Re: [PATCH v1 05/11] mm: convert VM_PFNMAP tracking to pfnmap_track() + pfnmap_untrack()

David Hildenbrand
2025-04-28T20:23:51Z

On 28.04.25 21:57, Suren Baghdasaryan wrote:
> On Mon, Apr 28, 2025 at 12:37 PM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.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