linux-kernel - Re: [PATCH v4 08/28] netfs: Provide readahead and readpage netfs helpers

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Date:   Sun, 21 Mar 2021 01:42:02 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Jeff Layton <jlayton@...hat.com>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org,
        David Wysochanski <dwysocha@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 08/28] netfs: Provide readahead and readpage netfs
 helpers

On Wed, Mar 10, 2021 at 04:56:13PM +0000, David Howells wrote:
> +void netfs_readahead(struct readahead_control *ractl,
> +		     const struct netfs_read_request_ops *ops,
> +		     void *netfs_priv)
> +{
> +	struct netfs_read_request *rreq;
> +	struct page *page;
> +	unsigned int debug_index = 0;
> +
> +	_enter("%lx,%x", readahead_index(ractl), readahead_count(ractl));
> +
> +	if (readahead_count(ractl) == 0)
> +		goto cleanup;
> +
> +	rreq = netfs_alloc_read_request(ops, netfs_priv, ractl->file);
> +	if (!rreq)
> +		goto cleanup;
> +	rreq->mapping	= ractl->mapping;
> +	rreq->start	= readahead_pos(ractl);
> +	rreq->len	= readahead_length(ractl);
> +
> +	netfs_rreq_expand(rreq, ractl);
> +
> +	atomic_set(&rreq->nr_rd_ops, 1);
> +	do {
> +		if (!netfs_rreq_submit_slice(rreq, &debug_index))
> +			break;
> +
> +	} while (rreq->submitted < rreq->len);
> +
> +	while ((page = readahead_page(ractl)))
> +		put_page(page);

You don't need this pair of lines (unless I'm missing something).
read_pages() in mm/readahead.c puts the reference and unlocks any
pages which are not read by the readahead op.  Indeed, I think doing
this is buggy because you don't unlock the page.

> +	/* If we decrement nr_rd_ops to 0, the ref belongs to us. */
> +	if (atomic_dec_and_test(&rreq->nr_rd_ops))
> +		netfs_rreq_assess(rreq, false);
> +	return;
> +
> +cleanup:
> +	if (netfs_priv)
> +		ops->cleanup(ractl->mapping, netfs_priv);
> +	return;
> +}
> +EXPORT_SYMBOL(netfs_readahead);

> +int netfs_readpage(struct file *file,
> +		   struct page *page,
> +		   const struct netfs_read_request_ops *ops,
> +		   void *netfs_priv)
> +{
> +	struct netfs_read_request *rreq;
> +	unsigned int debug_index = 0;
> +	int ret;
> +
> +	_enter("%lx", page->index);
> +
> +	rreq = netfs_alloc_read_request(ops, netfs_priv, file);
> +	if (!rreq) {
> +		if (netfs_priv)
> +			ops->cleanup(netfs_priv, page->mapping);
> +		unlock_page(page);
> +		return -ENOMEM;
> +	}
> +	rreq->mapping	= page->mapping;

FYI, this isn't going to work with swap-over-NFS.  You have to use
page_file_mapping().

> +	rreq->start	= page->index * PAGE_SIZE;

and page_index() here.

I rather dislike it that swap-over-NFS uses readpage which makes this
need to exist.  If somebody were to switch SWP_FS_OPS to using kiocbs,
some of this pain could go away.