linux-kernel - Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a superfluous page

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Date:   Tue, 19 Oct 2021 19:28:02 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     linux-cachefs@...hat.com,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.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-kernel@...r.kernel.org
Subject: Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a
 superfluous page

On Mon, Oct 18, 2021 at 03:50:32PM +0100, David Howells wrote:
> @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>  		if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
>  			iocb->ki_flags |= IOCB_NOWAIT;
>  
> +		isize = i_size_read(inode);
> +		if (unlikely(iocb->ki_pos >= isize))
> +			goto put_pages;
> +

Is there a good reason to assign to isize here?  I'd rather not,
because it complicates analysis, and a later change might look at
the isize read here, not realising it was a racy use.  So I'd
rather see:

		if (unlikely(iocb->ki_pos >= i_size_read(inode)))
			goto put_pages;