Date:   Wed, 21 Dec 2022 19:11:42 +0000
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, Baoquan He <bhe@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>
Subject: Re: [PATCH v2 1/3] mm: vmalloc: Avoid of calling __find_vmap_area()
 twise in __vunmap()

Some pedantic grammar/spelling stuff:-

(I know it can be a little annoying to get grammatical suggestions so I do hope
that it isn't too irritating!)

For the Subject line:-
'mm: vmalloc: Avoid of calling __find_vmap_area() twise in __vunmap()' ->
'mm: vmalloc: Avoid calling __find_vmap_area() twice in __vunmap()'

On Wed, Dec 21, 2022 at 06:44:52PM +0100, Uladzislau Rezki (Sony) wrote:
> Currently __vunmap() path calls __find_vmap_area() two times. One on
> entry to check that area exists, second time inside remove_vm_area()
> function that also performs a new search of VA.

Perhaps slightly tweak to:-

"Currently the __vunmap() path calls __find_vmap_area() twice. Once on entry
 to check that the area exists, then inside the remove_vm_area() function
 which also performs a new search for the VA."

>
> In order to improvie it from a performance point of view we split
> remove_vm_area() into two new parts:
>   - find_unlink_vmap_area() that does a search and unlink from tree;
>   - __remove_vm_area() that does a removing but without searching.

'that does a removing but without searching' reads better I think as
'that removes without searching'.

>
> In this case there is no any functional change for remove_vm_area()
> whereas vm_remove_mappings(), where a second search happens, switches
> to the __remove_vm_area() variant where already detached VA is passed
> as a parameter, so there is no need to find it again.
>

'where already detached VA' -> 'where the already detached VA' as a minor nit
here!

> Performance wise, i use test_vmalloc.sh with 32 threads doing alloc
> free on a 64-CPUs-x86_64-box:
>
> perf without this patch:
> -   31.41%     0.50%  vmalloc_test/10  [kernel.vmlinux]    [k] __vunmap
>    - 30.92% __vunmap
>       - 17.67% _raw_spin_lock
>            native_queued_spin_lock_slowpath
>       - 12.33% remove_vm_area
>          - 11.79% free_vmap_area_noflush
>             - 11.18% _raw_spin_lock
>                  native_queued_spin_lock_slowpath
>         0.76% free_unref_page
>
> perf with this patch:
> -   11.35%     0.13%  vmalloc_test/14  [kernel.vmlinux]    [k] __vunmap
>    - 11.23% __vunmap
>       - 8.28% find_unlink_vmap_area
>          - 7.95% _raw_spin_lock
>               7.44% native_queued_spin_lock_slowpath
>       - 1.93% free_vmap_area_noflush
>          - 0.56% _raw_spin_lock
>               0.53% native_queued_spin_lock_slowpath
>         0.60% __vunmap_range_noflush
>
> __vunmap() consumes around ~20% less CPU cycles on this test.

Very nice, amazing work!

>
> Reported-by: Roman Gushchin <roman.gushchin@...ux.dev>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  mm/vmalloc.c | 66 +++++++++++++++++++++++++++++++++-------------------
>  1 file changed, 42 insertions(+), 24 deletions(-)
>
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 9e30f0b39203..28030d2441f1 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1825,9 +1825,11 @@ static void free_vmap_area_noflush(struct vmap_area *va)
>  	unsigned long va_start = va->va_start;
>  	unsigned long nr_lazy;
>
> -	spin_lock(&vmap_area_lock);
> -	unlink_va(va, &vmap_area_root);
> -	spin_unlock(&vmap_area_lock);
> +	if (!list_empty(&va->list)) {
> +		spin_lock(&vmap_area_lock);
> +		unlink_va(va, &vmap_area_root);
> +		spin_unlock(&vmap_area_lock);
> +	}

Do we want to do the same in free_vmap_area()?

>
>  	nr_lazy = atomic_long_add_return((va->va_end - va->va_start) >>
>  				PAGE_SHIFT, &vmap_lazy_nr);
> @@ -1871,6 +1873,19 @@ struct vmap_area *find_vmap_area(unsigned long addr)
>  	return va;
>  }
>
> +static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
> +{
> +	struct vmap_area *va;
> +
> +	spin_lock(&vmap_area_lock);
> +	va = __find_vmap_area(addr, &vmap_area_root);
> +	if (va)
> +		unlink_va(va, &vmap_area_root);
> +	spin_unlock(&vmap_area_lock);
> +
> +	return va;
> +}
> +
>  /*** Per cpu kva allocator ***/
>
>  /*
> @@ -2591,6 +2606,20 @@ struct vm_struct *find_vm_area(const void *addr)
>  	return va->vm;
>  }
>
> +static struct vm_struct *__remove_vm_area(struct vmap_area *va)
> +{
> +	struct vm_struct *vm;
> +
> +	if (!va || !va->vm)
> +		return NULL;
> +
> +	vm = va->vm;
> +	kasan_free_module_shadow(vm);
> +	free_unmap_vmap_area(va);
> +
> +	return vm;
> +}
> +
>  /**
>   * remove_vm_area - find and remove a continuous kernel virtual area
>   * @addr:	    base address
> @@ -2607,22 +2636,8 @@ struct vm_struct *remove_vm_area(const void *addr)
>
>  	might_sleep();
>
> -	spin_lock(&vmap_area_lock);
> -	va = __find_vmap_area((unsigned long)addr, &vmap_area_root);
> -	if (va && va->vm) {
> -		struct vm_struct *vm = va->vm;
> -
> -		va->vm = NULL;
> -		spin_unlock(&vmap_area_lock);
> -
> -		kasan_free_module_shadow(vm);
> -		free_unmap_vmap_area(va);
> -
> -		return vm;
> -	}
> -
> -	spin_unlock(&vmap_area_lock);
> -	return NULL;
> +	va = find_unlink_vmap_area((unsigned long) addr);
> +	return __remove_vm_area(va);
>  }

Really nice separation of concerns and cleanup.

>
>  static inline void set_area_direct_map(const struct vm_struct *area,
> @@ -2637,15 +2652,16 @@ static inline void set_area_direct_map(const struct vm_struct *area,
>  }
>
>  /* Handle removing and resetting vm mappings related to the vm_struct. */
> -static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
> +static void vm_remove_mappings(struct vmap_area *va, int deallocate_pages)

Perhaps rename this to va_remove_mappings() or vmap_area_remove_mappings() since
it is now explicitly accepting a vmap_area rather than vm_struct?

>  {
> +	struct vm_struct *area = va->vm;
>  	unsigned long start = ULONG_MAX, end = 0;
>  	unsigned int page_order = vm_area_page_order(area);
>  	int flush_reset = area->flags & VM_FLUSH_RESET_PERMS;
>  	int flush_dmap = 0;
>  	int i;
>
> -	remove_vm_area(area->addr);
> +	__remove_vm_area(va);
>
>  	/* If this is not VM_FLUSH_RESET_PERMS memory, no need for the below. */
>  	if (!flush_reset)
> @@ -2690,6 +2706,7 @@ static void vm_remove_mappings(struct vm_struct *area, int deallocate_pages)
>  static void __vunmap(const void *addr, int deallocate_pages)
>  {
>  	struct vm_struct *area;

Feels like it's getting a bit confusing with 'va' representing vmap_area and
'area' which represents... vm_struct (this file has a bunch of naming
inconsistencies like this actually), perhaps rename this to 'vm'?

> +	struct vmap_area *va;
>
>  	if (!addr)
>  		return;
> @@ -2698,19 +2715,20 @@ static void __vunmap(const void *addr, int deallocate_pages)
>  			addr))
>  		return;
>
> -	area = find_vm_area(addr);
> -	if (unlikely(!area)) {
> +	va = find_unlink_vmap_area((unsigned long)addr);
> +	if (unlikely(!va)) {
>  		WARN(1, KERN_ERR "Trying to vfree() nonexistent vm area (%p)\n",
>  				addr);
>  		return;
>  	}
>
> +	area = va->vm;
>  	debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
>  	debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
>
>  	kasan_poison_vmalloc(area->addr, get_vm_area_size(area));
>
> -	vm_remove_mappings(area, deallocate_pages);
> +	vm_remove_mappings(va, deallocate_pages);
>
>  	if (deallocate_pages) {
>  		int i;
> --
> 2.30.2
>

Other than some pendatic points about grammar/naming this looks really good!

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