linux-kernel - Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge

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Date:   Tue, 12 Apr 2022 16:59:31 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, ying.huang@...el.com,
        songmuchun@...edance.com, hch@...radead.org, willy@...radead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page
 check

On 12.04.22 15:42, Miaohe Lin wrote:
> On 2022/4/12 16:59, Oscar Salvador wrote:
>> On Sat, Apr 09, 2022 at 05:34:53PM +0800, Miaohe Lin wrote:
>>> We don't need to check can_split_folio() because folio_maybe_dma_pinned()
>>> is checked before. It will avoid the long term pinned pages to be swapped
>>> out. And we can live with short term pinned pages. Without can_split_folio
>>> checking we can simplify the code. Also activate_locked can be changed to
>>> keep_locked as it's just short term pinning.
>>
>> What do you mean by "we can live with short term pinned pages"?
>> Does it mean that it was not pinned when we check
>> folio_maybe_dma_pinned() but now it is?
>>
>> To me it looks like the pinning is fluctuating and we rely on
>> split_folio_to_list() to see whether we succeed or not, and if not
>> we give it another spin in the next round?
> 
> Yes. Short term pinned pages is relative to long term pinned pages and these pages won't be
> pinned for a noticeable time. So it's expected to split the folio successfully in the next
> round as the pinning is really fluctuating. Or am I miss something?
> 

Just so we're on the same page. folio_maybe_dma_pinned() only capture
FOLL_PIN, but not FOLL_GET. You can have long-term FOLL_GET right now
via vmsplice().

can_split_folio() is more precise then folio_maybe_dma_pinned(), but
both are racy as long as the page is still mapped.


-- 
Thanks,

David / dhildenb