On Fri, Jun 13, 2025 at 02:59:06PM +0200, Pratyush Yadav wrote:
> From: Pratyush Yadav <ptya...@amazon.de>
>
> Currently, when restoring higher order folios, kho_restore_folio() only
> calls prep_compound_page() on all the pages. That is not enough to
> properly initialize the folios. The managed page count does not
> get updated, the reserved flag does not get dropped, and page count does
> not get initialized properly.
>
> Restoring a higher order folio with it results in the following BUG with
> CONFIG_DEBUG_VM when attempting to free the folio:
>
> BUG: Bad page state in process test pfn:104e2b
> page: refcount:1 mapcount:0 mapping:0000000000000000
> index:0xffffffffffffffff pfn:0x104e2b
> flags: 0x2fffff80000000(node=0|zone=2|lastcpupid=0x1fffff)
> raw: 002fffff80000000 0000000000000000 00000000ffffffff 0000000000000000
> raw: ffffffffffffffff 0000000000000000 00000001ffffffff 0000000000000000
> page dumped because: nonzero _refcount
> [...]
> Call Trace:
> <TASK>
> dump_stack_lvl+0x4b/0x70
> bad_page.cold+0x97/0xb2
> __free_frozen_pages+0x616/0x850
> [...]
>
> Combine the path for 0-order and higher order folios, initialize the
> tail pages with a count of zero, and call adjust_managed_page_count() to
> account for all the pages instead of just missing them.
>
> In addition, since all the KHO-preserved pages get marked with
> MEMBLOCK_RSRV_NOINIT by deserialize_bitmap(), the reserved flag is not
> actually set (as can also be seen from the flags of the dumped page in
> the logs above). So drop the ClearPageReserved() calls.
>
> Fixes: fc33e4b44b271 ("kexec: enable KHO support for memory preservation")
> Signed-off-by: Pratyush Yadav <ptya...@amazon.de>
Reviewed-by: Mike Rapoport (Microsoft) <r...@kernel.org>
> ---
>
> Changes in v2:
> - Declare i in the loop instead of at the top.
>
> kernel/kexec_handover.c | 29 +++++++++++++++++------------
> 1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/kexec_handover.c b/kernel/kexec_handover.c
> index eb305e7e61296..ca525f794f6be 100644
> --- a/kernel/kexec_handover.c
> +++ b/kernel/kexec_handover.c
> @@ -157,11 +157,21 @@ static int __kho_preserve_order(struct kho_mem_track
> *track, unsigned long pfn,
> }
>
> /* almost as free_reserved_page(), just don't free the page */
> -static void kho_restore_page(struct page *page)
> +static void kho_restore_page(struct page *page, unsigned int order)
> {
> - ClearPageReserved(page);
> - init_page_count(page);
> - adjust_managed_page_count(page, 1);
> + unsigned int nr_pages = (1 << order);
> +
> + /* Head page gets refcount of 1. */
> + set_page_count(page, 1);
> +
> + /* For higher order folios, tail pages get a page count of zero. */
> + for (unsigned int i = 1; i < nr_pages; i++)
> + set_page_count(page + i, 0);
> +
> + if (order > 0)
> + prep_compound_page(page, order);
> +
> + adjust_managed_page_count(page, nr_pages);
> }
>
> /**
> @@ -179,15 +189,10 @@ struct folio *kho_restore_folio(phys_addr_t phys)
> return NULL;
>
> order = page->private;
> - if (order) {
> - if (order > MAX_PAGE_ORDER)
> - return NULL;
> -
> - prep_compound_page(page, order);
> - } else {
> - kho_restore_page(page);
> - }
> + if (order > MAX_PAGE_ORDER)
> + return NULL;
>
> + kho_restore_page(page, order);
> return page_folio(page);
> }
> EXPORT_SYMBOL_GPL(kho_restore_folio);
> --
> 2.47.1
>
--
Sincerely yours,
Mike.
