Date:   Mon, 5 Jun 2023 08:43:39 +0800
From:   Baoquan He <bhe@...hat.com>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 8/9] mm: vmalloc: Offload free_vmap_area_lock global lock

On 05/22/23 at 01:08pm, Uladzislau Rezki (Sony) wrote:
......  
> +static unsigned long
> +this_cpu_zone_alloc_fill(struct cpu_vmap_zone *z,
> +	unsigned long size, unsigned long align,
> +	gfp_t gfp_mask, int node)
> +{
> +	unsigned long addr = VMALLOC_END;
> +	struct vmap_area *va;
> +
> +	/*
> +	 * It still can race. One task sets a progress to
> +	 * 1 a second one gets preempted on entry, the first
> +	 * zeroed the progress flag and second proceed with
> +	 * an extra prefetch.
> +	 */
> +	if (atomic_xchg(&z->fill_in_progress, 1))
> +		return addr;
> +
> +	va = kmem_cache_alloc_node(vmap_area_cachep, gfp_mask, node);
> +	if (unlikely(!va))
> +		goto out;
> +
> +	spin_lock(&free_vmap_area_lock);
> +	addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
> +		cvz_size, 1, VMALLOC_START, VMALLOC_END);
> +	spin_unlock(&free_vmap_area_lock);

The 'z' is passed in from this_cpu_zone_alloc(), and it's got with
raw_cpu_ptr(&cpu_vmap_zone). Here when we try to get chunk of cvz_size
from free_vmap_area_root/free_vmap_area_list, how can we guarantee it
must belong to the 'z' zone? With my understanding, __alloc_vmap_area()
will get efficient address range sequentially bottom up from
free_vmap_area_root. Please correct me if I am wrong.

static unsigned long
this_cpu_zone_alloc(unsigned long size, unsigned long align, gfp_t gfp_mask, int node)
{
        struct cpu_vmap_zone *z = raw_cpu_ptr(&cpu_vmap_zone);
	......
	if (addr == VMALLOC_END && left < 4 * PAGE_SIZE)
                addr = this_cpu_zone_alloc_fill(z, size, align, gfp_mask, node);
}

> +
> +	if (addr == VMALLOC_END) {
> +		kmem_cache_free(vmap_area_cachep, va);
> +		goto out;
> +	}
> +
> +	va->va_start = addr;
> +	va->va_end = addr + cvz_size;
> +
> +	fbl_lock(z, FREE);
> +	va = merge_or_add_vmap_area_augment(va,
> +		&fbl_root(z, FREE), &fbl_head(z, FREE));
> +	addr = va_alloc(va, &fbl_root(z, FREE), &fbl_head(z, FREE),
> +		size, align, VMALLOC_START, VMALLOC_END);
> +	fbl_unlock(z, FREE);
> +
> +out:
> +	atomic_set(&z->fill_in_progress, 0);
> +	return addr;
> +}
> +
> +static unsigned long
> +this_cpu_zone_alloc(unsigned long size, unsigned long align, gfp_t gfp_mask, int node)
> +{
> +	struct cpu_vmap_zone *z = raw_cpu_ptr(&cpu_vmap_zone);
> +	unsigned long extra = align > PAGE_SIZE ? align : 0;
> +	unsigned long addr = VMALLOC_END, left = 0;
> +
> +	/*
> +	 * It is disabled, fallback to a global heap.
> +	 */
> +	if (cvz_size == ULONG_MAX)
> +		return addr;
> +
> +	/*
> +	 * Any allocation bigger/equal than one half of
                          ~~~~~~typo~~~~~~  bigger than/equal to
> +	 * a zone-size will fallback to a global heap.
> +	 */
> +	if (cvz_size / (size + extra) < 3)
> +		return addr;
> +
> +	if (RB_EMPTY_ROOT(&fbl_root(z, FREE)))
> +		goto fill;
> +
> +	fbl_lock(z, FREE);
> +	addr = __alloc_vmap_area(&fbl_root(z, FREE), &fbl_head(z, FREE),
> +		size, align, VMALLOC_START, VMALLOC_END);
> +
> +	if (addr == VMALLOC_END)
> +		left = get_subtree_max_size(fbl_root(z, FREE).rb_node);
> +	fbl_unlock(z, FREE);
> +
> +fill:
> +	/*
> +	 * A low watermark is 3 pages.
> +	 */
> +	if (addr == VMALLOC_END && left < 4 * PAGE_SIZE)
> +		addr = this_cpu_zone_alloc_fill(z, size, align, gfp_mask, node);
> +
> +	return addr;
> +}
> +
>  /*
>   * Allocate a region of KVA of the specified size and alignment, within the
>   * vstart and vend.
> @@ -1678,11 +1765,21 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
>  	 */
>  	kmemleak_scan_area(&va->rb_node, SIZE_MAX, gfp_mask);
>  
> +	/*
> +	 * Fast path allocation, start with it.
> +	 */
> +	if (vstart == VMALLOC_START && vend == VMALLOC_END)
> +		addr = this_cpu_zone_alloc(size, align, gfp_mask, node);
> +	else
> +		addr = vend;
> +
>  retry:
> -	preload_this_cpu_lock(&free_vmap_area_lock, gfp_mask, node);
> -	addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
> -		size, align, vstart, vend);
> -	spin_unlock(&free_vmap_area_lock);
> +	if (addr == vend) {
> +		preload_this_cpu_lock(&free_vmap_area_lock, gfp_mask, node);
> +		addr = __alloc_vmap_area(&free_vmap_area_root, &free_vmap_area_list,
> +			size, align, vstart, vend);
> +		spin_unlock(&free_vmap_area_lock);
> +	}
>  
>  	trace_alloc_vmap_area(addr, size, align, vstart, vend, addr == vend);
>  
> @@ -1827,6 +1924,27 @@ purge_cpu_vmap_zone(struct cpu_vmap_zone *z)
>  	return num_purged_areas;
>  }
>  
> +static void
> +drop_cpu_vmap_cache(struct cpu_vmap_zone *z)
> +{
> +	struct vmap_area *va, *n_va;
> +	LIST_HEAD(free_head);
> +
> +	if (RB_EMPTY_ROOT(&fbl_root(z, FREE)))
> +		return;
> +
> +	fbl_lock(z, FREE);
> +	WRITE_ONCE(fbl(z, FREE, root.rb_node), NULL);
> +	list_replace_init(&fbl_head(z, FREE), &free_head);
> +	fbl_unlock(z, FREE);
> +
> +	spin_lock(&free_vmap_area_lock);
> +	list_for_each_entry_safe(va, n_va, &free_head, list)
> +		merge_or_add_vmap_area_augment(va,
> +			&free_vmap_area_root, &free_vmap_area_list);
> +	spin_unlock(&free_vmap_area_lock);
> +}
> +
>  /*
>   * Purges all lazily-freed vmap areas.
>   */
> @@ -1868,6 +1986,7 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
>  		for_each_possible_cpu(i) {
>  			z = per_cpu_ptr(&cpu_vmap_zone, i);
>  			num_purged_areas += purge_cpu_vmap_zone(z);
> +			drop_cpu_vmap_cache(z);
>  		}
>  	}
>  
> -- 
> 2.30.2
> 

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