Re: [PATCH v3 06/15] mm/slab: add alloc_flags to slab_alloc_context

[Suren Baghdasaryan]

On Mon, Jun 15, 2026 at 4:55 AM Vlastimil Babka (SUSE)
<vbabka@xxxxxxxxxx> wrote:
>
> Add alloc_flags as a new field to the slab_alloc_context helper struct,
> so we can pass it to more functions in the slab implementation without
> adding another function parameter.
>
> Start checking them via alloc_flags_allow_spinning() in
> alloc_single_from_new_slab() (where we can drop the allow_spin
> parameter), ___slab_alloc(), get_from_partial_node() and
> get_from_any_partial(). This further reduces false-positive
> spinning-not-allowed from allocations that are not kmalloc_nolock() but
> lack __GFP_RECLAIM flags.
>
> _kmalloc_nolock_noprof() initializes ac.alloc_flags using its flags that
> are SLAB_ALLOC_NOLOCK. slab_alloc_node() and __kmem_cache_alloc_bulk()
> are not reachable from kmalloc_nolock() and all their callers expect
> spinning to be allowed, so they can use SLAB_ALLOC_DEFAULT. This is
> temporary as the scope of slab_alloc_context will further move to the
> callers, making the alloc_flags usage more obvious.
>
> Also change how trynode_flags are constructed in ___slab_alloc() to
> achieve the same "do not upgrade to GFP_NOWAIT" by using masking instead
> of checking allow_spin. We need to do that because we now determine
> allow_spin from alloc_flags, and would otherwise start to upgrade e.g.
> kmalloc() allocations without __GFP_KSWAPD_RECLAIM (that however do
> allow spinning) to GFP_NOWAIT, thus including __GFP_KSWAPD_RECLAIM.
>
> During the masking keep also existing __GFP_NOMEMALLOC (pointed out by
> Sashiko) and __GFP_ACCOUNT. Previously the hardcoded GFP_NOWAIT would
> eliminate them, but it's not a big problem that would need a separate
> fix.
>
> Link: https://patch.msgid.link/20260610-slab_alloc_flags-v2-6-7190909db118@xxxxxxxxxx
> Reviewed-by: Harry Yoo (Oracle) <harry@xxxxxxxxxx>
> Reviewed-by: Hao Li <hao.li@xxxxxxxxx>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>


> ---
>  mm/slub.c | 28 +++++++++++++++-------------
>  1 file changed, 15 insertions(+), 13 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 6f6c15d796e1..3a34907b881b 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -217,6 +217,7 @@ static DEFINE_STATIC_KEY_FALSE(strict_numa);
>  struct slab_alloc_context {
>         unsigned long caller_addr;
>         size_t orig_size;
> +       unsigned int alloc_flags;
>  };
>
>  /* Structure holding parameters for get_partial_node_bulk() */
> @@ -3687,9 +3688,9 @@ static inline void init_slab_obj_iter(struct kmem_cache *s, struct slab *slab,
>   * and put the slab to the partial (or full) list.
>   */
>  static void *alloc_single_from_new_slab(struct kmem_cache *s, struct slab *slab,
> -                                       const struct slab_alloc_context *ac,
> -                                       bool allow_spin)
> +                                       const struct slab_alloc_context *ac)
>  {
> +       bool allow_spin = alloc_flags_allow_spinning(ac->alloc_flags);
>         struct kmem_cache_node *n;
>         struct slab_obj_iter iter;
>         bool needs_add_partial;
> @@ -3835,7 +3836,7 @@ static void *get_from_partial_node(struct kmem_cache *s,
>         if (!n || !n->nr_partial)
>                 return NULL;
>
> -       if (gfpflags_allow_spinning(gfp_flags))
> +       if (alloc_flags_allow_spinning(ac->alloc_flags))
>                 spin_lock_irqsave(&n->list_lock, flags);
>         else if (!spin_trylock_irqsave(&n->list_lock, flags))
>                 return NULL;
> @@ -3891,7 +3892,7 @@ static void *get_from_any_partial(struct kmem_cache *s, gfp_t gfp_flags,
>         struct zone *zone;
>         enum zone_type highest_zoneidx = gfp_zone(gfp_flags);
>         unsigned int cpuset_mems_cookie;
> -       bool allow_spin = gfpflags_allow_spinning(gfp_flags);
> +       bool allow_spin = alloc_flags_allow_spinning(ac->alloc_flags);
>
>         /*
>          * The defrag ratio allows a configuration of the tradeoffs between
> @@ -4449,7 +4450,7 @@ static unsigned int alloc_from_new_slab(struct kmem_cache *s, struct slab *slab,
>  static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>                            const struct slab_alloc_context *ac)
>  {
> -       bool allow_spin = gfpflags_allow_spinning(gfpflags);
> +       bool allow_spin = alloc_flags_allow_spinning(ac->alloc_flags);
>         gfp_t trynode_flags;
>         void *object;
>         struct slab *slab;
> @@ -4466,18 +4467,15 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>          * 1) try to get a partial slab from target node only by having
>          *    __GFP_THISNODE in trynode_flags for get_from_partial()
>          * 2) if 1) failed, try to allocate a new slab from target node with
> -        *    GPF_NOWAIT | __GFP_THISNODE opportunistically
> +        *    (at most) GFP_NOWAIT | __GFP_THISNODE opportunistically
>          * 3) if 2) failed, retry with original gfpflags which will allow
>          *    get_from_partial() try partial lists of other nodes before
>          *    potentially allocating new page from other nodes
>          */
>         if (unlikely(node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE)
>                      && try_thisnode)) {
> -               if (unlikely(!allow_spin))
> -                       /* Do not upgrade gfp to NOWAIT from more restrictive mode */
> -                       trynode_flags = gfpflags | __GFP_THISNODE;
> -               else
> -                       trynode_flags = GFP_NOWAIT | __GFP_THISNODE;
> +               trynode_flags &= GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_ACCOUNT;
> +               trynode_flags |= __GFP_NOWARN | __GFP_THISNODE;
>         }
>
>         object = get_from_partial(s, node, trynode_flags, ac);
> @@ -4499,7 +4497,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>         stat(s, ALLOC_SLAB);
>
>         if (IS_ENABLED(CONFIG_SLUB_TINY) || kmem_cache_debug(s)) {
> -               object = alloc_single_from_new_slab(s, slab, ac, allow_spin);
> +               object = alloc_single_from_new_slab(s, slab, ac);
>
>                 if (likely(object))
>                         goto success;
> @@ -4918,6 +4916,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, gfp_t gfp, size_t size,
>  static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list_lru *lru,
>                 gfp_t gfpflags, int node, unsigned long addr, size_t orig_size)
>  {
> +       const unsigned int alloc_flags = SLAB_ALLOC_DEFAULT;
>         void *object;
>
>         s = slab_pre_alloc_hook(s, gfpflags);
> @@ -4928,12 +4927,13 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
>         if (unlikely(object))
>                 goto out;
>
> -       object = alloc_from_pcs(s, gfpflags, SLAB_ALLOC_DEFAULT, node);
> +       object = alloc_from_pcs(s, gfpflags, alloc_flags, node);
>
>         if (unlikely(!object)) {
>                 const struct slab_alloc_context ac = {
>                         .caller_addr = addr,
>                         .orig_size = orig_size,
> +                       .alloc_flags = alloc_flags,
>                 };
>                 object = __slab_alloc_node(s, gfpflags, node, &ac);
>         }
> @@ -5366,6 +5366,7 @@ void *_kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_flags, in
>         const struct slab_alloc_context ac = {
>                 .caller_addr = _RET_IP_,
>                 .orig_size = orig_size,
> +               .alloc_flags = alloc_flags,
>         };
>
>         VM_WARN_ON_ONCE(gfp_flags & ~(__GFP_ACCOUNT | __GFP_ZERO |
> @@ -7254,6 +7255,7 @@ static bool __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags,
>                 const struct slab_alloc_context ac = {
>                         .caller_addr = _RET_IP_,
>                         .orig_size = s->object_size,
> +                       .alloc_flags = SLAB_ALLOC_DEFAULT,
>                 };
>                 for (i = 0; i < size; i++) {
>
>
> --
> 2.54.0
>