Re: [PATCH 2/3] migrate_pages: move split folios processing out of migrate_pages_batch()

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

On 3/1/2023 2:35 PM, Huang, Ying wrote:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> On 2/24/2023 10:11 PM, Huang Ying wrote:
>>> To simplify the code logic and reduce the line number.
>>> 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 | 76 ++++++++++++++++++----------------------------------
>>>    1 file changed, 26 insertions(+), 50 deletions(-)
>>> diff --git a/mm/migrate.c b/mm/migrate.c
>>> index 7ac37dbbf307..91198b487e49 100644
>>> --- a/mm/migrate.c
>>> +++ b/mm/migrate.c
>>> @@ -1605,9 +1605,10 @@ static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
>>>    static int migrate_pages_batch(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 migrate_pages_stats *stats)
>>> +		struct list_head *split_folios, struct migrate_pages_stats *stats,
>>> +		int nr_pass)
>>>    {
>>> -	int retry;
>>> +	int retry = 1;
>>>    	int large_retry = 1;
>>>    	int thp_retry = 1;
>>>    	int nr_failed = 0;
>>> @@ -1617,19 +1618,12 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    	bool is_large = false;
>>>    	bool is_thp = false;
>>>    	struct folio *folio, *folio2, *dst = NULL, *dst2;
>>> -	int rc, rc_saved, nr_pages;
>>> -	LIST_HEAD(split_folios);
>>> +	int rc, rc_saved = 0, nr_pages;
>>>    	LIST_HEAD(unmap_folios);
>>>    	LIST_HEAD(dst_folios);
>>>    	bool nosplit = (reason == MR_NUMA_MISPLACED);
>>> -	bool no_split_folio_counting = false;
>>>    -retry:
>>> -	rc_saved = 0;
>>> -	retry = 1;
>>> -	for (pass = 0;
>>> -	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
>>> -	     pass++) {
>>> +	for (pass = 0; pass < nr_pass && (retry || large_retry); pass++) {
>>>    		retry = 0;
>>>    		large_retry = 0;
>>>    		thp_retry = 0;
>>> @@ -1660,7 +1654,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    			if (!thp_migration_supported() && is_thp) {
>>>    				nr_large_failed++;
>>>    				stats->nr_thp_failed++;
>>> -				if (!try_split_folio(folio, &split_folios)) {
>>> +				if (!try_split_folio(folio, split_folios)) {
>>>    					stats->nr_thp_split++;
>>>    					continue;
>>>    				}
>>> @@ -1692,7 +1686,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    					stats->nr_thp_failed += is_thp;
>>>    					/* Large folio NUMA faulting doesn't split to retry. */
>>>    					if (!nosplit) {
>>> -						int ret = try_split_folio(folio, &split_folios);
>>> +						int ret = try_split_folio(folio, split_folios);
>>>      						if (!ret) {
>>>    							stats->nr_thp_split += is_thp;
>>> @@ -1709,18 +1703,11 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    							break;
>>>    						}
>>>    					}
>>> -				} else if (!no_split_folio_counting) {
>>> +				} else {
>>>    					nr_failed++;
>>>    				}
>>>      				stats->nr_failed_pages += nr_pages +
>>> nr_retry_pages;
>>> -				/*
>>> -				 * There might be some split folios of fail-to-migrate large
>>> -				 * folios left in split_folios list. Move them to ret_folios
>>> -				 * list so that they could be put back to the right list by
>>> -				 * the caller otherwise the folio refcnt will be leaked.
>>> -				 */
>>> -				list_splice_init(&split_folios, ret_folios);
>>>    				/* nr_failed isn't updated for not used */
>>>    				nr_large_failed += large_retry;
>>>    				stats->nr_thp_failed += thp_retry;
>>> @@ -1733,7 +1720,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    				if (is_large) {
>>>    					large_retry++;
>>>    					thp_retry += is_thp;
>>> -				} else if (!no_split_folio_counting) {
>>> +				} else {
>>>    					retry++;
>>>    				}
>>>    				nr_retry_pages += nr_pages;
>>> @@ -1756,7 +1743,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    				if (is_large) {
>>>    					nr_large_failed++;
>>>    					stats->nr_thp_failed += is_thp;
>>> -				} else if (!no_split_folio_counting) {
>>> +				} else {
>>>    					nr_failed++;
>>>    				}
>>>    @@ -1774,9 +1761,7 @@ static int migrate_pages_batch(struct
>>> list_head *from, new_page_t get_new_page,
>>>    	try_to_unmap_flush();
>>>      	retry = 1;
>>> -	for (pass = 0;
>>> -	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
>>> -	     pass++) {
>>> +	for (pass = 0; pass < nr_pass && (retry || large_retry); pass++) {
>>>    		retry = 0;
>>>    		large_retry = 0;
>>>    		thp_retry = 0;
>>> @@ -1805,7 +1790,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    				if (is_large) {
>>>    					large_retry++;
>>>    					thp_retry += is_thp;
>>> -				} else if (!no_split_folio_counting) {
>>> +				} else {
>>>    					retry++;
>>>    				}
>>>    				nr_retry_pages += nr_pages;
>>> @@ -1818,7 +1803,7 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>>>    				if (is_large) {
>>>    					nr_large_failed++;
>>>    					stats->nr_thp_failed += is_thp;
>>> -				} else if (!no_split_folio_counting) {
>>> +				} else {
>>>    					nr_failed++;
>>>    				}
>>>    @@ -1855,27 +1840,6 @@ static int migrate_pages_batch(struct
>>> list_head *from, new_page_t get_new_page,
>>>    		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 (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.
>>> -		 */
>>> -		list_splice_init(from, ret_folios);
>>> -		list_splice_init(&split_folios, from);
>>> -		/*
>>> -		 * Force async mode to avoid to wait lock or bit when we have
>>> -		 * locked more than one folios.
>>> -		 */
>>> -		mode = MIGRATE_ASYNC;
>>> -		no_split_folio_counting = true;
>>> -		goto retry;
>>> -	}
>>> -
>>>    	return rc;
>>>    }
>>>    @@ -1914,6 +1878,7 @@ int migrate_pages(struct list_head *from,
>>> new_page_t get_new_page,
>>>    	struct folio *folio, *folio2;
>>>    	LIST_HEAD(folios);
>>>    	LIST_HEAD(ret_folios);
>>> +	LIST_HEAD(split_folios);
>>>    	struct migrate_pages_stats stats;
>>>      	trace_mm_migrate_pages_start(mode, reason);
>>> @@ -1947,12 +1912,23 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>>    	else
>>>    		list_splice_init(from, &folios);
>>>    	rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
>>> -				 mode, reason, &ret_folios, &stats);
>>> +				 mode, reason, &ret_folios, &split_folios, &stats,
>>> +				 NR_MAX_MIGRATE_PAGES_RETRY);
>>>    	list_splice_tail_init(&folios, &ret_folios);
>>>    	if (rc < 0) {
>>>    		rc_gather = rc;
>>> +		list_splice_tail(&split_folios, &ret_folios);
>>
>> Can we still keep the original comments? Which can help to understand
>> the case, at least for me:)
>>   /*
>>    * There might be some split folios of fail-to-migrate large
>>    * folios left in split_folios list. Move them to ret_folios
>>    * list so that they could be put back to the right list by
>>    * the caller otherwise the folio refcnt will be leaked.
>>    */
> 
> Previously, the cleanup code is buried in a corner of a much more
> complex code path.  So the comments are necessary.  Now, it is an
> explicit and simple code path.  And, the rule is clear, every folio list
> needs to be cleaned up before return: folios, split_folios, then
> ret_folios.  And we have done this here and there in the series.

OK. Fair enough.

> 
>>>    		goto out;
>>>    	}
>>> +	if (!list_empty(&split_folios)) {
>>> +		/*
>>> +		 * Failure isn't counted since all split folios of a large folio
>>> +		 * is counted as 1 failure already.
>>> +		 */
>>> +		migrate_pages_batch(&split_folios, get_new_page, put_new_page, private,
>>> +				    MIGRATE_ASYNC, reason, &ret_folios, NULL, &stats, 1);
>>
>> Better to copy the original comments to explain why force to
>> MIGRATE_ASYNC mode for split folios.
> 
> Yes.  It's a good idea to explain that.  And now the rule to call
> migrate_pages_batch() has been changed.  If mode != MIGRATE_ASYNC, the
> length of "from" must be <= 1.  I will add a VM_WARN_ON() for that at
> the beginning of migrate_pages_batch().  And I would rather to add the
> comments to the header of migrate_pages().  Other callers of the
> function needs to follow that rule too.

Looks reasonable to me. Thanks.