Re: [PATCH 13/15] frontswap: Get rid of swap_lock dependency

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

Hi Minchan and Konrad,
On Fri, Feb 01, 2013 at 03:23:02PM -0500, Konrad Rzeszutek Wilk wrote:
>From: Minchan Kim <minchan@kernel.org>
>
>Frontswap initialization routine depends on swap_lock, which want
>to be atomic about frontswap's first appearance.
>IOW, frontswap is not present and will fail all calls OR frontswap is
>fully functional but if new swap_info_struct isn't registered
>by enable_swap_info, swap subsystem doesn't start I/O so there is no
>race
>between init procedure and page I/O working on frontswap.
>
>So let's remove unncessary swap_lock dependency.
>
>Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
>Signed-off-by: Minchan Kim <minchan@kernel.org>
>[v1: Rebased on my branch, reworked to work with backends loading late]
>[v2: Added a check for !map]
>Signed-off-by: Konrad Rzeszutek Wilk <konrad@darnok.org>
>
>squash
>---
> include/linux/frontswap.h |  6 +++---
> mm/frontswap.c            | 12 +++++++++---
> mm/swapfile.c             |  7 ++++++-
> 3 files changed, 18 insertions(+), 7 deletions(-)
>
>diff --git a/include/linux/frontswap.h b/include/linux/frontswap.h
>index 612c176..3d72f14 100644
>--- a/include/linux/frontswap.h
>+++ b/include/linux/frontswap.h
>@@ -20,7 +20,7 @@ extern void frontswap_writethrough(bool);
> #define FRONTSWAP_HAS_EXCLUSIVE_GETS
> extern void frontswap_tmem_exclusive_gets(bool);
>
>-extern void __frontswap_init(unsigned type);
>+extern void __frontswap_init(unsigned type, unsigned long *map);
> extern int __frontswap_store(struct page *page);
> extern int __frontswap_load(struct page *page);
> extern void __frontswap_invalidate_page(unsigned, pgoff_t);
>@@ -122,9 +122,9 @@ static inline void frontswap_invalidate_area(unsigned type)
> 	__frontswap_invalidate_area(type);
> }
>
>-static inline void frontswap_init(unsigned type)
>+static inline void frontswap_init(unsigned type, unsigned long *map)
> {
>-	__frontswap_init(type);
>+	__frontswap_init(type, map);
> }
>
> #endif /* _LINUX_FRONTSWAP_H */
>diff --git a/mm/frontswap.c b/mm/frontswap.c
>index ebf4c18..8254a6a 100644
>--- a/mm/frontswap.c
>+++ b/mm/frontswap.c
>@@ -127,8 +127,13 @@ struct frontswap_ops *frontswap_register_ops(struct frontswap_ops *ops)
> 	int i;
>
> 	for (i = 0; i < MAX_SWAPFILES; i++) {
>-		if (test_and_clear_bit(i, need_init))
>+		if (test_and_clear_bit(i, need_init)) {
>+			struct swap_info_struct *sis = swap_info[i];
>+			/* enable_swap_info _should_ have set it! */
>+			if (!sis->frontswap_map)
>+				return ERR_PTR(-EINVAL);
> 			ops->init(i);
>+		}
> 	}
> 	/*
> 	 * We MUST have frontswap_ops set _after_ the frontswap_init's
>@@ -166,14 +171,15 @@ EXPORT_SYMBOL(frontswap_tmem_exclusive_gets);
>  *
>  * Can be called without any backend driver is registered.
>  */
>-void __frontswap_init(unsigned type)
>+void __frontswap_init(unsigned type, unsigned long *map)
> {
> 	struct swap_info_struct *sis = swap_info[type];
>
> 	if (static_key_false(&frontswap_key)) {
> 		BUG_ON(sis == NULL);
>-		if (sis->frontswap_map == NULL)
>+		if (!map)
> 			return;
>+		frontswap_map_set(sis, map);
> 		frontswap_ops->init(type);
> 	}
> 	else {
>diff --git a/mm/swapfile.c b/mm/swapfile.c
>index e97a0e5..c1c3a62 100644
>--- a/mm/swapfile.c
>+++ b/mm/swapfile.c
>@@ -1454,6 +1454,10 @@ static void _enable_swap_info(struct swap_info_struct *p, int prio,
> 	else
> 		p->prio = --least_priority;
> 	p->swap_map = swap_map;
>+	/*
>+	 * This is required for frontswap to handle backends loading
>+	 * after the swap has been activated.
>+	 */
> 	frontswap_map_set(p, frontswap_map);

Why set frontswap_map twice?

> 	p->flags |= SWP_WRITEOK;
> 	nr_swap_pages += p->pages;
>@@ -1477,9 +1481,9 @@ static void enable_swap_info(struct swap_info_struct *p, int prio,
> 				unsigned char *swap_map,
> 				unsigned long *frontswap_map)
> {
>+	frontswap_init(p->type, frontswap_map);
> 	spin_lock(&swap_lock);
> 	_enable_swap_info(p, prio, swap_map, frontswap_map);
>-	frontswap_init(p->type);
> 	spin_unlock(&swap_lock);
> }
>
>@@ -1589,6 +1593,7 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
> 	p->swap_map = NULL;
> 	p->flags = 0;
> 	frontswap_invalidate_area(type);
>+	frontswap_map_set(p, NULL);

This will lead to memory leak, the memory will not be freed since
vfree(frontswap_map_get(p)); will miss it. :(

> 	spin_unlock(&swap_lock);
> 	mutex_unlock(&swapon_mutex);
> 	vfree(swap_map);
>-- 
>1.7.11.7
>
>--
>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>

--
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>