Re: [PATCH 1/2] Introduce copy_user_handle_tail routine

[Andi Kleen]

Vitaly Mayatskikh wrote:
> Andi Kleen <andi@xxxxxxxxxxxxxx> writes:
> 
>>>>>> rep movs can fail.
>>>> How? (if it's a byte copy?)
>>> Parameter len is a number of uncopied bytes,
>> But that is exactly what copy_*_user wants to return
> 
> Last experience showed, it doesn't.

?

> Ok, when unrolled version fails on reading quad word at unaligned
> address, it doesn't know where it was failed exactly. At this moment it
> hasn't correct number of uncopied bytes, because some bytes can still
> remain at the very end of the page. copy_user_handle_tail copies them
> and return correct value on uncopied bytes. Complicated logic for
> counting the number of these bytes is not necessary to optimize at
> assembly level, because we already missed performance. It's hard to
> complain against it.

Yes I'm talking about the "replay loop"

There's no complicated logic in a rep ; movs. And it's still
a byte copy. In fact it is far simpler than what you already have.

>> The original version I wrote returned "unfaulted bytes" which was wrong.
>> Correct is "uncopied" as fixed by Linus. rep ; movs returns uncopied.
> 
> It's not in C. If you have the proposal why it should be written in
> assembly, send it to Linus.

Well it would turn your 15+ lines C function in ~4 (well tested) lines or so.

> 
>>> Why do you think that zeroing can never fail, even in userspace?
>> There's no zeroing in user space, only in kernel space.
> 
> Agree.
> 
>> The only reason kernel does it is to avoid leaking uninitialized data,
>> but for user space it doesn't make sense (see above)
> 
> Ok, copy_in_user can pass zerorest=0 to copy_user_handle_tail. Is it ok
> for you?

My point was that for the zeroing you can just use memset(), there's
no need to support faulting there at all.

-Andi

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