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

[Mike Rapoport <rppt@kernel.org>]

On Mon, Feb 23, 2026 at 12:17:22PM +0100, Ard Biesheuvel wrote:
> 
> 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 :-)

Heh, EFI on x86 does a lot of, hmm, interesting things with memory, like
memremaping kmalloced memory and I it really begs for cleanups :)
 
> > 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.

initcall is long after SMP. It the real mode trampoline allocation is the
only thing that should happen pre-SMP?
 
> 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?

Right now we anyway don't free anything below 1M, I don't see why it should
change. 

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

Thanks for confirming. I'll look into extracting the allocation of the real
mode trampoline to a separate quirk and then making the entire
efi_free_boot_services() an initcall.

-- 
Sincerely yours,
Mike.