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Sampat wrote: >>> Agreed, I think Kiryl was hinting at pre-allocated bitmaps as well. >>> >>> Since, the overhead to do this upfront is fairly minimal, that should >>> certainly simplify things and have very little to no meddling with the >>> original EFI struct. >>> >> >> Taking another look at this suggestion, I think there may be more to it >> than I previously thought. Parsing e820 tables to know what the range >> are for allocating the bitmap to cover hotplug may be difficult. For e.g >> >> [ 0.000000] efi: mem110: [Unaccepted ] >> range=[0x0000000100000000-0x000000017fffffff] (2048MB) >> [ 0.000000] efi: mem111: [Reserved ] >> range=[0x000000fd00000000-0x000000ffffffffff] (12288MB) >> >> Parsing of the ACPI SRAT seems to be the one that gives us useful ranges >> to base the upfront bitmap allocation on. e.g. >> ... >> [ 0.018357] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x17fffffff] >> [ 0.018781] ACPI: SRAT: Node 0 PXM 0 [mem 0x180000000-0x2ffffffff] >> hotplug >> This is also where max_possible_pfn gets updated to reflect this range. > > Do I understand correctly that EFI memory map doesn't mention hot plug > range at all, but SRAT does? > > That's a mess. I thought, all hotpluggable range supposed to be declared > in the memory map. Not an EFI expert by a long shot, but seems so. EFI_MEMORY_HOT_PLUGGABLE attribute does exist for hot-removable regions of memory that must not be used for allocation during the boot context. However, I am unclear if this in principle is also supposed to span the entire range or just the cold-plugged regions of memory. > > I wounder if it is what BIOS provides, or is it result of EFI memmap > cleanup by kernel? I see we are doing bunch of them, like in > efi_remove_e820_mmio(). > >> One potential solution could be to parse the SRAT during unaccepted >> memory bitmap allocation in the EFI stub. However, this would fragment >> the implementation by duplicating the SRAT parsing. Alternatively, we >> could keep the current approach of dynamically allocating the bitmap on >> hotplug or I could also replace the entire memblock_reserved unaccepted >> table like Kiryl suggested if we must absolutely avoid changing the >> unaccepted structure? > > Other possible option would be to accept all memory on hotplug and don't > touch the bitmap at all. It might be not that bad: it doesn't block boot. > We can think of a better solution later, if needed. > Absolutely, accepting memory as soon as it's added is easy. Benchmarking it's effects may be a little tricky since unlike measuring boot-time in eager vs lazy we may have to find representative workloads to measure how much overheads accepting memory up-front adds. Thanks --Pratik