linux-kernel - Re: [PATCH] mm/rmap: do not add fully unmapped large folio to deferred split list

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2C698A64-268C-4E43-9EDE-6238B656A391@nvidia.com>
Date: Fri, 12 Apr 2024 10:35:42 -0400
From: Zi Yan <ziy@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Yang Shi <shy828301@...il.com>,
 Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 11 Apr 2024, at 11:46, David Hildenbrand wrote:

> On 11.04.24 17:32, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> In __folio_remove_rmap(), a large folio is added to deferred split list
>> if any page in a folio loses its final mapping. It is possible that
>> the folio is unmapped fully, but it is unnecessary to add the folio
>> to deferred split list at all. Fix it by checking folio mapcount before
>> adding a folio to deferred split list.
>>
>> Signed-off-by: Zi Yan <ziy@...dia.com>
>> ---
>>   mm/rmap.c | 9 ++++++---
>>   1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2608c40dffad..d599a772e282 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1494,7 +1494,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   		enum rmap_level level)
>>   {
>>   	atomic_t *mapped = &folio->_nr_pages_mapped;
>> -	int last, nr = 0, nr_pmdmapped = 0;
>> +	int last, nr = 0, nr_pmdmapped = 0, mapcount = 0;
>>   	enum node_stat_item idx;
>>    	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
>> @@ -1506,7 +1506,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>>   			break;
>>   		}
>>  -		atomic_sub(nr_pages, &folio->_large_mapcount);
>> +		mapcount = atomic_sub_return(nr_pages,
>> +					     &folio->_large_mapcount) + 1;
>
> That becomes a new memory barrier on some archs. Rather just re-read it below. Re-reading should be fine here.

Would atomic_sub_return_relaxed() work? Originally I was using atomic_read(mapped)
below, but to save an atomic op, I chose to read mapcount here.

--
Best Regards,
Yan, Zi

Download attachment "signature.asc" of type "application/pgp-signature" (855 bytes)