RE: [PATCH] use a nocache copy for bvecs in copy_from_iter_nocache()

[Boylston, Brian]

Kani, Toshimitsu wrote on 2016-10-10:
> On Fri, 2016-10-07 at 18:08 +0100, Al Viro wrote:
>> On Fri, Oct 07, 2016 at 10:55:11AM -0500, Brian Boylston wrote:
>>> 
>>> copy_from_iter_nocache() is only "nocache" for iovecs.ÂÂEnhance it
>>> to also use a nocache copy for bvecs.ÂÂThis improves performance by
>>> 2-3X when splice()ing to a file in a DAX-mounted, pmem-backed file
>>> system.
>> 
>>> 
>>> +static void memcpy_from_page_nocache(char *to, struct page *page,
>>> size_t offset, size_t len)
>>> +{
>>> +	char *from = kmap_atomic(page);
>>> +	__copy_from_user_inatomic_nocache(to, from, len);
>>> +	kunmap_atomic(from);
>>> +}
>> 
>> At the very least, it will blow up on any architecture with split
>> userland and kernel MMU contexts.ÂÂYou *can't* feed a kernel pointer
>> to things like that and expect it to work.ÂÂAt the very least, you
>> need to add memcpy_nocache() and have it default to memcpy(), with
>> non-dummy version on x86.ÂÂAnd use _that_, rather than messing with
>> __copy_from_user_inatomic_nocache()
> 
> Good point. ÂI think we can add memcpy_nocache() which calls
> __copy_from_user_inatomic_nocache() on x86 and defauts to memcpy() on
> other architectures.

Thanks, Al and Toshi, for the feedback.  I'll re-work and come back.

Brian