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

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Message-ID: <CA+CK2bCyTi+C+JaOjbaMhX=vnD7NTW-T4VDb9xJcTZTjwTJr6A@mail.gmail.com>
Date: Fri, 13 Jun 2025 10:02:57 -0400
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Alexander Graf <graf@...zon.com>, Mike Rapoport <rppt@...nel.org>, 
	Changyuan Lyu <changyuanl@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Pratyush Yadav <ptyadav@...zon.de>, kexec@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] kho: initialize tail pages for higher order folios properly

On Fri, Jun 13, 2025 at 8:59 AM Pratyush Yadav <pratyush@...nel.org> wrote:
>
> From: Pratyush Yadav <ptyadav@...zon.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 <ptyadav@...zon.de>

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