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-d3111b915...@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 <mho...@kernel.org>
> Cc: Michael Ellerman <m...@ellerman.id.au>
> Cc: Benjamin Herrenschmidt <b...@kernel.crashing.org>
> Cc: Paul Mackerras <pau...@samba.org>
> Cc: Rashmica Gupta <rashmic...@gmail.com>
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Mike Rapoport <r...@kernel.org>
> Cc: Michal Hocko <mho...@suse.com>
> Cc: Oscar Salvador <osalva...@suse.de>
> Cc: Wei Yang <richard.weiy...@linux.alibaba.com>
> Signed-off-by: David Hildenbrand <da...@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.
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
