linux-kernel - Re: [PATCH 2/6] lib/rhashtable: guarantee initial hashtable allocation

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Message-ID: <20180528094956.zaxusqqju3wtbdcz@gondor.apana.org.au>
Date:   Mon, 28 May 2018 17:49:56 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
        tgraf@...g.ch, manfred@...orfullife.com,
        guillaume.knispel@...ersonicimagine.com, linux-api@...r.kernel.org,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 2/6] lib/rhashtable: guarantee initial hashtable
 allocation

On Thu, May 24, 2018 at 02:11:31PM -0700, Davidlohr Bueso wrote:
>
> -static struct bucket_table *bucket_table_alloc(struct rhashtable *ht,
> -					       size_t nbuckets,
> -					       gfp_t gfp)
> +static struct bucket_table *__bucket_table_alloc(struct rhashtable *ht,
> +						 size_t nbuckets,
> +						 gfp_t gfp, bool retry)
>  {
>  	struct bucket_table *tbl = NULL;
>  	size_t size, max_locks;
>  	int i;
>  
>  	size = sizeof(*tbl) + nbuckets * sizeof(tbl->buckets[0]);
> +	if (retry) {
> +		gfp |= __GFP_NOFAIL;
> +		tbl = kzalloc(size, gfp);
> +	} /* fall-through */

I'd prefer this logic to be moved to the caller.  So just call the
function with GFP_KERNEL | __GFP_NOFAIL.

Of course you need to modify bucket_table_alloc so that it still
treats this as GFP_KERNEL (as opposed to GFP_ATOMIC).  That is,
instead of 

>  	if (gfp != GFP_KERNEL)

You will need
	if ((gfp & ~__GFP_NOFAIL) != GFP_KERNEL)

> @@ -1067,9 +1086,20 @@ int rhashtable_init(struct rhashtable *ht,
>  		}
>  	}
>  
> +	/*
> +	 * This is api initialization and thus we need to guarantee the
> +	 * initial rhashtable allocation. Upon failure, retry with a
> +	 * smallest possible size, otherwise we exhaust our options with
> +	 * __GFP_NOFAIL.
> +	 */
>  	tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
> -	if (tbl == NULL)
> -		return -ENOMEM;
> +	if (unlikely(tbl == NULL)) {
> +		size = HASH_MIN_SIZE;
> +
> +		tbl = bucket_table_alloc(ht, size, GFP_KERNEL);
> +		if (tbl == NULL)
> +			tbl = bucket_table_alloc_retry(ht, size, GFP_KERNEL);
> +	}

Perhaps you should also explain here why we don't just try the
minimum size with __GFP_NOFAIL as the second step rather than the
third.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt