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@xxxxxxxxxx > > 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@xxxxxxxxxx> > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx> > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> > Cc: Paul Mackerras <paulus@xxxxxxxxx> > Cc: Rashmica Gupta <rashmica.g@xxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Mike Rapoport <rppt@xxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Oscar Salvador <osalvador@xxxxxxx> > Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx> > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> 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. 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. -- Michal Hocko SUSE Labs
