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

Sergey Senozhatsky
2014-11-15T13:20:12Z

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
> >
> > --
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