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Peter Anvin" , "Thomas Gleixner" , linux-efi@vger.kernel.org, linux-mm@kvack.org, stable@vger.kernel.org Message-Id: In-Reply-To: References: <20260223075219.2348035-1-rppt@kernel.org> Subject: Re: [PATCH] x86/efi: defer freeing of boot services memory Content-Type: text/plain Content-Transfer-Encoding: 7bit On Mon, 23 Feb 2026, at 12:40, Mike Rapoport wrote: > 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)" >> >> > >> >> > 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 :) > Yeah. Sadly, all this has become ABI for kexec, so the EFI memory map abuse is hard to fix. >> > 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? > early_initcall() should be early enough, those run before SMP init. >> 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. > Thanks!