linux-kernel - Re: [PATCH v4 40/68] cachefiles: Implement cache registration and withdrawal

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Date:   Thu, 06 Jan 2022 17:44:04 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 40/68] cachefiles: Implement cache registration and withdrawal

Jeff Layton <jlayton@...nel.org> wrote:

> > +	/* check parameters */
> > +	ret = -EOPNOTSUPP;
> > +	if (d_is_negative(root) ||
> > +	    !d_backing_inode(root)->i_op->lookup ||
> > +	    !d_backing_inode(root)->i_op->mkdir ||
> > +	    !(d_backing_inode(root)->i_opflags & IOP_XATTR) ||
> > +	    !root->d_sb->s_op->statfs ||
> > +	    !root->d_sb->s_op->sync_fs ||
> > +	    root->d_sb->s_blocksize > PAGE_SIZE)
> > +		goto error_unsupported;
> > +
> 
> That's quite a collection of tests.
> 
> Most are obvious, but some comments explaining the need for others would
> not be a bad thing. In particular, why is s_blocksize > PAGE_SIZE
> unsupported?

It can't do page-sized DIO requests to a filesystem with a block size larger
than a page.  In the future I can work around that in conjunction with
netfslib by expanding read and write sizes.

> Also, should you vet whether the fs supports i_op->tmpfile here ?

That's a good idea.

David