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

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Date:   Tue, 19 Oct 2021 21:04:51 +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 Tue, Oct 19, 2021 at 07:48:15PM +0100, David Howells wrote:
> Matthew Wilcox <willy@...radead.org> wrote:
> 
> > > +		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 we don't set isize, the loop will never end.  Actually, maybe we can just
> break out at that point rather than going to put_pages.

Umm, yes, of course.  Sorry.

It makes more sense to just break because we haven't got any pages,
so putting pages that we haven't got seems unnecessary.
>