Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to allocate more pages per call

[Al Viro <viro-3bDd1+5oDREiFSDQTTA3OLVCufUGDwFn@public.gmane.org>]

On Thu, Feb 02, 2017 at 11:49:01PM -0800, Christoph Hellwig wrote:
> On Thu, Feb 02, 2017 at 08:00:52AM -0500, Jeff Layton wrote:
> > Yeah, that might work. You could kmalloc the buffer array according to
> > the maxsize value. For small ones we could even consider using an on-
> > stack buffer.
> 
> For the block direct I/O code we defintively want to avoid any
> allocations for small I/O a that shows up in the performance numbers.
> And we'd like to reuse the on-stack bio_vec so that the defintion of
> a small I/O can be as big as possible without blowing up the stack.

Hmm...  Reuse part is really nasty ;-/  OTOH, it might make sense to have
a "fill bio_vec array" as separate primitive - having that sucker come
from bio looks like an artificial restriction.

OK, next question, seeing that you've dealt with O_DIRECT guts more than
I have.  When we have iov_iter_get_pages() fail on do_direct_IO() write
with some blocks already allocated, we pick zero page as data source.
So far, so good, but:
	* should we bother zeroing anything unless buffer_new() is true?
	* why, in case of more than a page worth of pending allocated
blocks, do we bother with calling iov_iter_get_pages() again and again?
We *do* take care not to allocate anything else after that point, but
dio_get_page() will be calling iov_iter_get_pages() every time in that
case - there's only one page in queue.
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