Re: [PATCH v2] kho: initialize tail pages for higher order folios properly

[Pasha Tatashin]

On Fri, Jun 13, 2025 at 8:59 AM Pratyush Yadav <pratyush@xxxxxxxxxx> wrote:
>
> From: Pratyush Yadav <ptyadav@xxxxxxxxx>
>
> 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 <ptyadav@xxxxxxxxx>

Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>