linux-kernel - Re: [PATCH v20 03/32] splice: Make direct_read

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Message-ID: <ZGc3vUU/bUpt+3Tm@infradead.org>
Date:   Fri, 19 May 2023 01:47:57 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        Al Viro <viro@...iv.linux.org.uk>,
        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,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v20 03/32] splice: Make direct_read_splice() limit to eof
 where appropriate

On Fri, May 19, 2023 at 09:43:34AM +0100, David Howells wrote:
> > direct_read_splice (which also appears a little misnamed) really is
> > a splice by calling ->read_iter helper.
> 
> It can be renamed if you want a different name.  copy_splice_read() or
> copy_splice_read_iter()?

Maybe something like that, yes.

> 
> > I we don't do any of this validtion we can just call it directly from
> > splice.c instead of calling into ->splice_read for direct I/O and DAX and
> > remove a ton of boilerplate code.
> 
> There's a couple of places where we might not want to do that - at least for
> non-DAX.  shmem and f2fs for example.  f2fs calls back to buffered reading
> under some circumstances.  shmem ignores O_DIRECT and always splices from the
> pagecache.

So?  even if ->read_iter does buffered I/O for O_DIRECT it will still
work.  This can in fact happen for many other file systems due when
they fall back to buffeed I/O due to various reasons.