Re: [PATCH 1/6] zsmalloc: switch from alloc_vm_area to get_vm_area

[Christoph Hellwig <hch@lst.de>]

On Mon, Sep 21, 2020 at 10:42:56AM -0700, Minchan Kim wrote:
> IIRC, the problem was runtime pte popluating needs GFP_KERNEL but
> zs_map_object API runs under non-preemtible section.

Make sense.

> > -	area->vm = alloc_vm_area(PAGE_SIZE * 2, NULL);
> > +	area->vm = get_vm_area(PAGE_SIZE * 2, 0);
> >  	if (!area->vm)
> >  		return -ENOMEM;
> >  	return 0;
> 
> I think it shoud work.
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 05789aa4af12..6a1e4d854593 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2232,7 +2232,7 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
>  	arch_enter_lazy_mmu_mode();
>  
>  	do {
> -		if (create || !pte_none(*pte)) {
> +		if ((create || !pte_none(*pte)) && fn) {
>  			err = fn(pte++, addr, data);
>  			if (err)
>  				break;
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 3e4fe3259612..9ef7daf3d279 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1116,6 +1116,8 @@ static struct zspage *find_get_zspage(struct size_class *class)
>  #ifdef CONFIG_ZSMALLOC_PGTABLE_MAPPING
>  static inline int __zs_cpu_up(struct mapping_area *area)
>  {
> +	int ret;
> +
>  	/*
>  	 * Make sure we don't leak memory if a cpu UP notification
>  	 * and zs_init() race and both call zs_cpu_up() on the same cpu
> @@ -1125,7 +1127,13 @@ static inline int __zs_cpu_up(struct mapping_area *area)
>  	area->vm = get_vm_area(PAGE_SIZE * 2, 0);
>  	if (!area->vm)
>  		return -ENOMEM;
> -	return 0;
> +
> +	/*
> +	 * Populate ptes in advance to avoid pte allocation with GFP_KERNEL
> +	 * in non-preemtible context of zs_map_object.
> +	 */
> +	ret = apply_to_page_range(&init_mm, NULL, PAGE_SIZE * 2, NULL, NULL);
> +	return ret;

I think this needs the addr from the vm area somewhere..

We probaby want to add a trivial helper to prefault an area instead of
the open coded variant.