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@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.
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
