Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic()

[afzal mohammed <afzal.mohd.ma@gmail.com>]

Hi,

On Fri, Jun 12, 2020 at 10:07:28PM +0200, Arnd Bergmann wrote:

> I think a lot
> of usercopy calls are only for a few bytes, though this is of course
> highly workload dependent and you might only care about the large
> ones.

Observation is that max. pages reaching copy_{from,to}_user() is 2,
observed maximum of n (number of bytes) being 1 page size. i think C
library cuts any size read, write to page size (if it exceeds) &
invokes the system call. Max. pages reaching 2, happens when 'n'
crosses page boundary, this has been observed w/ small size request
as well w/ ones of exact page size (but not page aligned).

Even w/ dd of various size >4K, never is the number of pages required
to be mapped going greater than 2 (even w/ 'dd' 'bs=1M')

i have a worry (don't know whether it is an unnecessary one): even
if we improve performance w/ large copy sizes, it might end up in a
sluggishness w.r.t user experience due to most (hence a high amount)
of user copy calls being few bytes & there the penalty being higher.
And benchmark would not be able to detect anything abnormal since
usercopy are being tested on large sizes.

Quickly comparing boot-time on Beagle Bone White, boot time increases
by only 4%, perhaps this worry is irrelevant, but just thought will
put it across.

> There is also still hope of optimizing small aligned copies like
> 
> set_ttbr0(user_ttbr);
> ldm();
> set_ttbr0(kernel_ttbr);
> stm();

Hmm, more needs to be done to be in a position to test it.

Regards
afzal