linux-kernel - Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

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Message-ID: <3ceb9fc6-47cd-4715-ab86-517ed873dc04@redhat.com>
Date: Wed, 13 Nov 2024 19:45:19 +0100
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>
Cc: Jann Horn <jannh@...gle.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, "Kirill A . Shutemov" <kirill@...temov.name>,
 Nicholas Piggin <npiggin@...il.com>, Matthew Wilcox <willy@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 James Houghton <jthoughton@...gle.com>, Huang Ying <ying.huang@...el.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
 Vlastimil Babka <vbabka@...e.cz>,
 Rick P Edgecombe <rick.p.edgecombe@...el.com>,
 Hugh Dickins <hughd@...gle.com>, Borislav Petkov <bp@...en8.de>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Michael Ellerman <mpe@...erman.id.au>, Rik van Riel <riel@...riel.com>,
 Dan Williams <dan.j.williams@...el.com>,
 Mel Gorman <mgorman@...hsingularity.net>, x86@...nel.org,
 Ingo Molnar <mingo@...hat.com>, linuxppc-dev@...ts.ozlabs.org,
 Dave Hansen <dave.hansen@...ux.intel.com>, Dave Jiang
 <dave.jiang@...el.com>, Oscar Salvador <osalvador@...e.de>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5 0/7] mm/mprotect: Fix dax puds

On 13.11.24 18:56, Peter Xu wrote:
> On Wed, Nov 13, 2024 at 05:42:15PM +0100, David Hildenbrand wrote:
>> On 13.11.24 17:39, Peter Xu wrote:
>>> On Mon, Nov 11, 2024 at 10:20:59PM +0100, Jann Horn wrote:
>>>> On Mon, Aug 12, 2024 at 8:12 PM Peter Xu <peterx@...hat.com> wrote:
>>>>> Dax supports pud pages for a while, but mprotect on puds was missing since
>>>>> the start.  This series tries to fix that by providing pud handling in
>>>>> mprotect().  The goal is to add more types of pud mappings like hugetlb or
>>>>> pfnmaps.  This series paves way for it by fixing known pud entries.
>>>>
>>>> Do people actually use hardware where they can use PUD THP mappings
>>>> for DAX? I thought that was just some esoteric feature that isn't
>>>> actually usable on almost any system.
>>>> Was I wrong about that?
>>>
>>> I did run it with a qemu emulated nvdimm device.  Though in reality I've no
>>> idea on how many people are using it..
>>
>> I wonder if we still have to support it ... or could disable it+rip it out.
> 
> Note that in my previous email, I also mentioned mremap() for PMD on dax
> too.  If that's a real problem, it won't be fixed even if dropping dax PUD
> support.
>

Very true.

> And we definitely want to understand whether there're still users on pud
> dax to consider dropping anything.. it could still be that both mprotect()
> and mremap() are not yet used in the current use cases.

Right, but at least NVDIMMs are getting less important. Just a thought 
if this is really still worth having.

-- 
Cheers,

David / dhildenb