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Date: Mon, 25 Mar 2024 21:25:29 +0800
From: Baoquan He <bhe@redhat.com>
To: Heiko Carstens <hca@linux.ibm.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	"Uladzislau Rezki (Sony)" <urezki@gmail.com>, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Matthew Wilcox <willy@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>, linux-s390@vger.kernel.org
Subject: Re: [PATCH 1/1] mm: vmalloc: Bail out early in find_vmap_area() if
 vmap is not init
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References: <20240323141544.4150-1-urezki@gmail.com>
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In-Reply-To: <20240325111650.16056-A-hca@linux.ibm.com>

On 03/25/24 at 12:16pm, Heiko Carstens wrote:
> On Mon, Mar 25, 2024 at 06:09:26PM +0800, Baoquan He wrote:
> > On 03/25/24 at 10:39am, Heiko Carstens wrote:
> > > On Sun, Mar 24, 2024 at 04:32:00PM -0700, Christoph Hellwig wrote:
> > > > On Sat, Mar 23, 2024 at 03:15:44PM +0100, Uladzislau Rezki (Sony) wrote:
> > ......snip
> > > > I guess this is ok as an urgend bandaid to get s390 booting again,
> > > > but calling find_vmap_area before the vmap area is initialized
> > > > seems an actual issue in the s390 mm init code.
> > > > 
> > > > Adding the s390 maintainers to see if they have and idea how this could
> > > > get fixed in a better way.
> > > 
> > > I'm going to push the patch below to the s390 git tree later. This is not a
> > > piece of art, but I wanted to avoid to externalize vmalloc's vmap_initialized,
> > > or come up with some s390 specific change_page_attr_alias_early() variant where
> > > sooner or later nobody remembers what "early" means.
> > > 
> > > So this seems to be "good enough".
> ...
> > > Add a slab_is_available() check to change_page_attr_alias() in order to
> > > avoid early calls into vmalloc code. slab_is_available() is not exactly
> > > what is needed, but there is currently no other way to tell if the vmalloc
> > > code is initialized or not, and there is no reason to expose
> > > e.g. vmap_initialized from vmalloc to achieve the same.
> > 
> > If so, I would rather add a vmalloc_is_available() to achieve the same.
> > The added code and the code comment definitely will confuse people and
> > make people to dig why.
> 
> So after having given this a bit more thought I think Uladzislau's patch is
> probably the best way to address this.
> 
> It seems to be better that the vmalloc code would just do the right thing,
> regardless how early it is called, instead of adding yet another
> subsystem_xyz_is_available() call.
> 
> Alternatively this could be addressed in s390 code with some sort of
> "early" calls, but as already stated, sooner or later nobody would remember
> what "early" means, and even if that would be remembered: would that
> restriction still be valid?

I agree, it's better to let vmalloc code do the thing right whether it's
early ot not with Uladzislau's patch.