Re: [PATCH v3 5/6] mm: Add mirror flag back on initrd memory

From: Mike Rapoport <rppt@kernel.org>
To: mawupeng <mawupeng1@huawei.com>
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Subject: Re: [PATCH v3 5/6] mm: Add mirror flag back on initrd memory
Date: Wed, 8 Jun 2022 13:02:42 +0300	[thread overview]
Message-ID: <YqBzwgTgWEAx8J/C@kernel.org> (raw)
In-Reply-To: <29900b05-ec44-76a2-645a-22a13399d7fd@huawei.com>

On Wed, Jun 08, 2022 at 03:27:09PM +0800, mawupeng wrote:
> 
> 在 2022/6/7 22:49, Ard Biesheuvel 写道:
> > On Tue, 7 Jun 2022 at 14:22, David Hildenbrand <david@redhat.com> wrote:
> > > 
> > > On 07.06.22 11:38, Wupeng Ma wrote:
> > > > From: Ma Wupeng <mawupeng1@huawei.com>
> > > > 
> > > > Initrd memory will be removed and then added in arm64_memblock_init() and this
> > > > will cause it to lose all of its memblock flags. The lost of MEMBLOCK_MIRROR
> > > > flag will lead to error log printed by find_zone_movable_pfns_for_nodes if
> > > > the lower 4G range has some non-mirrored memory.
> > > > 
> > > > In order to solve this problem, the lost MEMBLOCK_MIRROR flag will be
> > > > reinstalled if the origin memblock has this flag.
> > > > 
> > > > Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
> > > > ---
> > > >   arch/arm64/mm/init.c     |  9 +++++++++
> > > >   include/linux/memblock.h |  1 +
> > > >   mm/memblock.c            | 20 ++++++++++++++++++++
> > > >   3 files changed, 30 insertions(+)
> > > > 
> > > > diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> > > > index 339ee84e5a61..11641f924d08 100644
> > > > --- a/arch/arm64/mm/init.c
> > > > +++ b/arch/arm64/mm/init.c
> > > > @@ -350,9 +350,18 @@ void __init arm64_memblock_init(void)
> > > >                        "initrd not fully accessible via the linear mapping -- please check your bootloader ...\n")) {
> > > >                        phys_initrd_size = 0;
> > > >                } else {
> > > > +                     int flags, ret;
> > > > +
> > > > +                     ret = memblock_get_flags(base, &flags);
> > > > +                     if (ret)
> > > > +                             flags = 0;
> > > > +
> > > >                        memblock_remove(base, size); /* clear MEMBLOCK_ flags */
> > > >                        memblock_add(base, size);
> > > >                        memblock_reserve(base, size);
> > > 
> > > Can you explain why we're removing+re-adding here exactly? Is it just to
> > > clear flags as the comment indicates?
> > > 
> > 
> > This should only happen if the placement of the initrd conflicts with
> > a mem= command line parameter or it is not covered by memblock for
> > some other reason.
> > 
> > IOW, this should never happen, and if re-memblock_add'ing this memory
> > unconditionally is causing problems, we should fix that instead of
> > working around it.
> 
> This will happen if we use initrdmem=3G,100M to reserve initrd memory below
> the 4G limit to test this scenario(just for testing, I have trouble to boot
> qemu with initrd enabled and memory below 4G are all mirror memory).
> 
> Re-memblock_add'ing this memory unconditionally seems fine but clear all
> flags(especially MEMBLOCK_MIRROR) may lead to some error log.
> 
> > 
> > > If it's really just about clearing flags, I wonder if we rather want to
> > > have an interface that does exactly that, and hides the way this is
> > > actually implemented (obtain flags, remove, re-add ...), internally.
> > > 
> > > But most probably there is more magic in the code and clearing flags
> > > isn't all it ends up doing.
> > > 
> > 
> > I don't remember exactly why we needed to clear the flags, but I think
> > it had to do with some corner case we hit when the initrd was
> > partially covered.
> If "mem=" is set in command line, memblock_mem_limit_remove_map() will
> remove all memory block without MEMBLOCK_NOMAP. Maybe this will bring the
> memory back if this initrd mem has the MEMBLOCK_NOMAP flag?
> 
> The rfc version [1] introduce and use memblock_clear_nomap() to clear the
> MEMBLOCK_NOMAP of this initrd memblock.
> So maybe the usage of memblock_remove() is just to avoid introducing new
> function(memblock_clear_nomap)?
> 
> Since commit 4c546b8a3469 ("memblock: add memblock_clear_nomap()") already
> introduced memblock_clear_nomap(). Can we use this to remove flag MEMBLOCK_NOMAP
> to solve this problem rather than bring flag MEMBLOCK_MIRROR back?

AFAICT, there are two corner cases that re-adding initrd memory covers:
* initrd memory is not a part of the memory reported to memblock, either
because of firmware weirdness or because it was cut out with mem=
* initrd memory overlaps a NOMAP region

So to make sure initrd memory is mapped properly and retains
MEMBLOCK_MIRROR I think the best we can do is

	memblock_add();
	memblock_clear_nomap();
	memblock_reserve();

 
> [1] https://lore.kernel.org/linux-arm-kernel/20160202180622.GP10166@arm.com/T/#t
> > .

-- 
Sincerely yours,
Mike.

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