Messages in this thread | | | Subject | Re: [PATCH v1 4/4] powernv/memtrace: don't abuse memory hot(un)plug infrastructure for memory allocations | From | David Hildenbrand <> | 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
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