Re: [PATCH 0/1] Pages not released from memblock to the buddy allocator

[David Rientjes <rientjes@google.com>]

On Wed, 4 Jan 2023, Aaron Thompson wrote:

> Hi all,
> 
> (I've CC'ed the KMSAN and x86 EFI maintainers as an FYI; the only code change
> I'm proposing is in memblock.)
> 
> I've run into a case where pages are not released from memblock to the buddy
> allocator. If deferred struct page init is enabled, and memblock_free_late() is
> called before page_alloc_init_late() has run, and the pages being freed are in
> the deferred init range, then the pages are never released. memblock_free_late()
> calls memblock_free_pages() which only releases the pages if they are not in the
> deferred range. That is correct for free pages because they will be initialized
> and released by page_alloc_init_late(), but memblock_free_late() is dealing with
> reserved pages. If memblock_free_late() doesn't release those pages, they will
> forever be reserved. All reserved pages were initialized by memblock_free_all(),
> so I believe the fix is to simply have memblock_free_late() call
> __free_pages_core() directly instead of memblock_free_pages().
> 
> In addition, there was a recent change (3c20650982609 "init: kmsan: call KMSAN
> initialization routines") that added a call to kmsan_memblock_free_pages() in
> memblock_free_pages(). It looks to me like it would also be incorrect to make
> that call in the memblock_free_late() case, because the KMSAN metadata was
> already initialized for all reserved pages by kmsan_init_shadow(), which runs
> before memblock_free_all(). Having memblock_free_late() call __free_pages_core()
> directly also fixes this issue.
> 
> I encountered this issue when I tried to switch some x86_64 VMs I was running
> from BIOS boot to EFI boot. The x86 EFI code reserves all EFI boot services
> ranges via memblock_reserve() (part of setup_arch()), and it frees them later
> via memblock_free_late() (part of efi_enter_virtual_mode()). The EFI
> implementation of the VM I was attempting this on, an Amazon EC2 t3.micro
> instance, maps north of 170 MB in boot services ranges that happen to fall in
> the deferred init range. I certainly noticed when that much memory went missing
> on a 1 GB VM.
> 
> I've tested the patch on EC2 instances, qemu/KVM VMs with OVMF, and some real
> x86_64 EFI systems, and they all look good to me. However, the physical systems
> that I have don't actually trigger this issue because they all have more than 4
> GB of RAM, so their deferred init range starts above 4 GB (it's always in the
> highest zone and ZONE_DMA32 ends at 4 GB) while their EFI boot services mappings
> are below 4 GB.
> 
> Deferred struct page init can't be enabled on x86_32 so those systems are
> unaffected. I haven't found any other code paths that would trigger this issue,
> though I can't promise that there aren't any. I did run with this patch on an
> arm64 VM as a sanity check, but memblock=debug didn't show any calls to
> memblock_free_late() so that system was unaffected as well.
> 
> I am guessing that this change should also go the stable kernels but it may not
> apply cleanly (__free_pages_core() was __free_pages_boot_core() and
> memblock_free_pages() was __free_pages_bootmem() when this issue was first
> introduced). I haven't gone through that process before so please let me know if
> I can help with that.
> 
> This is the end result on an EC2 t3.micro instance booting via EFI:
> 
> v6.2-rc2:
>   # grep -E 'Node|spanned|present|managed' /proc/zoneinfo
>   Node 0, zone      DMA
>           spanned  4095
>           present  3999
>           managed  3840
>   Node 0, zone    DMA32
>           spanned  246652
>           present  245868
>           managed  178867
> 
> v6.2-rc2 + patch:
>   # grep -E 'Node|spanned|present|managed' /proc/zoneinfo
>   Node 0, zone      DMA
>           spanned  4095
>           present  3999
>           managed  3840
>   Node 0, zone    DMA32
>           spanned  246652
>           present  245868
>           managed  222816
> 

The above before + after seems useful information to include in the commit 
description of the change.