linux-kernel - Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages

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Date:   Fri, 3 Feb 2017 03:09:30 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nfs@...r.kernel.org, ceph-devel@...r.kernel.org,
        lustre-devel@...ts.lustre.org,
        v9fs-developer@...ts.sourceforge.net,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v3 0/2] iov_iter: allow iov_iter_get_pages_alloc to
 allocate more pages per call

On Fri, Feb 03, 2017 at 08:54:15AM +0000, Al Viro wrote:
> 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.

Or just the only usecase :)  But yes, it could be generalized to take a
bio_vec without too much effort.

> 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?

I don't think so.

> 	* 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.

There shouldn't be a need for it.

But take it with a grain of salt - fs/direct-io.c is a hairy mess,
that's one of the reasons why I replaced it with the new iomap
code instead of trying to gradually move it to iomap API.