linux-kernel - Re: [PATCH v3 20/20] mm: stop maintaining the per-page mapcount of large folios (CONFIG_NO_PAGE

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: <71380b43-c23c-42b5-8aab-f158bb37bc75@redhat.com>
Date: Tue, 14 Oct 2025 14:59:30 +0200
From: David Hildenbrand <david@...hat.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 cgroups@...r.kernel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-api@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Tejun Heo <tj@...nel.org>,
 Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Muchun Song <muchun.song@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH v3 20/20] mm: stop maintaining the per-page mapcount of
 large folios (CONFIG_NO_PAGE_MAPCOUNT)

On 14.10.25 14:23, Wei Yang wrote:
> On Mon, Mar 03, 2025 at 05:30:13PM +0100, David Hildenbrand wrote:
> [...]
>> @@ -1678,6 +1726,22 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>> 		break;
>> 	case RMAP_LEVEL_PMD:
>> 	case RMAP_LEVEL_PUD:
>> +		if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
>> +			last = atomic_add_negative(-1, &folio->_entire_mapcount);
>> +			if (level == RMAP_LEVEL_PMD && last)
>> +				nr_pmdmapped = folio_large_nr_pages(folio);
>> +			nr = folio_dec_return_large_mapcount(folio, vma);
>> +			if (!nr) {
>> +				/* Now completely unmapped. */
>> +				nr = folio_large_nr_pages(folio);
>> +			} else {
>> +				partially_mapped = last &&
>> +						   nr < folio_large_nr_pages(folio);
> 
> Hi, David

Hi!

> 
> Do you think this is better to be?
> 
> 	partially_mapped = last && nr < nr_pmdmapped;

I see what you mean, it would be similar to the CONFIG_PAGE_MAPCOUNT 
case below.

But probably it could then be

	partially_mapped = nr < nr_pmdmapped;

because nr_pmdmapped is only set when "last = true".

I'm not sure if there is a good reason to change it at this point 
though. Smells like a micro-optimization for PUD, which we probably 
shouldn't worry about.

> 
> As commit 349994cf61e6 mentioned, we don't support partially mapped PUD-sized
> folio yet.

We do support partially mapped PUD-sized folios I think, but not 
anonymous PUD-sized folios.

So consequently the partially_mapped variable will never really be used 
later on, because the folio_test_anon() will never hit in the PUD case.

-- 
Cheers

David / dhildenb