Re: [PATCH] mm/damon/ops-common: ignore migration request to invalid nodes

[Joshua Hahn]

On Sun, 20 Jul 2025 11:58:22 -0700 SeongJae Park <sj@xxxxxxxxxx> wrote:

> damon_migrate_pages() try migration even if the target node is invalid.
> If users mistakenly make such invalid requests via
> DAMOS_MIGRATE_{HOT,COLD} action, below kernel BUG can happen.
> 
>     [ 7831.883495] BUG: unable to handle page fault for address: 0000000000001f48
>     [ 7831.884160] #PF: supervisor read access in kernel mode
>     [ 7831.884681] #PF: error_code(0x0000) - not-present page
>     [ 7831.885203] PGD 0 P4D 0
>     [ 7831.885468] Oops: Oops: 0000 [#1] SMP PTI
>     [ 7831.885852] CPU: 31 UID: 0 PID: 94202 Comm: kdamond.0 Not tainted 6.16.0-rc5-mm-new-damon+ #93 PREEMPT(voluntary)
>     [ 7831.886913] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-4.el9 04/01/2014
>     [ 7831.887777] RIP: 0010:__alloc_frozen_pages_noprof (include/linux/mmzone.h:1724 include/linux/mmzone.h:1750 mm/page_alloc.c:4936 mm/page_alloc.c:5137)
>     [...]
>     [ 7831.895953] Call Trace:
>     [ 7831.896195]  <TASK>
>     [ 7831.896397] __folio_alloc_noprof (mm/page_alloc.c:5183 mm/page_alloc.c:5192)
>     [ 7831.896787] migrate_pages_batch (mm/migrate.c:1189 mm/migrate.c:1851)
>     [ 7831.897228] ? __pfx_alloc_migration_target (mm/migrate.c:2137)
>     [ 7831.897735] migrate_pages (mm/migrate.c:2078)
>     [ 7831.898141] ? __pfx_alloc_migration_target (mm/migrate.c:2137)
>     [ 7831.898664] damon_migrate_folio_list (mm/damon/ops-common.c:321 mm/damon/ops-common.c:354)
>     [ 7831.899140] damon_migrate_pages (mm/damon/ops-common.c:405)
>     [...]
> 
> Add a target node validity check in damon_migrate_pages().  The validity
> check is stolen from that of do_pages_move(), which is being used for
> move_pages() system call.
> 
> Fixes: b51820ebea65 ("mm/damon/paddr: introduce DAMOS_MIGRATE_COLD action for demotion") # 6.11.x
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Honggyu Kim <honggyu.kim@xxxxxx>
> Signed-off-by: SeongJae Park <sj@xxxxxxxxxx>
> ---

LGTM, thank you SJ!

On a side note... This seems like it would be a common check. However, doing a
(quick) search seems to return no function that checks whether a node is valid.
Perhaps it would make sense to look deeper and see how many other functions
make this check, and export this as a function? I can try spinning something
if it makes sense to you : -)

Reviewed-by: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>

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