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

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Date:   Tue, 10 Jan 2023 12:37:51 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     "Huang, Ying" <ying.huang@...el.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


"Huang, Ying" <ying.huang@...el.com> writes:

> 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?

Sure. Part of my problem when reviewing this series is that everytime I
look at migrate_pages(), and in particular the number of conditionals
that are sufficiently non-obvious to require extensive comments, I can't
help but think it all needs some refactoring before making it any more
complicated. However perhaps I am alone in that.

Either way this kind of refactoring has been on my TODO list for a while
- I have a WIP series to converge some of the migrate_device.c code
which I will need to rebase on this anyway so as you suggest I could
make a lot of my suggested changes on top of this series.

Regards,

Alistair

> 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++;
>>>>>>>  				}