Re: [PATCH 0/2] vmalloc: Introduce vmap_file()

[Brendan Jackman]

On Mon Feb 3, 2025 at 6:53 PM UTC, Vishal Moola wrote:
> On Thu, Jan 30, 2025 at 4:48 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> On Thu, 30 Jan 2025 16:18:04 -0800 "Vishal Moola (Oracle)" <vishal.moola@xxxxxxxxx> wrote:
>>
>> > Currently, users have to call vmap() or vmap_pfn() to map pages to
>> > kernel virtual space. vmap() requires the page references, and
>> > vmap_pfn() requires page pfns. If we have a file but no page references,
>> > we have to do extra work to map them.
>> >
>> > Create a function, vmap_file(), to map a specified range of a given
>> > file to kernel virtual space. Also convert a user that benefits from
>> > vmap_file().
>> >
>>
>> Seems like a pretty specialized thing.  Have you identified any other
>> potential users of vmap_file()?  I couldn't see any.
>>
>> If drm is likely to remain the only user of this, perhaps we should
>> leave the code down in drivers/gpu/drm for now?
>
> This function is generally useful for file-systems that use the pagecache.
> I simply chose to highlight the most obvious user that benefits from it (and
> so that the function is introduced with a user).
>
> I haven't identified any other specific users of vmap_file() myself. I know
> Matthew has some other ideas for it; I've cc-ed him so he can chime in.

Not much to add but just to confirm - yep, this seems like it might be
useful as a part of the solution to the page cache perf issue[1] with
ASI that I spoke about (briefly and chaotically) at the end of the
LSF/MM/BPF session[0] on ASI this year.

[0] https://lwn.net/Articles/1016013/
[1] https://lore.kernel.org/linux-mm/20250129144320.2675822-1-jackmanb@xxxxxxxxxx/

But, for the moment this is all still pretty vague stuff, not at all
clear yet that this idea makes total sense. Hopefully I'll be able to
follow up in a few weeks after I've made some time to stare at/prototype
things.