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A single fix for debugobjects.

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The deferred page initialization prevents debug objects from allocating
 slab pages until the initialization is complete. That causes depletion of
 the pool and disabling of debugobjects.
 
 The reason is that debugobjects uses __GFP_HIGH for allocations as it might
 be invoked from arbitrary contexts. When PREEMPT_COUNT is disabled there is
 no way to know whether the context is safe to set __GFP_KSWAPD_RECLAIM.
 
 This worked until v6.18. Since then 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.
 
 Work around that when PREEMPT_COUNT is enabled as the preempt counter
 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.
 
 That makes debugobjects depend on PREEMPT_COUNT || !DEFERRED_STRUCT_PAGE_INIT,
 which limits the coverage slightly, but keeps it functional for most cases.
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Merge tag 'core-debugobjects-2026-03-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull debugobjects fix from Thomas Gleixner:
 "A single fix for debugobjects.

  The deferred page initialization prevents debug objects from
  allocating slab pages until the initialization is complete. That
  causes depletion of the pool and disabling of debugobjects.

  The reason is that debugobjects uses __GFP_HIGH for allocations as it
  might be invoked from arbitrary contexts. When PREEMPT_COUNT is
  disabled there is no way to know whether the context is safe to set
  __GFP_KSWAPD_RECLAIM.

  This worked until v6.18. Since then 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.

  Work around that when PREEMPT_COUNT is enabled as the preempt counter
  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.

  That makes debugobjects depend on PREEMPT_COUNT ||
  !DEFERRED_STRUCT_PAGE_INIT, which limits the coverage slightly, but
  keeps it functional for most cases"

* tag 'core-debugobjects-2026-03-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  debugobject: Make it work with deferred page initialization - again
This commit is contained in:
Linus Torvalds 2026-03-01 13:32:32 -08:00
commit e2bd1b1369
2 changed files with 19 additions and 1 deletions
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1
lib/Kconfig.debug
View file

@ -760,6 +760,7 @@ 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

19
lib/debugobjects.c
View file

@ -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);