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

[Minchan Kim <minchan@kernel.org>]

On Fri, Sep 18, 2020 at 06:37:19PM +0200, Christoph Hellwig wrote:
> There is no obvious reason why zsmalloc needs to pre-fault the PTEs
> given that it later uses map_kernel_range to just like vmap().

IIRC, the problem was runtime pte popluating needs GFP_KERNEL but
zs_map_object API runs under non-preemtible section.

> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  mm/zsmalloc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index c36fdff9a37131..3e4fe3259612fd 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1122,7 +1122,7 @@ static inline int __zs_cpu_up(struct mapping_area *area)
>  	 */
>  	if (area->vm)
>  		return 0;
> -	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;
 }
 
 static inline void __zs_cpu_down(struct mapping_area *area)