linux-kernel - Re: [PATCH v1 04/18] mm: track mapcount of large folios in single value

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Date: Thu, 18 Apr 2024 17:09:41 +0200
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, cgroups@...r.kernel.org, chris@...kel.net,
 corbet@....net, dalias@...c.org, fengwei.yin@...el.com,
 glaubitz@...sik.fu-berlin.de, hughd@...gle.com, jcmvbkbc@...il.com,
 linmiaohe@...wei.com, linux-doc@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, linux-sh@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, muchun.song@...ux.dev,
 naoya.horiguchi@....com, peterx@...hat.com, richardycc@...gle.com,
 ryan.roberts@....com, shy828301@...il.com, willy@...radead.org,
 ysato@...rs.sourceforge.jp, ziy@...dia.com
Subject: Re: [PATCH v1 04/18] mm: track mapcount of large folios in single
 value

On 18.04.24 16:50, Lance Yang wrote:
> Hey David,
> 
> FWIW, just a nit below.

Hi!

Thanks, but that was done on purpose.

This way, we'll have a memory barrier (due to at least one 
atomic_inc_and_test()) between incrementing the folio refcount 
(happening before the rmap change) and incrementing the mapcount.

Is it required? Not 100% sure, refcount vs. mapcount checks are always a 
bit racy. But doing it this way let me sleep better at night ;)

[with no subpage mapcounts, we'd do the atomic_inc_and_test on the large 
mapcount and have the memory barrier there again; but that's stuff for 
the future]

Thanks!

> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2608c40dffad..08bb6834cf72 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1143,7 +1143,6 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
>   		int *nr_pmdmapped)
>   {
>   	atomic_t *mapped = &folio->_nr_pages_mapped;
> -	const int orig_nr_pages = nr_pages;
>   	int first, nr = 0;
>   
>   	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
> @@ -1155,6 +1154,7 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
>   			break;
>   		}
>   
> +		atomic_add(nr_pages, &folio->_large_mapcount);
>   		do {
>   			first = atomic_inc_and_test(&page->_mapcount);
>   			if (first) {
> @@ -1163,7 +1163,6 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
>   					nr++;
>   			}
>   		} while (page++, --nr_pages > 0);
> -		atomic_add(orig_nr_pages, &folio->_large_mapcount);
>   		break;
>   	case RMAP_LEVEL_PMD:
>   		first = atomic_inc_and_test(&folio->_entire_mapcount);
> 
> Thanks,
> Lance
> 

-- 
Cheers,

David / dhildenb