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    SubjectRe: [PATCH] mm: don't call should_failslab() for !CONFIG_FAILSLAB
    On Tue, 5 Oct 2021 09:31:43 -0600 Jens Axboe <axboe@kernel.dk> wrote:

    > Allocations can be a very hot path, and this out-of-line function
    > call is noticeable.
    >
    > --- a/include/linux/fault-inject.h
    > +++ b/include/linux/fault-inject.h
    > @@ -64,8 +64,8 @@ static inline struct dentry *fault_create_debugfs_attr(const char *name,
    >
    > struct kmem_cache;
    >
    > -int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
    > #ifdef CONFIG_FAILSLAB
    > +int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
    > extern bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags);
    > #else
    > static inline bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
    > diff --git a/mm/slab.h b/mm/slab.h
    > index 58c01a34e5b8..92fd6fe01877 100644
    > --- a/mm/slab.h
    > +++ b/mm/slab.h
    > @@ -491,8 +491,10 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
    >
    > might_alloc(flags);
    >
    > +#ifdef CONFIG_FAILSLAB
    > if (should_failslab(s, flags))
    > return NULL;
    > +#endif

    Can we avoid the ifdefs here?

    >
    > if (!memcg_slab_pre_alloc_hook(s, objcgp, size, flags))
    > return NULL;
    > diff --git a/mm/slab_common.c b/mm/slab_common.c
    > index ec2bb0beed75..c21bd447f237 100644
    > --- a/mm/slab_common.c
    > +++ b/mm/slab_common.c
    > @@ -1323,6 +1323,7 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc_node);
    > EXPORT_TRACEPOINT_SYMBOL(kfree);
    > EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
    >
    > +#ifdef CONFIG_FAILSLAB
    > int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
    > {
    > if (__should_failslab(s, gfpflags))
    > @@ -1330,3 +1331,4 @@ int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
    > return 0;
    > }
    > ALLOW_ERROR_INJECTION(should_failslab, ERRNO);
    > +#endif

    Like,

    --- a/include/linux/fault-inject.h~mm-dont-call-should_failslab-for-config_failslab-fix
    +++ a/include/linux/fault-inject.h
    @@ -68,6 +68,10 @@ struct kmem_cache;
    int should_failslab(struct kmem_cache *s, gfp_t gfpflags);
    extern bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags);
    #else
    +static inline int should_failslab(struct kmem_cache *s, gfp_t gfpflags)
    +{
    + return 0;
    +}
    static inline bool __should_failslab(struct kmem_cache *s, gfp_t gfpflags)
    {
    return false;
    --- a/mm/slab.h~mm-dont-call-should_failslab-for-config_failslab-fix
    +++ a/mm/slab.h
    @@ -491,10 +491,8 @@ static inline struct kmem_cache *slab_pr

    might_alloc(flags);

    -#ifdef CONFIG_FAILSLAB
    if (should_failslab(s, flags))
    return NULL;
    -#endif

    if (!memcg_slab_pre_alloc_hook(s, objcgp, size, flags))
    return NULL;
    _
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