_resched();
>> - if (folio_test_hugetlb(folio))
>> - rc = unmap_and_move_huge_page(get_new_page,
>> - put_new_page, private,
>> - &folio->page, pass > 2, mode,
>> - reason,
>> - &ret_folios);
>> - else
>> - rc = unmap_and_move(get_new_page, put_new_page,
>> - private, folio, pass > 2, mode,
>> - reason, &ret_folios);
>> + rc = unmap_and_move(get_new_page, put_new_page,
>> + private, folio, pass > 2, mode,
>> + reason, &ret_folios);
>> /*
>> * The rules are:
>> - * Success: non hugetlb folio will be freed, hugetlb
>> - * folio will be put back
>> + * Success: folio will be freed
>> * -EAGAIN: stay on the from list
>> * -ENOMEM: stay on the from list
>> * -ENOSYS: stay on the from list
>> @@ -1514,7 +1612,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> stats.nr_thp_split += is_thp;
>> break;
>> }
>> - /* Hugetlb migration is unsupported */
>> } else if (!no_split_folio_counting) {
>> nr_failed++;
>> }
>> @@ -1608,8 +1705,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>> */
>> if (!list_empty(&split_folios)) {
>> /*
>> - * Move non-migrated folios (after 10 retries) to ret_folios
>> - * to avoid migrating them again.
>> + * Move non-migrated folios (after NR_MAX_MIGRATE_PAGES_RETRY
>> + * retries) to ret_folios to avoid migrating them again.
>> */
>> list_splice_init(from, &ret_folios);
>> list_splice_init(&split_folios, from);
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