On Thu, 9 Oct 2025 at 09:22, Kiryl Shutsemau <kirill@shutemov.name> wrote:
>
> Objtool is not happy about calling random stuff within UACCESS. I
> ignored it for now.

Yeah, that needs to be done inside the other stuff - including, very
much, the folio lookup.

> I am not sure if I use user_access_begin()/_end() correctly. Let me know
> if I misunderstood or misimplemented your idea.

Close. Except I'd have gotten rid of the iov stuff by making the inner
helper just get a 'void __user *' pointer and a length, and then
updating the iov state outside that helper.

> This patch brings 4k reads from 512k files to ~60GiB/s. Making the
> buffer 4k, brings it ~95GiB/s (baseline is 100GiB/s).

Note that right now, 'unsafe_copy_to_user()' is a horrible thing. It's
almost entirely unoptimized, see the hacky unsafe_copy_loop
implementation in <asm/uaccess.h>.

Because before this code, it was only used for readdir() to copy
individual filenames, I think.

Anyway, I'd have organized things a bit differently. Incremental
UNTESTED patch attached.

objtool still complains about SMAP issues, because
memcpy_from_file_folio() ends up resulting in a external call to
memcpy. Not great.

I don't love how complicated this all got, and even with your bigger
buffer it's slower than the baseline/

So honestly I'd be inclined to go back to "just deal with the
trivially small reads", and scratch this extra complexity.

       Linus