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Date: Fri, 14 Nov 2014 09:07:32 -0600
From: Seth Jennings <sjennings@...iantweb.net>
To: Minchan Kim <minchan@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Nitin Gupta <ngupta@...are.org>,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Dan Streetman <ddstreet@...e.org>,
Jerome Marchand <jmarchan@...hat.com>
Subject: Re: [PATCH] zsmalloc: correct fragile [kmap|kunmap]_atomic use
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@...iantweb.net>
>
> Signed-off-by: Minchan Kim <minchan@...nel.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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