linux-kernel - Re: [PATCH 2/8] migrate_pages: separate hugetlb folios migration

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Message-ID: <87fsck5fzc.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Mon, 09 Jan 2023 15:23:03 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Alistair Popple <apopple@...dia.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>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Oscar Salvador" <osalvador@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        "Bharata B Rao" <bharata@....com>, haoxin <xhao@...ux.alibaba.com>
Subject: Re: [PATCH 2/8] migrate_pages: separate hugetlb folios migration

Alistair Popple <apopple@...dia.com> writes:

> "Huang, Ying" <ying.huang@...el.com> writes:
>
>> [snip]
>>
>>>
>>>>>> @@ -1462,30 +1549,28 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>>>>>>  		nr_retry_pages = 0;
>>>>>>  
>>>>>>  		list_for_each_entry_safe(folio, folio2, from, lru) {
>>>>>> +			if (folio_test_hugetlb(folio)) {
>>>>>
>>>>> How do we hit this case? Shouldn't migrate_hugetlbs() have already moved
>>>>> any hugetlb folios off the from list?
>>>>
>>>> Retried hugetlb folios will be kept in from list.
>>>
>>> Couldn't migrate_hugetlbs() remove the failing retried pages from the
>>> list on the final pass? That seems cleaner to me.
>>
>> To do that, we need to go through the folio list again to remove all
>> hugetlb pages.  It could be time-consuming in some cases.  So I think
>> that it's better to keep this.
>
> Why? Couldn't we test pass == 9 and remove it from the list if it fails
> the final retry in migrate_hugetlbs()? In any case if it's on the list
> due to failed retries we have already passed over it 10 times, so the
> extra loop hardly seems like a problem.

Yes.  That's possible.  But "test pass == 9" looks more tricky than the
current code.

Feel free to change the code as you suggested on top this series.  If no
others object, I'm OK with that.  OK?

Best Regards,
Huang, Ying

>>
>>>>>> +				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
>>>>>> @@ -1512,7 +1597,6 @@ 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++;
>>>>>>  				}