linux-kernel - Re: [PATCH] mm: folio-compat: fix bug for pagecache_get

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Message-ID: <Y0/38zsvEgShWjLw@casper.infradead.org>
Date:   Wed, 19 Oct 2022 14:13:23 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Tuo Cao <91tuocao@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: folio-compat: fix bug for pagecache_get_page

On Tue, Oct 18, 2022 at 01:48:34PM -0700, Andrew Morton wrote:
> On Tue, 18 Oct 2022 15:39:39 +0100 Matthew Wilcox <willy@...radead.org> wrote:
> 
> > On Tue, Oct 18, 2022 at 10:36:39PM +0800, Tuo Cao wrote:
> > > The folio returned from __filemap_get_folio may be a NULL, it will
> > > causes the kernel crash when access folio->page.
> > 
> > This is not a bug.  &folio->page does not dereference folio but performs
> > pointer arithmetic.
> 
> This is the third attempt to "fix" this.  So far.  I think we should
> take the hint and make the check for a null pointer more explicit.

I think it's doing a great job of pointing out how many alleged
kernel programmers don't actually understand C and aren't capable of
searching mailing lists to see if somebody else already sent a patch.
Hopefully somebody's keeping track and is feeding this information into
their hiring algorithms.

The real solution is to convert all the callers of pagecache_get_page()
to use folios, and then we can delete this code.  I have a small series
in progress.  The amount of text savings is immense; half a kilobyte
from one conversion alone.  Our compound_head() has got very bloated.