linux-kernel - Re: [PATCH] mm: fix hugetlb bug due to user_shm

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Message-ID: <Pine.LNX.4.64.0909121212560.488@sister.anvils>
Date:	Sat, 12 Sep 2009 12:17:49 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	Mike Frysinger <vapier.adi@...il.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Stefan Huber <shuber2@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Meerwald <pmeerw@...y.sbg.ac.at>,
	James Morris <jmorris@...ei.org>,
	William Irwin <wli@...ementarian.org>,
	Mel Gorman <mel@....ul.ie>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix hugetlb bug due to user_shm_unlock call

On Fri, 11 Sep 2009, Mike Frysinger wrote:
> On Mon, Aug 24, 2009 at 11:30, Hugh Dickins wrote:
> >
> >  no_id:
> > +       if (shp->mlock_user)    /* shmflg & SHM_HUGETLB case */
> > +               user_shm_unlock(size, shp->mlock_user);
> >        fput(file);
> >  no_file:
> >        security_shm_free(shp);
> 
> this breaks on no-mmu systems due to user_shm_unlock() being
> mmu-specific.  normally gcc is smart enough to do dead code culling so
> it hasnt caused problems, but not here.  hugetlb support is not
> available on no-mmu systems, so the stubbed hugepage functions prevent
> calls to user_shm_unlock() and such, but here gcc cant figure it out:
> 
...
> 
> hugetlb_file_setup() expands to nothing and so mlock_user will never
> come back from NULL, but gcc still emits a reference to
> user_shm_unlock() in the error path.  perhaps the best thing here is
> to just add an #ifdef ?
>  no_id:
> +#ifdef CONFIG_HUGETLB_PAGE
> +    /* gcc isn't smart enough to see that mlock_user goes non-NULL
> only by hugetlb */
>     if (shp->mlock_user)    /* shmflg & SHM_HUGETLB case */
>         user_shm_unlock(size, shp->mlock_user);
> +#endif

Many thanks for reporting that, Mike.
Sorry, I've messed up both 2.6.31 final and 2.6.30.6 stable.
My preference is to avoid the #ifdef and use precisely the same
optimization technique as is working for it elsewhere.
Patch follows immediately in separate mail.

Hugh