Re: [PATCH] mm/page_alloc: skip debug_check_no_{obj,locks}_freed with FPI_TRYLOCK

[Vlastimil Babka <vbabka@suse.cz>]

On 2/6/26 18:20, Harry Yoo wrote:
> On Fri, Feb 06, 2026 at 12:08:04PM -0500, Zi Yan wrote:
>> On 6 Feb 2026, at 11:58, Harry Yoo wrote:
>> 
>> > When CONFIG_DEBUG_OBJECTS_FREE is enabled,
>> > debug_check_no_{obj,locks}_freed() functions are called.
>> >
>> > Since both of them spin on a lock, they are not safe to be called
>> > if the FPI_TRYLOCK flag is specified. This leads to a lockdep splat:
>> >
>> >   ================================
>> >   WARNING: inconsistent lock state
>> >   6.19.0-rc5-slab-for-next+ #326 Tainted: G                 N
>> >   --------------------------------
>> >   inconsistent {INITIAL USE} -> {IN-NMI} usage.
>> >   kunit_try_catch/9046 [HC2[2]:SC0[0]:HE0:SE1] takes:
>> >   ffffffff84ed6bf8 (&obj_hash[i].lock){-.-.}-{2:2}, at: __debug_check_no_obj_freed+0xe0/0x300
>> >   {INITIAL USE} state was registered at:
>> >     lock_acquire+0xd9/0x2f0
>> >     _raw_spin_lock_irqsave+0x4c/0x80
>> >     __debug_object_init+0x9d/0x1f0
>> >     debug_object_init+0x34/0x50
>> >     __init_work+0x28/0x40
>> >     init_cgroup_housekeeping+0x151/0x210
>> >     init_cgroup_root+0x3d/0x140
>> >     cgroup_init_early+0x30/0x240
>> >     start_kernel+0x3e/0xcd0
>> >     x86_64_start_reservations+0x18/0x30
>> >     x86_64_start_kernel+0xf3/0x140
>> >     common_startup_64+0x13e/0x148
>> >   irq event stamp: 2998
>> >   hardirqs last  enabled at (2997): [<ffffffff8298b77a>] exc_nmi+0x11a/0x240
>> >   hardirqs last disabled at (2998): [<ffffffff8298b991>] sysvec_irq_work+0x11/0x110
>> >   softirqs last  enabled at (1416): [<ffffffff813c1f72>] __irq_exit_rcu+0x132/0x1c0
>> >   softirqs last disabled at (1303): [<ffffffff813c1f72>] __irq_exit_rcu+0x132/0x1c0
>> >
>> >   other info that might help us debug this:
>> >    Possible unsafe locking scenario:
>> >
>> >          CPU0
>> >          ----
>> >     lock(&obj_hash[i].lock);
>> >     <Interrupt>
>> >       lock(&obj_hash[i].lock);
>> >
>> >    *** DEADLOCK ***
>> >
>> > Fix this by adding an fpi_t parameter to free_pages_prepare() and
>> > skipping those checks if FPI_TRYLOCK is set. Since mm/compaction.c
>> > calls free_pages_prepare(), move the fpi_t definition to mm/internal.h.
>> >
>> > Fixes: 8c57b687e833 ("mm, bpf: Introduce free_pages_nolock()")
>> > Cc: <stable@vger.kernel.org>
>> > Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
>> > ---
>> >  mm/compaction.c |  2 +-
>> >  mm/internal.h   | 35 ++++++++++++++++++++++++++++++++++-
>> >  mm/page_alloc.c | 42 ++++++------------------------------------
>> >  3 files changed, 41 insertions(+), 38 deletions(-)
>> >
>> > diff --git a/mm/compaction.c b/mm/compaction.c
>> > index 1e8f8eca318c..9ffeb7c6d2b0 100644
>> > --- a/mm/compaction.c
>> > +++ b/mm/compaction.c
>> > @@ -1859,7 +1859,7 @@ static void compaction_free(struct folio *dst, unsigned long data)
>> >  	struct page *page = &dst->page;
>> >
>> >  	if (folio_put_testzero(dst)) {
>> > -		free_pages_prepare(page, order);
>> > +		free_pages_prepare(page, order, FPI_NONE);
>> 
>> Is it OK to add something like free_pages_prepare_fpi_none() for this one
>> to avoid the FPI flag move?
> 
> Yeah, moving FPI flag definition isn't great :)
> 
> I'm totally fine with your suggestion,
> as long as page allocator/compaction folks are fine with it!

Maybe even like this?
free_pages_prepare() which calls __free_pages_prepare(FPI_NONE)