Re: debugobject: Make it work with deferred page initialization - again

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 2/7/26 14:27, Thomas Gleixner wrote:
> debugobjects uses __GFP_HIGH for allocations as it might be invoked
> within locked regions. That worked perfectly fine until v6.18. It still
> works correctly when deferred page initialization is disabled and works
> by chance when no page allocation is required before deferred page
> initialization has completed.
> 
> Since v6.18 allocations w/o a reclaim flag cause new_slab() to end up in
> alloc_frozen_pages_nolock_noprof(), which returns early when deferred
> page initialization has not yet completed. As the deferred page
> initialization takes quite a while the debugobject pool is depleted and
> debugobjects are disabled.
> 
> This can be worked around when PREEMPT_COUNT is enabled as that allows
> debugobjects to add __GFP_KSWAPD_RECLAIM to the GFP flags when the context
> is preemtible. When PREEMPT_COUNT is disabled the context is unknown and
> the reclaim bit can't be set because the caller might hold locks which
> might deadlock in the allocator.
> 
> In preemptible context the reclaim bit is harmless and not a performance
> issue as that's usually invoked from slow path initialization context.
> 
> That makes debugobjects depend on PREEMPT_COUNT || !DEFERRED_STRUCT_PAGE_INIT.
> 
> Fixes: af92793e52c3 ("slab: Introduce kmalloc_nolock() and kfree_nolock().")
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

FYI, it should now be possible to revert this workaround:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=335c347686e76df9d2c7d7f61b5ea627a4c5cb4c

> ---
>  lib/Kconfig.debug  |    1 +
>  lib/debugobjects.c |   19 ++++++++++++++++++-
>  2 files changed, 19 insertions(+), 1 deletion(-)
> 
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -753,6 +753,7 @@ source "mm/Kconfig.debug"
>  
>  config DEBUG_OBJECTS
>  	bool "Debug object operations"
> +	depends on PREEMPT_COUNT || !DEFERRED_STRUCT_PAGE_INIT
>  	depends on DEBUG_KERNEL
>  	help
>  	  If you say Y here, additional code will be inserted into the
> --- a/lib/debugobjects.c
> +++ b/lib/debugobjects.c
> @@ -398,9 +398,26 @@ static void fill_pool(void)
>  
>  	atomic_inc(&cpus_allocating);
>  	while (pool_should_refill(&pool_global)) {
> +		gfp_t gfp = __GFP_HIGH | __GFP_NOWARN;
>  		HLIST_HEAD(head);
>  
> -		if (!kmem_alloc_batch(&head, obj_cache, __GFP_HIGH | __GFP_NOWARN))
> +		/*
> +		 * Allow reclaim only in preemptible context and during
> +		 * early boot. If not preemptible, the caller might hold
> +		 * locks causing a deadlock in the allocator.
> +		 *
> +		 * If the reclaim flag is not set during early boot then
> +		 * allocations, which happen before deferred page
> +		 * initialization has completed, will fail.
> +		 *
> +		 * In preemptible context the flag is harmless and not a
> +		 * performance issue as that's usually invoked from slow
> +		 * path initialization context.
> +		 */
> +		if (preemptible() || system_state < SYSTEM_SCHEDULING)
> +			gfp |= __GFP_KSWAPD_RECLAIM;
> +
> +		if (!kmem_alloc_batch(&head, obj_cache, gfp))
>  			break;
>  
>  		guard(raw_spinlock_irqsave)(&pool_lock);
>