Re: [PATCH v11 03/15] powerpc, kexec_file: factor out memblock-based arch_kexec_walk_mem()

From: Dave Young <dyoung@redhat.com>
To: AKASHI Takahiro <takahiro.akashi@linaro.org>,
	James Morse <james.morse@arm.com>,
	catalin.marinas@arm.com, will.deacon@arm.com,
	dhowells@redhat.com, vgoyal@redhat.com,
	herbert@gondor.apana.org.au, davem@davemloft.net, bhe@redhat.com,
	arnd@arndb.de, ard.biesheuvel@linaro.org, bhsharma@redhat.com,
	kexec@lists.infradead.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org,
	"Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH v11 03/15] powerpc, kexec_file: factor out memblock-based arch_kexec_walk_mem()
Date: Fri, 20 Jul 2018 13:57:27 +0800	[thread overview]
Message-ID: <20180720055727.GA9487@dhcp-128-65.nay.redhat.com> (raw)
In-Reply-To: <20180720053333.GJ11258@linaro.org>

On 07/20/18 at 02:33pm, AKASHI Takahiro wrote:
> Dave,
> 
> On Wed, Jul 18, 2018 at 02:45:19PM +0800, Dave Young wrote:
> > On 07/18/18 at 03:40pm, AKASHI Takahiro wrote:
> > > On Wed, Jul 18, 2018 at 02:13:50PM +0800, Dave Young wrote:
> > > > Hi AKASHI,
> > > > 
> > > > On 07/18/18 at 02:38pm, AKASHI Takahiro wrote:
> > > > > Dave,
> > > > > 
> > > > > On Tue, Jul 17, 2018 at 03:49:23PM +0800, Dave Young wrote:
> > > > > > Hi AKASHI,
> > > > > > On 07/17/18 at 02:31pm, AKASHI Takahiro wrote:
> > > > > > > Hi Dave,
> > > > > > > 
> > > > > > > On Mon, Jul 16, 2018 at 08:24:12PM +0800, Dave Young wrote:
> > > > > > > > On 07/16/18 at 12:04pm, James Morse wrote:
> > > > > > > > > Hi Dave,
> > > > > > > > > 
> > > > > > > > > On 14/07/18 02:52, Dave Young wrote:
> > > > > > > > > > On 07/11/18 at 04:41pm, AKASHI Takahiro wrote:
> > > > > > > > > >> Memblock list is another source for usable system memory layout.
> > > > > > > > > >> So powerpc's arch_kexec_walk_mem() is moved to kexec_file.c so that
> > > > > > > > > >> other memblock-based architectures, particularly arm64, can also utilise
> > > > > > > > > >> it. A moved function is now renamed to kexec_walk_memblock() and merged
> > > > > > > > > >> into the existing arch_kexec_walk_mem() for general use, either resource
> > > > > > > > > >> list or memblock list.
> > > > > > > > > >>
> > > > > > > > > >> A consequent function will not work for kdump with memblock list, but
> > > > > > > > > >> this will be fixed in the next patch.
> > > > > > > > > 
> > > > > > > > > >> diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
> > > > > > > > > 
> > > > > > > > > >> @@ -513,6 +563,10 @@ static int locate_mem_hole_callback(struct resource *res, void *arg)
> > > > > > > > > >>  int __weak arch_kexec_walk_mem(struct kexec_buf *kbuf,
> > > > > > > > > >>  			       int (*func)(struct resource *, void *))
> > > > > > > > > >>  {
> > > > > > > > > >> +	if (IS_ENABLED(CONFIG_HAVE_MEMBLOCK) &&
> > > > > > > > > >> +			!IS_ENABLED(CONFIG_ARCH_DISCARD_MEMBLOCK))
> > > > > > > > > >> +		return kexec_walk_memblock(kbuf, func);
> > > > > > > > > > 
> > > > > > > > > > AKASHI, I'm not sure if this works on all arches, for example I chekced
> > > > > > > > > > the .config on my Nokia N900 kernel tree, there is HAVE_MEMBLOCK=y and
> > > > > > > > > > no CONFIG_ARCH_DISCARD_MEMBLOCK, in 32bit arm code no arch_kexec_walk_mem()
> > > > > > > > > By doesn't work you mean it's a change in behaviour?
> > > > > > > > > I think this is fine because 32bit arm doesn't support KEXEC_FILE, (this file is
> > > > > > > > > kexec_file specific right?).
> > > > > > > > 
> > > > > > > > Ah, replied on a train, I forgot this is only for kexec_file, sorry
> > > > > > > > about that.  Please ignore the comment.
> > > > > > > > 
> > > > > > > > But since we have a weak function arch_kexec_walk_mem, adding another
> > > > > > > > condition branch within this weak function looks not good.
> > > > > > > > Something like below would be better:
> > > > > > > 
> > > > > > > I see your concern here, but
> > > > > > > 
> > > > > > > 
> > > > > > > > int kexec_locate_mem_hole(struct kexec_buf *kbuf)
> > > > > > > > {
> > > > > > > >         int ret;
> > > > > > > > 
> > > > > > > > 	+ if use memblock
> > > > > > > > 	+	ret = kexec_walk_memblock()
> > > > > > > > 	+ else
> > > > > > > >         	ret = arch_kexec_walk_mem(kbuf, locate_mem_hole_callback);
> > > > > > > > 
> > > > > > > >         return ret == 1 ? 0 : -EADDRNOTAVAIL;
> > > > > > > > }
> > > > > > > 
> > > > > > > what if yet another architecture comes to kexec_file and wanna
> > > > > > > take a third approach? How can it override those functions?
> > > > > > > Depending on kernel configuration, it might re-define either
> > > > > > > kexec_walk_memblock() or arch_kexec_walk_mem(). It sounds weird to me.
> > > > > > 
> > > > > > I also feel this weird, but it is slightly better because currently no
> > > > > > user need another overriding requirement, and I feel it is not expected to have in
> > > > > > the future for the memblock use.
> > > > > > 
> > > > > > Rethinking about this issue, we can just remove the weak function and
> > > > > > just use general function.
> > > > > 
> > > > > Do you really want to remove "weak" attribute?
> > > > > 
> > > > > > Currently with your patch applied only s390 use arch_kexec_walk_mem like
> > > > > > below:
> > > > > > /*
> > > > > >  * The kernel is loaded to a fixed location. Turn off kexec_locate_mem_hole
> > > > > >  * and provide kbuf->mem by hand.
> > > > > >  */
> > > > > > int arch_kexec_walk_mem(struct kexec_buf *kbuf,
> > > > > >                         int (*func)(struct resource *, void *))
> > > > > > {
> > > > > >         return 1;
> > > > > > }
> > > > > > 
> > > > > > AFAIK, all other users initialize kbuf->mem as NULL, so we can check
> > > > > 
> > > > > As a matter of fact, nobody initializes kbuf->mem before calling
> > > > > kexec_add_buffer (in turn, kexec_locate_mem_hole()).
> > > > 
> > > > Not sure we understand each other..
> > > > Let's take an example in arch/x86/kernel/kexec-bzimage64.c:
> > > > bzImage64_load() :
> > > > 	struct kexec_buf kbuf = { .image = image, .buf_max = ULONG_MAX,
> > > > 				.top_down = true };
> > > > 
> > > > Except the three fields above other members will be initialized as zero
> > > > when compiling including the kbuf->mem
> > > 
> > > Ah, you're right.
> > > (My armr64 patch doesn't use struct initializer, though.)
> > > 
> > > > > 
> > > > > > kbuf->mem in int kexec_locate_mem_hole:
> > > > > > 
> > > > > > if (kbuf->mem)
> > > > > > 	return 0;
> > > > > > 
> > > > > > if use memblock
> > > > > > 	kexec_walk_memblock
> > > > > > else
> > > > > > 	kexec_walk_mem
> > > > 
> > > > kexec_walk_resource will be better than kexec_walk_mem
> > > > 
> > > > > 
> > > > > I think that your solution will work for existing architectures
> > > > > with appropriate patches, but to take your approach, as I said above,
> > > > > we will have to modify every call site on all kexec_file-capable architectures.
> > > > > 
> > > > > If this is what you expect, I will work on it, but I don't think
> > > > > that it would be a better idea.
> > > 
> > > So you would expect me to modify my own arm64 code as well as s390.
> > 
> > Yes :)  But I had not get time to read all your patches so I was not
> > aware the struct initialization in arm64 code so I assumed only s390
> > need a change..
> 
> Okay, but I don't want to mix cross-arch changes into a single patch,
> prefer to leave the current patch as it is and add an additional patch
> as you suggested here.
Hi AKASHI,

Maybe add another patch to drop s390 walk function first, then follow
with this patch with the modification about common code restructure.

Is this better? For example:
03/15 s390, drop s390 arch_kexec_mem_walk
04/15 powerpc, kexec_file: factor out memblock-based arch_kexec_walk_mem
> 
> Is that OK for you?
> 
> Thanks,
> -Takahiro AKASHI
> 
> 
> > Thanks
> > Dave

Thanks
Dave