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Message-ID: <a07525c6-a8a5-2d8b-1fec-eb25a3503a3d@redhat.com>
Date: Tue, 3 Nov 2020 10:29:06 +0100
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linuxppc-dev@...ts.ozlabs.org,
Michael Ellerman <mpe@...erman.id.au>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Paul Mackerras <paulus@...ba.org>,
Rashmica Gupta <rashmica.g@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Rapoport <rppt@...nel.org>,
Oscar Salvador <osalvador@...e.de>,
Wei Yang <richard.weiyang@...ux.alibaba.com>
Subject: Re: [PATCH v1 4/4] powernv/memtrace: don't abuse memory hot(un)plug
infrastructure for memory allocations
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@...nel.org>
>> Cc: Michael Ellerman <mpe@...erman.id.au>
>> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
>> Cc: Paul Mackerras <paulus@...ba.org>
>> Cc: Rashmica Gupta <rashmica.g@...il.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: Mike Rapoport <rppt@...nel.org>
>> Cc: Michal Hocko <mhocko@...e.com>
>> Cc: Oscar Salvador <osalvador@...e.de>
>> Cc: Wei Yang <richard.weiyang@...ux.alibaba.com>
>> Signed-off-by: David Hildenbrand <david@...hat.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
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