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

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

[Minchan Kim]

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.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 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

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@variantweb.net>

> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  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
> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

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

[Sergey Senozhatsky]

On (11/14/14 09:07), Seth Jennings wrote:
> 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:
> 

both solutions look good to me:
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

	-ss

> ---
> 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@variantweb.net>
> 
> > 
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  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
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
> 

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

[Ganesh Mahendran]

Hello

2014-11-14 9:11 GMT+08:00 Minchan Kim <minchan@kernel.org>:
> 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.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  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);
Just a nitpick:
I think
                    link = (struct link_free *)(vaddr + off);
is better.
To use the same method as in zs_free()
                   link = (struct link_free *)(vaddr + f_offset);

>
>                 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);
            link = (struct link_free *)(vaddr + m_offset)

>         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);
Yes, I think this method to calc *link* is better. (:

Thanks

>         link->next = first_page->freelist;
> -       kunmap_atomic(link);
> +       kunmap_atomic(vaddr);
>         first_page->freelist = (void *)obj;
>
>         first_page->inuse--;
> --
> 2.0.0
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/

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

[Andrew Morton]

On Fri, 14 Nov 2014 09:07:32 -0600 Seth Jennings <sjennings@variantweb.net> wrote:

> 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.

It is a bit alarming, but I've seen code elsewhere in which a modified
pointer is passed to kunmap_atomic().  So the kunmap_atomic() interface
is "kvaddr should point somewhere into the page" and that won't be
changing without a big effort.

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

I think Minchan's approach is better: it explicitly retains the
kmap_atomic() return value for passing to kunmap_atomic().  That's
nicer than modifying it and then setting it back again.

I mean, a cleaner way of implementing your suggestion would be

void kunmap_atomic_unaligned(void *p)
{
	kunmap_atomic(void *)((unsigned long)p & PAGE_MASK);
}

but then one looks at

void __kunmap_atomic(void *kvaddr)
{
	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;

and asks "what the heck".


So I dunno.  We could leave the code as-is.  I have no strong feelings
either way.  Minchan's patch has no effect on zsmalloc.o section sizes
with my compiler.

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

[Minchan Kim]

Hello Andrew,

On Tue, Nov 18, 2014 at 03:01:38PM -0800, Andrew Morton wrote:
> On Fri, 14 Nov 2014 09:07:32 -0600 Seth Jennings <sjennings@variantweb.net> wrote:
> 
> > 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.
> 
> It is a bit alarming, but I've seen code elsewhere in which a modified
> pointer is passed to kunmap_atomic().  So the kunmap_atomic() interface
> is "kvaddr should point somewhere into the page" and that won't be
> changing without a big effort.
> 
> > Seems like you could just use PAGE_MASK to get the base page address
> > from link like this:
> 
> I think Minchan's approach is better: it explicitly retains the
> kmap_atomic() return value for passing to kunmap_atomic().  That's
> nicer than modifying it and then setting it back again.
> 
> I mean, a cleaner way of implementing your suggestion would be
> 
> void kunmap_atomic_unaligned(void *p)
> {
> 	kunmap_atomic(void *)((unsigned long)p & PAGE_MASK);
> }
> 
> but then one looks at
> 
> void __kunmap_atomic(void *kvaddr)
> {
> 	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
> 
> and asks "what the heck".
> 
> 
> So I dunno.  We could leave the code as-is.  I have no strong feelings
> either way.  Minchan's patch has no effect on zsmalloc.o section sizes
> with my compiler.

I hope to merge my patch.

Main reason I sent the patch is I got a subtle bug when I implement
new feature of zsmalloc(ie, compaction) due to link's mishandling
(ie, link was over page boundary by my fault).
Although it was totally my mistake, it took time for a while
to find a root cause because unpredictable kmapped address should
be unmapped so it's almost random crash.

IOW, it's fragile to depend on kunmap_atomic's internal and at least,
I wanted to make zram code more robust.

Thanks.

> 
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org.  For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>

-- 
Kind regards,
Minchan Kim

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

[Andrew Morton]

On Wed, 19 Nov 2014 08:21:39 +0900 Minchan Kim <minchan@kernel.org> wrote:

> Main reason I sent the patch is I got a subtle bug when I implement
> new feature of zsmalloc(ie, compaction) due to link's mishandling
> (ie, link was over page boundary by my fault).
> Although it was totally my mistake, it took time for a while
> to find a root cause because unpredictable kmapped address should
> be unmapped so it's almost random crash.

Fair enough.

That's pretty rude behaviour from kunmap_atomic().  Unfortunately it
just doesn't have anything with which to check the address - we'd need
to create a special per-cpu array[KM_TYPE_NR] just for the purpose.