Re: [patch 2/2] fs: buffer: move allocation failure loop into theallocator

[Andrew Morton]

On Wed, 4 Dec 2013 10:52:18 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:

> SLUB already try to allocate high order page with clearing __GFP_NOFAIL.
> But, when allocating shadow page for kmemcheck, it missed clearing
> the flag. This trigger WARN_ON_ONCE() reported by Christian Casteyde.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=65991
> 
> This patch fix this situation by using same allocation flag as original
> allocation.
> 
> Reported-by: Christian Casteyde <casteyde.christian@xxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 545a170..3dd28b1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1335,11 +1335,12 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  	page = alloc_slab_page(alloc_gfp, node, oo);
>  	if (unlikely(!page)) {
>  		oo = s->min;

What is the value of s->min?  Please tell me it's zero.

> +		alloc_gfp = flags;
>  		/*
>  		 * Allocation may have failed due to fragmentation.
>  		 * Try a lower order alloc if possible
>  		 */
> -		page = alloc_slab_page(flags, node, oo);
> +		page = alloc_slab_page(alloc_gfp, node, oo);
>  
>  		if (page)
>  			stat(s, ORDER_FALLBACK);

This change doesn't actually do anything.

> @@ -1349,7 +1350,7 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>  		&& !(s->flags & (SLAB_NOTRACK | DEBUG_DEFAULT_FLAGS))) {
>  		int pages = 1 << oo_order(oo);
>  
> -		kmemcheck_alloc_shadow(page, oo_order(oo), flags, node);
> +		kmemcheck_alloc_shadow(page, oo_order(oo), alloc_gfp, node);

That seems reasonable, assuming kmemcheck can handle the allocation
failure.


Still I dislike this practice of using unnecessarily large allocations.
What does it gain us?  Slightly improved object packing density. 
Anything else?

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