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

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Date:   Wed, 13 Apr 2022 09:26:13 +0800
From:   "ying.huang@...el.com" <ying.huang@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Oscar Salvador <osalvador@...e.de>
Cc:     akpm@...ux-foundation.org, songmuchun@...edance.com,
        hch@...radead.org, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH v2 2/9] mm/vmscan: remove unneeded can_split_huge_page
 check

On Tue, 2022-04-12 at 16:59 +0200, David Hildenbrand wrote:
> 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().

Per my original understanding, folio_maybe_dma_pinned() can be used to
detect long-term pinned pages.  And it seems reasonable to skip the
long-term pinned pages and try short-term pinned pages during page
reclaiming.  But as you pointed out, vmsplice() doesn't use FOLL_PIN. 
So if vmsplice() is expected to pin pages for long time, and we have no
way to detect it, then we should keep can_split_folio() in the original
code.

Copying more people who have worked on long-term pinning for comments.

Best Regards,
Huang, Ying 

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