linux-kernel - Re: [PATCH v2 0/5] add static PMD zero page support

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: <a35629df-290f-4adf-9f18-661ea86ed59d@redhat.com>
Date: Tue, 15 Jul 2025 17:27:02 +0200
From: David Hildenbrand <david@...hat.com>
To: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>,
 Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Michal Hocko <mhocko@...e.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>,
 Dev Jain <dev.jain@....com>, "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
 willy@...radead.org, linux-mm@...ck.org, x86@...nel.org,
 linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org,
 gost.dev@...sung.com, hch@....de, Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 0/5] add static PMD zero page support

On 15.07.25 17:25, Pankaj Raghav (Samsung) wrote:
> On Tue, Jul 15, 2025 at 04:16:44PM +0200, David Hildenbrand wrote:
>> On 15.07.25 16:12, Lorenzo Stoakes wrote:
>>> On Tue, Jul 15, 2025 at 04:06:29PM +0200, David Hildenbrand wrote:
>>>> I think at some point we discussed "when does the PMD-sized zeropage make
>>>> *any* sense on these weird arch configs" (512MiB on arm64 64bit)
>>>>
>>>> No idea who wants to waste half a gig on that at runtime either.
>>>
>>> Yeah this is a problem we _really_ need to solve. But obviously somewhat out of
>>> scope here.
>>>
>>>>
>>>> But yeah, we should let the arch code opt in whether it wants it or not (in
>>>> particular, maybe only on arm64 with CONFIG_PAGE_SIZE_4K)
>>>
>>> I don't think this should be an ARCH_HAS_xxx.
>>>
>>> Because that's saying 'this architecture has X', this isn't architecture
>>> scope.
>>>
>>> I suppose PMDs may vary in terms of how huge they are regardless of page
>>> table size actually.
>>>
>>> So maybe the best solution is a semantic one - just rename this to
>>> ARCH_WANT_STATIC_PMD_ZERO_PAGE
>>>
>>> And then put the page size selector in the arch code.
>>>
>>> For example in arm64 we have:
>>>
>>> 	select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36)
>>>
>>> So doing something similar here like:
>>>
>>> 	select ARCH_WANT_STATIC_PMD_ZERO_PAGE if ARM64_4K_PAGES
>>>
>>> Would do thie job and sort everything out.
>>
>> Yes.
> 
> Actually I had something similar in one of my earlier versions[1] where we
> can opt in from arch specific Kconfig with *WANT* instead *HAS*.
> 
> For starters, I will enable this only from x86. We can probably extend
> this once we get the base patches up.

Makes sense.

-- 
Cheers,

David / dhildenb