Re: [PATCH v1 4/4] powernv/memtrace: don't abuse memory hot(un)plug infrastructure for memory allocations

[David Hildenbrand]

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