Re: [PATCH] x86/efi: defer freeing of boot services memory

["Ard Biesheuvel" <ardb@kernel.org>]

On Mon, 23 Feb 2026, at 11:55, Mike Rapoport wrote:
> Hi Ard,
>
> On Mon, Feb 23, 2026 at 09:08:29AM +0100, Ard Biesheuvel wrote:
>> Hi Mike,
>> 
>> On Mon, 23 Feb 2026, at 08:52, Mike Rapoport wrote:
>> > From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>> >
>> > efi_free_boot_services() frees memory occupied by EFI_BOOT_SERVICES_CODE
>> > and EFI_BOOT_SERVICES_DATA using memblock_free_late().
>> >
>> > There are two issue with that: memblock_free_late() should be used for
>> > memory allocated with memblock_alloc() while the memory reserved with
>> > memblock_reserve() should be freed with free_reserved_area().
>> >
>> > More acutely, with CONFIG_DEFERRED_STRUCT_PAGE_INIT=y
>> > efi_free_boot_services() is called before deferred initialization of the
>> > memory map is complete.
>> >
>> > Benjamin Herrenschmidt reports that this causes a leak of ~140MB of
>> > RAM on EC2 t3a.nano instances which only have 512MB or RAM.
>> >
>> > If the freed memory resides in the areas that memory map for them is
>> > still uninitialized, they won't be actually freed because
>> > memblock_free_late() calls memblock_free_pages() and the latter skips
>> > uninitialized pages.
>> >
>> > Using free_reserved_area() at this point is also problematic because
>> > __free_page() accesses the buddy of the freed page and that again might
>> > end up in uninitialized part of the memory map.
>> >
>> > Delaying the entire efi_free_boot_services() could be problematic
>> > because in addition to freeing boot services memory it updates
>> > efi.memmap without any synchronization and that's undesirable late in
>> > boot when there is concurrency.
>> >
>> > More robust approach is to only defer freeing of the EFI boot services
>> > memory.
>> >
>> > Make efi_free_boot_services() collect ranges that should be freed into
>> > an array and add an initcall efi_free_boot_services_memory() that walks
>> > that array and actually frees the memory using free_reserved_area().
>> >
>> 
>> Instead of creating another table, could we just traverse the EFI memory
>> map again in the arch_initcall(), and free all boot services code/data
>> above 1M with EFI_MEMORY_RUNTIME cleared ?
> 
> Currently efi_free_boot_services() unmaps all boot services code/data with
> EFI_MEMORY_RUNTIME cleared and removes them from the efi.memmap.
>

Ah yes, I failed to spot that those entries are long gone by initcall time. Other architectures don't touch the EFI memory map at all, but x86 mangles it beyond recognition :-)

> I wasn't sure it's Ok to only unmap them, but leave in efi.memmap, that's
> why I didn't use the existing EFI memory map.
>
> Now thinking about it, if the unmapping can happen later, maybe we'll just
> move the entire efi_free_boot_services() to an initcall?
>

As long as it is pre-SMP, as that code also contains a quirk to allocate the real mode trampoline if all memory below 1 MB is used for boot services.

But actually, that should be a separate quirk to begin with, rather than being integrated into an unrelated function that happens to iterate over the boot services regions. The only problem, I guess, is that memblock_reserve()'ing that sub-1MB region in the old location in the ordinary way would cause it to be freed again in the initcall?

But yes, in general I think it is fine to unmap those regions from the EFI page tables during an initcall.