linux-kernel - Re: [PATCH 1/6] block: manage bio slab cache by xarray

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Date:   Mon, 4 Jan 2021 09:54:21 +0100
From:   Christoph Hellwig <hch@....de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] block: manage bio slab cache by xarray

On Wed, Dec 30, 2020 at 08:32:50AM +0800, Ming Lei wrote:
> Managing bio slab cache via xarray by using slab cache size as xarray index, and

Overly long line in the commit log above.

> +static struct bio_slab *create_bio_slab(unsigned int size)
> +{
> +	struct bio_slab *bslab = kzalloc(sizeof(*bslab), GFP_KERNEL);
> +	if (!bslab)
> +		return NULL;

Missing whitespace after the variable declaration.

> +
> +	snprintf(bslab->name, sizeof(bslab->name), "bio-%d", size);
> +	bslab->slab = kmem_cache_create(bslab->name, size,
> +			ARCH_KMALLOC_MINALIGN, SLAB_HWCACHE_ALIGN, NULL);
> +	if (bslab->slab) {
> +		bslab->slab_ref = 1;
> +		bslab->slab_size = size;
> +	} else {
> +		kfree(bslab);
> +		bslab = NULL;
> +	}
> +	return bslab;
> +}

I'd simply this to:

	if (!bslab->slab) {
		kfree(bslab);
		return NULL;
	}

	bslab->slab_ref = 1;
	bslab->slab_size = size;
	return bslab;
}

>  
>  static struct kmem_cache *bio_find_or_create_slab(unsigned int extra_size)
>  {
>  	unsigned int sz = sizeof(struct bio) + extra_size;
>  	struct kmem_cache *slab = NULL;
> +	struct bio_slab *bslab;
>  
>  	mutex_lock(&bio_slab_lock);
> +	bslab = xa_load(&bio_slabs, sz);
> +	if (bslab) {
> +		slab = bslab->slab;
> +		bslab->slab_ref++;
> +	} else {
> +		bslab = create_bio_slab(sz);
> +		if(bslab && !xa_err(xa_store(&bio_slabs, sz, bslab,
> +						GFP_KERNEL)))

Missing whitespace after the "if"

But more importantly, I'd expect the xa_store to go into create_bio_slab
to make the code a little more readable and to consolidate the error
handling.  Also we really shouldn't need the slab local variable in
this function.

>  static void bio_put_slab(struct bio_set *bs)
>  {
>  	struct bio_slab *bslab = NULL;
> +	unsigned int slab_size = bs->front_pad + sizeof(struct bio) +
> +		BIO_INLINE_VECS * sizeof(struct bio_vec);

This calculation would look nice factored out into a little helper with
a comment explaining it.