Re: [PATCH v2 15/16] mm/slab: remove __GFP_NO_OBJ_EXT usage from alloc_slab_obj_exts()

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

On 6/10/26 17:40, Vlastimil Babka (SUSE) wrote:
> __GFP_NO_OBJ_EXT has limited scope within the slab allocator itself and
> gfp flags are a scarce resource, unlike slab's alloc_flags.
> 
> Introduce SLAB_ALLOC_NO_RECURSE alloc flag that has the same intent as
> __GFP_NO_OBJ_EXT but a more generic name, meaning that a kmalloc()
> family function should not recurse into another kmalloc*() for the
> purposes of allocating auxiliary structures (obj_ext arrays or sheaves).
> 
> First, replace the __GFP_NO_OBJ_EXT for allocating obj_ext arrays in
> alloc_slab_obj_exts(). Make use of the newly added kmalloc_flags()
> function, where we can pass alloc_flags with SLAB_ALLOC_NO_RECURSE
> added. This will also pass through SLAB_ALLOC_TRYLOCK so we don't need
> to special case kmalloc_nolock() anymore.
> 
> Note that until now the kmalloc_nolock() ignored the incoming gfp flags
> and hardcoded __GFP_ZERO | __GFP_NO_OBJ_EXT. But it's correct to pass on
> the incoming gfp flags (only augmented with __GFP_ZERO), because if
> alloc_flags contain SLAB_ALLOC_TRYLOCK, the incoming gfp flags have to
> be also compatible with it.
> 
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

As pointed out by Sashiko, this piecemeal approach creates a bisection
hazard where sheaves -> obj_ext -> sheaves -> ... recursion can happen.
So I'll changes this as follows to make obj_ext accept and pass both the
gfp and alloc_flags preventing recursion, and change the next patch
to revert that temporary change again.

diff --git a/mm/slub.c b/mm/slub.c
index a81f1f6bad67..c60f3a252ae5 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2167,6 +2167,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
 
 	gfp &= ~OBJCGS_CLEAR_MASK;
 	/* Prevent recursive extension vector allocation */
+	gfp |= __GFP_NO_OBJ_EXT;
 	alloc_flags |= SLAB_ALLOC_NO_RECURSE;
 
 	sz = obj_exts_alloc_size(s, slab, gfp);
@@ -2371,7 +2372,7 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags,
 	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
 		return;
 
-	if (alloc_flags & SLAB_ALLOC_NO_RECURSE)
+	if (alloc_flags & SLAB_ALLOC_NO_RECURSE || flags & __GFP_NO_OBJ_EXT)
 		return;
 
 	slab = virt_to_slab(object);