linux-kernel - Re: [PATCH v1 05/11] mm: convert VM_PFNMAP tracking to pfnmap_track() + pfnmap

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Message-ID: <bd8ce421-cf2a-451b-a3eb-613e6856bb9e@redhat.com>
Date: Mon, 28 Apr 2025 22:23:42 +0200
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, x86@...nel.org, intel-gfx@...ts.freedesktop.org,
 dri-devel@...ts.freedesktop.org, linux-trace-kernel@...r.kernel.org,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
 Rodrigo Vivi <rodrigo.vivi@...el.com>, Tvrtko Ursulin
 <tursulin@...ulin.net>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, Andrew Morton <akpm@...ux-foundation.org>,
 Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH v1 05/11] mm: convert VM_PFNMAP tracking to pfnmap_track()
 + pfnmap_untrack()

On 28.04.25 21:57, Suren Baghdasaryan wrote:
> On Mon, Apr 28, 2025 at 12:37 PM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.com> wrote:
>>
>> On Mon, Apr 28, 2025 at 07:23:18PM +0200, David Hildenbrand wrote:
>>> On 28.04.25 18:24, Peter Xu wrote:
>>>> On Mon, Apr 28, 2025 at 06:16:21PM +0200, David Hildenbrand wrote:
>>>>>> Probably due to what config you have.  E.g., when I'm looking mine it's
>>>>>> much bigger and already consuming 256B, but it's because I enabled more
>>>>>> things (userfaultfd, lockdep, etc.).
>>>>>
>>>>> Note that I enabled everything that you would expect on a production system
>>>>> (incld. userfaultfd, mempolicy, per-vma locks), so I didn't enable lockep.
>>>>
>>>> I still doubt whether you at least enabled userfaultfd, e.g., your previous
>>>> paste has:
>>>>
>>>>     struct vm_userfaultfd_ctx  vm_userfaultfd_ctx;   /*   176     0 */
>>>>
>>>> Not something that matters.. but just in case you didn't use the expected
>>>> config file you wanted to use..
>>>
>>> You're absolutely right. I only briefly rechecked for this purpose here on
>>> my notebook, and only looked for the existence of members, not expecting
>>> that we have confusing stuff like vm_userfaultfd_ctx.
>>>
>>> I checked again and the size stays at 192 with allyesconfig and then
>>> disabling debug options.
>>
>> I think a reasonable case is everything on, except CONFIG_DEBUG_LOCK_ALLOC and I
>> don't care about nommu.
> 
> I think it's safe to assume that production systems would disable
> lockdep due to the performance overhead. At least that's what we do on
> Android - enable it on development branches but disable in production.

Right, and "struct lockdep_map" is ... significantly larger than 8 
bytes. With that enabled, one is already paying for extra VMA space ...

-- 
Cheers,

David / dhildenb