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

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Date:   Wed, 8 Jun 2022 15:27:09 +0800
From:   mawupeng <mawupeng1@...wei.com>
To:     <ardb@...nel.org>, <david@...hat.com>
CC:     <corbet@....net>, <will@...nel.org>, <catalin.marinas@....com>,
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        <mawupeng1@...wei.com>
Subject: Re: [PATCH v3 5/6] mm: Add mirror flag back on initrd memory



在 2022/6/7 22:49, Ard Biesheuvel 写道:
> On Tue, 7 Jun 2022 at 14:22, David Hildenbrand <david@...hat.com> wrote:
>>
>> On 07.06.22 11:38, Wupeng Ma wrote:
>>> From: Ma Wupeng <mawupeng1@...wei.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@...wei.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?

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