On 16.06.22 04:45, Muchun Song wrote: > On Wed, Jun 15, 2022 at 11:51:49AM +0200, David Hildenbrand wrote: >> On 20.05.22 04:55, Muchun Song wrote: >>> For now, the feature of hugetlb_free_vmemmap is not compatible with the >>> feature of memory_hotplug.memmap_on_memory, and hugetlb_free_vmemmap >>> takes precedence over memory_hotplug.memmap_on_memory. However, someone >>> wants to make memory_hotplug.memmap_on_memory takes precedence over >>> hugetlb_free_vmemmap since memmap_on_memory makes it more likely to >>> succeed memory hotplug in close-to-OOM situations. So the decision >>> of making hugetlb_free_vmemmap take precedence is not wise and elegant. >>> The proper approach is to have hugetlb_vmemmap.c do the check whether >>> the section which the HugeTLB pages belong to can be optimized. If >>> the section's vmemmap pages are allocated from the added memory block >>> itself, hugetlb_free_vmemmap should refuse to optimize the vmemmap, >>> otherwise, do the optimization. Then both kernel parameters are >>> compatible. So this patch introduces SECTION_CANNOT_OPTIMIZE_VMEMMAP >>> to indicate whether the section could be optimized. >>> >> >> In theory, we have that information stored in the relevant memory block, >> but I assume that lookup in the xarray + locking is impractical. >> >> I wonder if we can derive that information simply from the vmemmap pages >> themselves, because *drumroll* >> >> For one vmemmap page (the first one), the vmemmap corresponds to itself >> -- what?! >> >> >> [ hotplugged memory ] >> [ memmap ][ usable memory ] >> | | | >> ^--- | | >> ^------- | >> ^---------------------- >> >> The memmap of the first page of hotplugged memory falls onto itself. >> We'd have to derive from actual "usable memory" that condition. >> >> >> We currently support memmap_on_memory memory only within fixed-size >> memory blocks. So "hotplugged memory" is guaranteed to be aligned to >> memory_block_size_bytes() and the size is memory_block_size_bytes(). >> >> If we'd have a page falling into usbale memory, we'd simply lookup the >> first page and test if the vmemmap maps to itself. >> > > I think this can work. Should we use this approach in next version? > Either that or more preferable, flagging the vmemmap pages eventually. That's might be future proof. >> >> Of course, once we'd support variable-sized memory blocks, it would be >> different. >> >> >> An easier/future-proof approach might simply be flagging the vmemmap >> pages as being special. We reuse page flags for that, which don't have >> semantics yet (i.e., PG_reserved indicates a boot-time allocation via >> memblock). >> > > I think you mean flag vmemmap pages' struct page as PG_reserved if it > can be optimized, right? When the vmemmap pages are allocated in > hugetlb_vmemmap_alloc(), is it valid to flag them as PG_reserved (they > are allocated from buddy allocator not memblock)? > Sorry I wasn't clear. I'd flag them with some other not-yet-used-for-vmemmap-pages flag. Reusing PG_reserved could result in trouble. -- Thanks, David / dhildenb
