Re: [PATCH 3/3] migrate_pages: try migrate in batch asynchronously firstly

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2/24/2023 10:11 PM, Huang Ying wrote:
> When we have locked more than one folios, we cannot wait the lock or
> bit (e.g., page lock, buffer head lock, writeback bit) synchronously.
> Otherwise deadlock may be triggered.  This make it hard to batch the
> synchronous migration directly.
> 
> This patch re-enables batching synchronous migration via trying to
> migrate in batch asynchronously firstly.  And any folios that are
> failed to be migrated asynchronously will be migrated synchronously
> one by one.
> 
> Test shows that this can restore the TLB flushing batching performance
> for synchronous migration effectively.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: "Xu, Pengfei" <pengfei.xu@intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Stefan Roesch <shr@devkernel.io>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Xin Hao <xhao@linux.alibaba.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>   mm/migrate.c | 65 ++++++++++++++++++++++++++++++++++++++++++++--------
>   1 file changed, 55 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 91198b487e49..c17ce5ee8d92 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1843,6 +1843,51 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>   	return rc;
>   }
>   
> +static int migrate_pages_sync(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 list_head *ret_folios,
> +		struct list_head *split_folios, struct migrate_pages_stats *stats)
> +{
> +	int rc, nr_failed = 0;
> +	LIST_HEAD(folios);
> +	struct migrate_pages_stats astats;
> +
> +	memset(&astats, 0, sizeof(astats));
> +	/* Try to migrate in batch with MIGRATE_ASYNC mode firstly */
> +	rc = migrate_pages_batch(from, get_new_page, put_new_page, private, MIGRATE_ASYNC,
> +				 reason, &folios, split_folios, &astats,
> +				 NR_MAX_MIGRATE_PAGES_RETRY);
> +	stats->nr_succeeded += astats.nr_succeeded;
> +	stats->nr_thp_succeeded += astats.nr_thp_succeeded;
> +	stats->nr_thp_split += astats.nr_thp_split;
> +	if (rc < 0) {
> +		stats->nr_failed_pages += astats.nr_failed_pages;
> +		stats->nr_thp_failed += astats.nr_thp_failed;
> +		list_splice_tail(&folios, ret_folios);
> +		return rc;
> +	}
> +	stats->nr_thp_failed += astats.nr_thp_split;
> +	nr_failed += astats.nr_thp_split;
> +	/*
> +	 * Fall back to migrate all failed folios one by one synchronously. All
> +	 * failed folios except split THPs will be retried, so their failure
> +	 * isn't counted
> +	 */
> +	list_splice_tail_init(&folios, from);
> +	while (!list_empty(from)) {
> +		list_move(from->next, &folios);
> +		rc = migrate_pages_batch(&folios, get_new_page, put_new_page,
> +					 private, mode, reason, ret_folios,
> +					 split_folios, stats, NR_MAX_MIGRATE_PAGES_RETRY);
> +		list_splice_tail_init(&folios, ret_folios);
> +		if (rc < 0)
> +			return rc;
> +		nr_failed += rc;
> +	}
> +
> +	return nr_failed;
> +}
> +
>   /*
>    * migrate_pages - migrate the folios specified in a list, to the free folios
>    *		   supplied as the target for the page migration
> @@ -1874,7 +1919,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   		enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
>   {
>   	int rc, rc_gather;
> -	int nr_pages, batch;
> +	int nr_pages;
>   	struct folio *folio, *folio2;
>   	LIST_HEAD(folios);
>   	LIST_HEAD(ret_folios);
> @@ -1890,10 +1935,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   	if (rc_gather < 0)
>   		goto out;
>   
> -	if (mode == MIGRATE_ASYNC)
> -		batch = NR_MAX_BATCHED_MIGRATION;
> -	else
> -		batch = 1;
>   again:
>   	nr_pages = 0;
>   	list_for_each_entry_safe(folio, folio2, from, lru) {
> @@ -1904,16 +1945,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>   		}
>   
>   		nr_pages += folio_nr_pages(folio);
> -		if (nr_pages >= batch)
> +		if (nr_pages >= NR_MAX_BATCHED_MIGRATION)
>   			break;
>   	}
> -	if (nr_pages >= batch)
> +	if (nr_pages >= NR_MAX_BATCHED_MIGRATION)
>   		list_cut_before(&folios, from, &folio2->lru);
>   	else
>   		list_splice_init(from, &folios);
> -	rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
> -				 mode, reason, &ret_folios, &split_folios, &stats,
> -				 NR_MAX_MIGRATE_PAGES_RETRY);
> +	if (mode == MIGRATE_ASYNC)
> +		rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
> +					 mode, reason, &ret_folios, &split_folios, &stats,
> +					 NR_MAX_MIGRATE_PAGES_RETRY);
> +	else
> +		rc = migrate_pages_sync(&folios, get_new_page, put_new_page, private,
> +					mode, reason, &ret_folios, &split_folios, &stats);

For split folios, it seems also reasonable to use migrate_pages_sync() 
instead of always using fixed MIGRATE_ASYNC mode?

>   	list_splice_tail_init(&folios, &ret_folios);
>   	if (rc < 0) {
>   		rc_gather = rc;