linux-kernel - Re: [PATCH] iov_iter: Add automatic-alloc for ITER_BVEC and use in direct_splice

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Date:   Fri, 19 May 2023 01:18:29 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] iov_iter: Add automatic-alloc for ITER_BVEC and use in
 direct_splice_read()

On Fri, May 19, 2023 at 08:49:18AM +0100, David Howells wrote:
> direct_splice_read() is then modified to make use of this.  This is less
> efficient if we know in advance that we want to allocate the full buffer as
> we can't so easily use bulk allocation, but it does mean in cases where we
> might not want the full buffer (say we hit a hole in DIO), we don't have to
> allocate it.

Can you eplain the workloads we're trying to optimize for here?

To me splice on O_DIRECT is more of a historic accident than an actually
good use case..