Re: [PATCH] zsmalloc: correct fragile [kmap|kunmap]_atomic use

[Seth Jennings]

On Fri, Nov 14, 2014 at 10:11:01AM +0900, Minchan Kim wrote:
> The kunmap_atomic should use virtual address getting by kmap_atomic.
> However, some pieces of code in zsmalloc uses modified address,
> not the one got by kmap_atomic for kunmap_atomic.
> 
> It's okay for working because zsmalloc modifies the address
> inner PAGE_SIZE bounday so it works with current kmap_atomic's
> implementation. But it's still fragile with potential changing
> of kmap_atomic so let's correct it.

Seems like you could just use PAGE_MASK to get the base page address
from link like this:

---
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index b3b57ef..d6ca05a 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -654,7 +654,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
                 */
                next_page = get_next_page(page);
                link->next = obj_location_to_handle(next_page, 0);
-               kunmap_atomic(link);
+               kunmap_atomic((void *)((unsigned long)link & PAGE_MASK));
                page = next_page;
                off %= PAGE_SIZE;
        }
@@ -1087,7 +1087,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
                                        m_offset / sizeof(*link);
        first_page->freelist = link->next;
        memset(link, POISON_INUSE, sizeof(*link));
-       kunmap_atomic(link);
+       kunmap_atomic((void *)((unsigned long)link & PAGE_MASK));
 
        first_page->inuse++;
        /* Now move the zspage to another fullness group, if required */
@@ -1124,7 +1124,7 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
        link = (struct link_free *)((unsigned char *)kmap_atomic(f_page)
                                                        + f_offset);
        link->next = first_page->freelist;
-       kunmap_atomic(link);
+       kunmap_atomic((void *)((unsigned long)link & PAGE_MASK));
        first_page->freelist = (void *)obj;
 
        first_page->inuse--;
---

This seems cleaner, but, at the same time, it isn't obvious that we are
passing the same value to kunmap_atomic() that we got from
kmap_atomic().  Just a thought.

Either way:

Reviewed-by: Seth Jennings <sjennings@xxxxxxxxxxxxxx>

> 
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
>  mm/zsmalloc.c | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index b3b57ef85830..85e14f584048 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -629,6 +629,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
>  		struct page *next_page;
>  		struct link_free *link;
>  		unsigned int i = 1;
> +		void *vaddr;
>  
>  		/*
>  		 * page->index stores offset of first object starting
> @@ -639,8 +640,8 @@ static void init_zspage(struct page *first_page, struct size_class *class)
>  		if (page != first_page)
>  			page->index = off;
>  
> -		link = (struct link_free *)kmap_atomic(page) +
> -						off / sizeof(*link);
> +		vaddr = kmap_atomic(page);
> +		link = (struct link_free *)vaddr + off / sizeof(*link);
>  
>  		while ((off += class->size) < PAGE_SIZE) {
>  			link->next = obj_location_to_handle(page, i++);
> @@ -654,7 +655,7 @@ static void init_zspage(struct page *first_page, struct size_class *class)
>  		 */
>  		next_page = get_next_page(page);
>  		link->next = obj_location_to_handle(next_page, 0);
> -		kunmap_atomic(link);
> +		kunmap_atomic(vaddr);
>  		page = next_page;
>  		off %= PAGE_SIZE;
>  	}
> @@ -1055,6 +1056,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>  	unsigned long obj;
>  	struct link_free *link;
>  	struct size_class *class;
> +	void *vaddr;
>  
>  	struct page *first_page, *m_page;
>  	unsigned long m_objidx, m_offset;
> @@ -1083,11 +1085,11 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size)
>  	obj_handle_to_location(obj, &m_page, &m_objidx);
>  	m_offset = obj_idx_to_offset(m_page, m_objidx, class->size);
>  
> -	link = (struct link_free *)kmap_atomic(m_page) +
> -					m_offset / sizeof(*link);
> +	vaddr = kmap_atomic(m_page);
> +	link = (struct link_free *)vaddr + m_offset / sizeof(*link);
>  	first_page->freelist = link->next;
>  	memset(link, POISON_INUSE, sizeof(*link));
> -	kunmap_atomic(link);
> +	kunmap_atomic(vaddr);
>  
>  	first_page->inuse++;
>  	/* Now move the zspage to another fullness group, if required */
> @@ -1103,6 +1105,7 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
>  	struct link_free *link;
>  	struct page *first_page, *f_page;
>  	unsigned long f_objidx, f_offset;
> +	void *vaddr;
>  
>  	int class_idx;
>  	struct size_class *class;
> @@ -1121,10 +1124,10 @@ void zs_free(struct zs_pool *pool, unsigned long obj)
>  	spin_lock(&class->lock);
>  
>  	/* Insert this object in containing zspage's freelist */
> -	link = (struct link_free *)((unsigned char *)kmap_atomic(f_page)
> -							+ f_offset);
> +	vaddr = kmap_atomic(f_page);
> +	link = (struct link_free *)(vaddr + f_offset);
>  	link->next = first_page->freelist;
> -	kunmap_atomic(link);
> +	kunmap_atomic(vaddr);
>  	first_page->freelist = (void *)obj;
>  
>  	first_page->inuse--;
> -- 
> 2.0.0
> 
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