Message-ID: <1336043475.13013.47.camel@sauron.fi.intel.com>
Subject: Re: [PATCH] vmalloc: add warning in __vmalloc
From: Artem Bityutskiy <dedekind1@gmail.com>
Reply-To: dedekind1@gmail.com
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Minchan Kim <minchan@kernel.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kosaki.motohiro@gmail.com,
        rientjes@google.com, Neil Brown <neilb@suse.de>,
        David Woodhouse <dwmw2@infradead.org>, "Theodore Ts'o" <tytso@mit.edu>,
        Adrian Hunter <adrian.hunter@intel.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        James Morris <jmorris@namei.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Sage Weil <sage@newdream.net>
Date: Thu, 03 May 2012 14:11:15 +0300
In-Reply-To: <20120502124610.175e099c.akpm@linux-foundation.org>
References: <1335932890-25294-1-git-send-email-minchan@kernel.org>
	 <20120502124610.175e099c.akpm@linux-foundation.org>
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On Wed, 2012-05-02 at 12:46 -0700, Andrew Morton wrote:
> On Wed,  2 May 2012 13:28:09 +0900
> Minchan Kim <minchan@kernel.org> wrote:
>=20
> > Now there are several places to use __vmalloc with GFP_ATOMIC,
> > GFP_NOIO, GFP_NOFS but unfortunately __vmalloc calls map_vm_area
> > which calls alloc_pages with GFP_KERNEL to allocate page tables.
> > It means it's possible to happen deadlock.
> > I don't know why it doesn't have reported until now.
> >=20
> > Firstly, I tried passing gfp_t to lower functions to support __vmalloc
> > with such flags but other mm guys don't want and decided that
> > all of caller should be fixed.
> >=20
> > http://marc.info/?l=3Dlinux-kernel&m=3D133517143616544&w=3D2
> >=20
> > To begin with, let's listen other's opinion whether they can fix it
> > by other approach without calling __vmalloc with such flags.
> >=20
> > So this patch adds warning in __vmalloc_node_range to detect it and
> > to be fixed hopely. __vmalloc_node_range isn't random chocie because
> > all caller which has gfp_mask of map_vm_area use it through __vmalloc_a=
rea_node.
> > And __vmalloc_area_node is current static function and is called by onl=
y
> > __vmalloc_node_range. So warning in __vmalloc_node_range would cover al=
l
> > vmalloc functions which have gfp_t argument.
> >
> > I Cced related maintainers.
> > If I miss someone, please Cced them.
> >=20
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -1648,6 +1648,10 @@ void *__vmalloc_node_range(unsigned long size, u=
nsigned long align,
> >  	void *addr;
> >  	unsigned long real_size =3D size;
> > =20
> > +	WARN_ON_ONCE(!(gfp_mask & __GFP_WAIT) ||
> > +			!(gfp_mask & __GFP_IO) ||
> > +			!(gfp_mask & __GFP_FS));
> > +
> >  	size =3D PAGE_ALIGN(size);
> >  	if (!size || (size >> PAGE_SHIFT) > totalram_pages)
> >  		goto fail;
>=20
> Well.  What are we actually doing here?  Causing the kernel to spew a
> warning due to known-buggy callsites, so that users will report the
> warnings, eventually goading maintainers into fixing their stuff.
>=20
> This isn't very efficient :(
>=20
> It would be better to fix that stuff first, then add the warning to
> prevent reoccurrences.  Yes, maintainers are very naughty and probably
> do need cattle prods^W^W warnings to motivate them to fix stuff, but we
> should first make an effort to get these things fixed without
> irritating and alarming our users. =20
>=20
> Where are these offending callsites?

OK, I checked my part - both UBI and UBIFS call __vmalloc() with
GFP_NOFS in several places of the _debugging_ code, and this is why we
do not see any issues - the debugging code is used very rarely for
validating purposes. All the places look fixable, I'll fix them a bit
later.

WARN_ON_ONCE() looks like a good first step. An I think it is better if
maintainers fix their areas rather than if someone who does not know how
the subsystem works starts trying to do that.

--=20
Best Regards,
Artem Bityutskiy

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