Re: [PATCH] x86/efi: Do not release sub-1MB memory regions when the crashkernel option is specified

[Baoquan He]

On 04/12/21 at 08:24am, Andy Lutomirski wrote:
> On Mon, Apr 12, 2021 at 2:52 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
> >
> > On 04/11/21 at 06:49pm, Andy Lutomirski wrote:
> > >
> > >
> > > > On Apr 11, 2021, at 6:14 PM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> > > >
> > > > On 04/09/21 at 07:59pm, H. Peter Anvin wrote:
> > > >> Why don't we do this unconditionally? At the very best we gain half a megabyte of memory (except the trampoline, which has to live there, but it is only a few kilobytes.)
> > > >
> > > > This is a great suggestion, thanks. I think we can fix it in this way to
> > > > make code simpler. Then the specific caring of real mode in
> > > > efi_free_boot_services() can be removed too.
> > > >
> > >
> > > This whole situation makes me think that the code is buggy before and buggy after.
> > >
> > > The issue here (I think) is that various pieces of code want to reserve specific pieces of otherwise-available low memory for their own nefarious uses. I don’t know *why* crash kernel needs this, but that doesn’t matter too much.
> >
> > Kdump kernel also need go through real mode code path during bootup. It
> > is not different than normal kernel except that it skips the firmware
> > resetting. So kdump kernel needs low 1M as system RAM just as normal
> > kernel does. Here we reserve the whole low 1M with memblock_reserve()
> > to avoid any later kernel or driver data reside in this area. Otherwise,
> > we need dump the content of this area to vmcore. As we know, when crash
> > happened, the old memory of 1st kernel should be untouched until vmcore
> > dumping read out its content. Meanwhile, kdump kernel need reuse low 1M.
> > In the past, we used a back up region to copy out the low 1M area, and
> > map the back up region into the low 1M area in vmcore elf file. In
> > 6f599d84231fd27 ("x86/kdump: Always reserve the low 1M when the crashkernel
> > option is specified"), we changed to lock the whole low 1M to avoid
> > writting any kernel data into, like this we can skip this area when
> > dumping vmcore.
> >
> > Above is why we try to memblock reserve the whole low 1M. We don't want
> > to use it, just don't want anyone to use it in 1st kernel.
> >
> > >
> > > I propose that the right solution is to give low-memory-reserving code paths two chances to do what they need: once at the very beginning and once after EFI boot services are freed.
> > >
> > > Alternatively, just reserve *all* otherwise unused sub 1M memory up front, then release it right after releasing boot services, and then invoke the special cases exactly once.
> >
> > I am not sure if I got both suggested ways clearly. They look a little
> > complicated in our case. As I explained at above, we want the whole low
> > 1M locked up, not one piece or some pieces of it.
> 
> My second suggestion is probably the better one.  Here it is, concretely:
> 
> The early (pre-free_efi_boot_services) code just reserves all
> available sub-1M memory unconditionally, but it specially marks it as
> reserved-but-available-later.  We stop allocating the trampoline page
> at this stage.
> 
> In free_efi_boot_services, instead of *freeing* the sub-1M memory, we
> stick it in the pile of reserved memory created in the early step.
> This may involve splitting a block, kind of like the current
> trampoline late allocation works.
> 
> Then, *after* free_efi_boot_services(), we run a single block of code
> that lets everything that wants sub-1M code claim some.  This means
> that the trampoline gets allocated and, if crashkernel wants to claim
> everything else, it can.  After that, everything still unclaimed gets
> freed.

void __init setup_arch(char **cmdline_p)
{
...
	efi_reserve_boot_services();
	e820__memblock_alloc_reserved_mpc_new();
#ifdef CONFIG_X86_CHECK_BIOS_CORRUPTION
        setup_bios_corruption_check();
#endif
        reserve_real_mode();                                                                                                                      

        trim_platform_memory_ranges();
        trim_low_memory_range();
...
}

After efi_reserve_boot_services(), there are several function calling to
require memory reservation under low 1M.


asmlinkage __visible void __init __no_sanitize_address start_kernel(void)                                                                         
{
...
	setup_arch(&command_line);
...
	mm_init();
		--> mem_init();
			 -->memblock_free_all();

...
#ifdef CONFIG_X86
        if (efi_enabled(EFI_RUNTIME_SERVICES))
                efi_enter_virtual_mode();
			-->efi_free_boot_services();
				-->memblock_free_late();
#endif
...
}

So from the code flow, we can see that buddy allocator is built in
mm_init() which puts all memory from memblock.memory excluding
memblock.reserved into buddy. And much later, we call
efi_free_boot_services() to release those reserved efi boot memory into
buddy too.

Are you suggesting we should do the memory reservation from low 1M
after efi_free_boot_services()? To require memory pages from buddy for
them? Please help point out my misunderstanding if have any.

With my understanding, in non-efi case, we have done the memory
reservation with memblock_reserve(), e.g
e820__memblock_alloc_reserved_mpc_new, reserve_real_mode() are calling
to do. Just efi_reserve|free_boot_services() break them when efi is
enabled. We can do them again in efi_free_boot_services() just like the
real_mode reservation does.

Thanks
Baoquan