Re: [PATCH v2] kho: align kho_scratch to MAX_ORDER_NR_PAGES pages

["Michał Cłapiński" <mclapinski@google.com>]

On Fri, Jul 17, 2026 at 6:21 PM Pasha Tatashin
<pasha.tatashin@soleen.com> wrote:
>
> On 07-17 15:40, Michal Clapinski wrote:
> > While booting with KHO, the following crash was observed:
> >
> > BUG: unable to handle page fault for address: ff19164fffff8328
> > RIP: 0010:__free_one_page+0x1a1/0x6b0
> > Call Trace:
> >  <TASK>
> >  [<ffffffff913208bf>] free_one_page+0xaf/0x240
> >  [<ffffffff93973288>] deferred_free_pages+0xa8/0xd0
> >  [<ffffffff93971b4f>] deferred_init_memmap_chunk+0x10f/0x1b0
> >  [<ffffffff9396e265>] padata_mt_helper+0x65/0xa0
> >  [<ffffffff90fac402>] process_scheduled_works+0x202/0x410
> >  [<ffffffff90fae739>] worker_thread+0x1f9/0x2d0
> >  [<ffffffff90fb62fd>] kthread+0x27d/0x2f0
> >  [<ffffffff90fae540>] ? __pfx_worker_thread+0x10/0x10
> >  [<ffffffff90fb6080>] ? __pfx_kthread+0x10/0x10
> >  [<ffffffff90efdc55>] ret_from_fork+0x145/0x280
> >  [<ffffffff90fb6080>] ? __pfx_kthread+0x10/0x10
> >  [<ffffffff90e2e46a>] ret_from_fork_asm+0x1a/0x30
> >  </TASK>
> >
> > deferred_init_memmap_chunk() interleaves initialization of struct pages
> > with freeing them. This works fine without KHO because free regions
> > will never be neighbors. However, with KHO, free memory will be split
> > into (free && scratch) and (free && !scratch), which can be neighbors.
> >
> > KHO scratch is aligned to CMA_MIN_ALIGNMENT_PAGES pages but buddy looks
> > at the neighborhood of MAX_ORDER_NR_PAGES pages. These values are
> > configurable but CMA_MIN_ALIGNMENT_PAGES is always less or equal to
> > MAX_ORDER_NR_PAGES. In the crashing configuration they were set as
> > follows:
> > CMA_MIN_ALIGNMENT_PAGES = 1 << 9
> > MAX_ORDER_NR_PAGES = 1 << 10
> >
> > So while freeing one chunk, buddy accessed uninitialized struct pages
> > from another chunk, tried to merge the blocks and crashed.
> >
> > To fix this, let's just align KHO scratch to MAX_ORDER_NR_PAGES pages.
> >
> > Fixes: c6073743d0c7 ("kho: make preserved pages compatible with deferred struct page init")
> > Signed-off-by: Michal Clapinski <mclapinski@google.com>
> > ---
> > Unfortunately, this is very hard to catch, so I don't have a good
> > reproducer. But I run the code with the fix through extensive testing
> > and it seems fine.
> > I think I've never caught it before because enabling HUGETLB is what
> > causes CMA_MIN_ALIGNMENT_PAGES to be 1 << 9, instead of 1 << 10.
> >
> > If this passes review, please cherry-pick it to 7.2 so we don't release
> > a broken kernel.
>
> A changelog would be super helful here.

Sorry, forgot.

Changelog:
- functionally it's exactly the same
- changed SCRATCH_ALIGNMENT_BYTES to be calculated using
multiplication instead of binary shift
- updated the commit msg as Mike asked
- rebased on top of liveupdate/next

> Pasha
>
> > ---
> >  kernel/liveupdate/kexec_handover.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
> > index 4834a809985a..175c08a6e41e 100644
> > --- a/kernel/liveupdate/kexec_handover.c
> > +++ b/kernel/liveupdate/kexec_handover.c
> > @@ -38,6 +38,16 @@
> >  #include "../kexec_internal.h"
> >  #include "kexec_handover_internal.h"
> >
> > +/*
> > + * This is the minimal alignment required by deferred struct page init.
> > + * deferred_init_memmap_chunk frees memory to the buddy allocator, which looks
> > + * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them.
> > + * If KHO scratch is not aligned to that value, buddy can access uninitialized
> > + * struct pages, which can cause a crash.
> > + */
> > +#define SCRATCH_ALIGNMENT_BYTES (PAGE_SIZE * MAX_ORDER_NR_PAGES)
> > +static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES);
> > +
> >  /* The magic token for preserved pages */
> >  #define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */
> >
> > @@ -640,8 +650,8 @@ static void __init scratch_size_update(void)
> >        * Scratch areas are released as MIGRATE_CMA. Round them up to the right
> >        * size.
> >        */
> > -     scratch_size_lowmem = round_up(scratch_size_lowmem, CMA_MIN_ALIGNMENT_BYTES);
> > -     scratch_size_global = round_up(scratch_size_global, CMA_MIN_ALIGNMENT_BYTES);
> > +     scratch_size_lowmem = round_up(scratch_size_lowmem, SCRATCH_ALIGNMENT_BYTES);
> > +     scratch_size_global = round_up(scratch_size_global, SCRATCH_ALIGNMENT_BYTES);
> >  }
> >
> >  static phys_addr_t __init scratch_size_node(int nid)
> > @@ -656,7 +666,7 @@ static phys_addr_t __init scratch_size_node(int nid)
> >               size = scratch_size_pernode;
> >       }
> >
> > -     return round_up(size, CMA_MIN_ALIGNMENT_BYTES);
> > +     return round_up(size, SCRATCH_ALIGNMENT_BYTES);
> >  }
> >
> >  /**
> > @@ -692,7 +702,7 @@ static void __init kho_reserve_scratch(void)
> >        * next kernel
> >        */
> >       size = scratch_size_lowmem;
> > -     addr = memblock_phys_alloc_range(size, CMA_MIN_ALIGNMENT_BYTES, 0,
> > +     addr = memblock_phys_alloc_range(size, SCRATCH_ALIGNMENT_BYTES, 0,
> >                                        ARCH_LOW_ADDRESS_LIMIT);
> >       if (!addr) {
> >               pr_err("Failed to reserve lowmem scratch buffer\n");
> > @@ -705,7 +715,7 @@ static void __init kho_reserve_scratch(void)
> >
> >       /* reserve large contiguous area for allocations without nid */
> >       size = scratch_size_global;
> > -     addr = memblock_phys_alloc(size, CMA_MIN_ALIGNMENT_BYTES);
> > +     addr = memblock_phys_alloc(size, SCRATCH_ALIGNMENT_BYTES);
> >       if (!addr) {
> >               pr_err("Failed to reserve global scratch buffer\n");
> >               goto err_free_scratch_areas;
> > @@ -721,7 +731,7 @@ static void __init kho_reserve_scratch(void)
> >        */
> >       for_each_node_state(nid, N_MEMORY) {
> >               size = scratch_size_node(nid);
> > -             addr = memblock_alloc_range_nid(size, CMA_MIN_ALIGNMENT_BYTES,
> > +             addr = memblock_alloc_range_nid(size, SCRATCH_ALIGNMENT_BYTES,
> >                                               0, MEMBLOCK_ALLOC_ACCESSIBLE,
> >                                               nid, true);
> >               if (!addr) {
> > --
> > 2.55.0.229.g6434b31f56-goog
> >