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Subject: Re: [PATCH v1 4/4] powernv/memtrace: don't abuse memory hot(un)plug
 infrastructure for memory allocations
To:     Michal Hocko <mhocko@suse.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linuxppc-dev@lists.ozlabs.org,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Rashmica Gupta <rashmica.g@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Oscar Salvador <osalvador@suse.de>,
        Wei Yang <richard.weiyang@linux.alibaba.com>
References: <20201029162718.29910-1-david@redhat.com>
 <20201029162718.29910-5-david@redhat.com>
 <20201103092309.GD21990@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <a07525c6-a8a5-2d8b-1fec-eb25a3503a3d@redhat.com>
Date:   Tue, 3 Nov 2020 10:29:06 +0100
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On 03.11.20 10:23, Michal Hocko wrote:
> On Thu 29-10-20 17:27:18, David Hildenbrand wrote:
>> Let's use alloc_contig_pages() for allocating memory and remove the
>> linear mapping manually via arch_remove_linear_mapping(). Mark all pages
>> PG_offline, such that they will definitely not get touched - e.g.,
>> when hibernating. When freeing memory, try to revert what we did.
>>
>> The original idea was discussed in:
>>   https://lkml.kernel.org/r/48340e96-7e6b-736f-9e23-d3111b915b6e@redhat.com
>>
>> This is similar to CONFIG_DEBUG_PAGEALLOC handling on other
>> architectures, whereby only single pages are unmapped from the linear
>> mapping. Let's mimic what memory hot(un)plug would do with the linear
>> mapping.
>>
>> We now need MEMORY_HOTPLUG and CONTIG_ALLOC as dependencies.
>>
>> Simple test under QEMU TCG (10GB RAM, single NUMA node):
>>
>> sh-5.0# mount -t debugfs none /sys/kernel/debug/
>> sh-5.0# cat /sys/devices/system/memory/block_size_bytes
>> 40000000
>> sh-5.0# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
>> [   71.052836][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
>> sh-5.0# echo 0x80000000 > /sys/kernel/debug/powerpc/memtrace/enable
>> [   75.424302][  T356] radix-mmu: Mapped 0x0000000080000000-0x00000000c0000000 with 64.0 KiB pages
>> [   75.430549][  T356] memtrace: Freed trace memory back on node 0
>> [   75.604520][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
>> sh-5.0# echo 0x100000000 > /sys/kernel/debug/powerpc/memtrace/enable
>> [   80.418835][  T356] radix-mmu: Mapped 0x0000000080000000-0x0000000100000000 with 64.0 KiB pages
>> [   80.430493][  T356] memtrace: Freed trace memory back on node 0
>> [   80.433882][  T356] memtrace: Failed to allocate trace memory on node 0
>> sh-5.0# echo 0x40000000 > /sys/kernel/debug/powerpc/memtrace/enable
>> [   91.920158][  T356] memtrace: Allocated trace memory on node 0 at 0x0000000080000000
>>
>> Note 1: We currently won't be allocating from ZONE_MOVABLE - because our
>> 	pages are not movable. However, as we don't run with any memory
>> 	hot(un)plug mechanism around, we could make an exception to
>> 	increase the chance of allocations succeeding.
>>
>> Note 2: PG_reserved isn't sufficient. E.g., kernel_page_present() used
>> 	along PG_reserved in hibernation code will always return "true"
>> 	on powerpc, resulting in the pages getting touched. It's too
>> 	generic - e.g., indicates boot allocations.
>>
>> Note 3: For now, we keep using memory_block_size_bytes() as minimum
>> 	granularity. I'm not able to come up with a better guess (most
>> 	probably, doing it on a section basis could be possible).
>>
>> Suggested-by: Michal Hocko <mhocko@kernel.org>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Rashmica Gupta <rashmica.g@gmail.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Mike Rapoport <rppt@kernel.org>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Oscar Salvador <osalvador@suse.de>
>> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> Thanks! This looks like a move into the right direction. I cannot really
> judge implementation details because I am not familiar with the code.
> I have only one tiny concern:
> [...]
>> -/* called with device_hotplug_lock held */
>> -static bool memtrace_offline_pages(u32 nid, u64 start_pfn, u64 nr_pages)
>> +static u64 memtrace_alloc_node(u32 nid, u64 size)
>>   {
>> -	const unsigned long start = PFN_PHYS(start_pfn);
>> -	const unsigned long size = PFN_PHYS(nr_pages);
>> +	const unsigned long nr_pages = PHYS_PFN(size);
>> +	unsigned long pfn, start_pfn;
>> +	struct page *page;
>>   
>> -	if (walk_memory_blocks(start, size, NULL, check_memblock_online))
>> -		return false;
>> -
>> -	walk_memory_blocks(start, size, (void *)MEM_GOING_OFFLINE,
>> -			   change_memblock_state);
>> -
>> -	if (offline_pages(start_pfn, nr_pages)) {
>> -		walk_memory_blocks(start, size, (void *)MEM_ONLINE,
>> -				   change_memblock_state);
>> -		return false;
>> -	}
>> +	/*
>> +	 * Trace memory needs to be aligned to the size, which is guaranteed
>> +	 * by alloc_contig_pages().
>> +	 */
>> +	page = alloc_contig_pages(nr_pages, __GFP_THISNODE | __GFP_NOWARN,
>> +				  nid, NULL);
> 
> __GFP_THISNODE without other modifiers looks suspicious. I suspect you
> want to enfore node locality and exclude movable zones by this. While
> this works it is an antipattern. I would rather use GFP_KERNEL |
> __GFP_THISNODE | __GFP_NOWARN to be more in line with other gfp usage.

Agreed GFP_KERNEL should be the right thing to do here.

> 
> If for no other reasons we want to be able to work inside a normal
> compaction context (comparing to effectively GFP_NOIO which the above
> implies). Also this looks like a sleepable context.
> 

Yes it is. Thanks!

-- 
Thanks,

David / dhildenb