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

[Aaron Thompson <dev@aaront.org>]

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


Aaron Thompson (1):
  mm: Always release pages to the buddy allocator in
    memblock_free_late().

 mm/memblock.c                     | 2 +-
 tools/testing/memblock/internal.h | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

-- 
2.30.2