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

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Message-ID: <ZGcusJQfz68H1s7S@infradead.org>
Date:   Fri, 19 May 2023 01:09:20 -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,
        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 08:40:18AM +0100, David Howells wrote:
> Make direct_read_splice() limit the read to the end of the file for regular
> files and block devices, thereby reducing the amount of allocation it will
> do in such a case.
> 
> This means that the blockdev code doesn't require any special handling as
> filemap_read_splice() also limits to i_size.

I'm really not sure if this is a good idea.  Right now
direct_read_splice (which also appears a little misnamed) really is
a splice by calling ->read_iter helper.  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.

How often do we even call into splice beyond i_size and for how much?

> +	if (S_ISREG(file_inode(in)->i_mode) ||
> +	    S_ISBLK(file_inode(in)->i_mode)) {

Btw, having these kinds of checks in supposedly generic code is always
a marked for a bit of a layering problem.