[PATCH] mm: do not consider SWAP to calculate available when not necessary

[PATCH] mm: do not consider SWAP to calculate available when not necessary

[Yang Yang]

When si_mem_available() calculates 'available', it takes SWAP
into account. But if CONFIG_SWAP is N or SWAP is off(some embedded system
would like to do that), there is no need to consider it.

Signed-off-by: Yang Yang <yang.yang29@zte.com.cn>
---
 mm/page_alloc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 6847177..10e186b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4700,6 +4700,7 @@ static inline void show_node(struct zone *zone)
 
 long si_mem_available(void)
 {
+	struct sysinfo i;
 	long available;
 	unsigned long pagecache;
 	unsigned long wmark_low = 0;
@@ -4708,6 +4709,7 @@ long si_mem_available(void)
 	struct zone *zone;
 	int lru;
 
+	si_swapinfo(&i);
 	for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
 		pages[lru] = global_node_page_state(NR_LRU_BASE + lru);
 
@@ -4724,9 +4726,13 @@ long si_mem_available(void)
 	 * Not all the page cache can be freed, otherwise the system will
 	 * start swapping. Assume at least half of the page cache, or the
 	 * low watermark worth of cache, needs to stay.
+	 * But if CONFIG_SWAP is N or SWAP is off, do not consider it.
 	 */
 	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
-	pagecache -= min(pagecache / 2, wmark_low);
+#ifdef CONFIG_SWAP
+	if (i.totalswap > 0)
+		pagecache -= min(pagecache / 2, wmark_low);
+#endif
 	available += pagecache;
 
 	/*
-- 
2.15.2

Re: [PATCH] mm: do not consider SWAP to calculate available when not necessary

[Matthew Wilcox]

On Mon, Nov 26, 2018 at 07:58:23AM +0800, Yang Yang wrote:
> When si_mem_available() calculates 'available', it takes SWAP
> into account. But if CONFIG_SWAP is N or SWAP is off(some embedded system
> would like to do that), there is no need to consider it.

I don't understand this patch.  The pagecache can be written back to
storage if it is dirty, regardless of whether there is swap space.

> @@ -4724,9 +4726,13 @@ long si_mem_available(void)
>  	 * Not all the page cache can be freed, otherwise the system will
>  	 * start swapping. Assume at least half of the page cache, or the
>  	 * low watermark worth of cache, needs to stay.
> +	 * But if CONFIG_SWAP is N or SWAP is off, do not consider it.
>  	 */
>  	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
> -	pagecache -= min(pagecache / 2, wmark_low);
> +#ifdef CONFIG_SWAP
> +	if (i.totalswap > 0)
> +		pagecache -= min(pagecache / 2, wmark_low);
> +#endif
>  	available += pagecache;
>  
>  	/*
> -- 
> 2.15.2
> 

Re: [PATCH] mm: do not consider SWAP to calculate available when not necessary

[Vlastimil Babka]

On 11/26/18 3:01 AM, Matthew Wilcox wrote:
> On Mon, Nov 26, 2018 at 07:58:23AM +0800, Yang Yang wrote:
>> When si_mem_available() calculates 'available', it takes SWAP
>> into account. But if CONFIG_SWAP is N or SWAP is off(some embedded system
>> would like to do that), there is no need to consider it.
> 
> I don't understand this patch.  The pagecache can be written back to
> storage if it is dirty, regardless of whether there is swap space.
> 
>> @@ -4724,9 +4726,13 @@ long si_mem_available(void)
>>  	 * Not all the page cache can be freed, otherwise the system will
>>  	 * start swapping. Assume at least half of the page cache, or the

I guess the first sentence in the comment above might be misleading by
using the word 'swapping', where 'thrashing' would be more accurate and
unambiguous.

So this is not related to the swap, but to the assumption that somebody
is accessing the pages in pagecache, and if too much would be freed,
most accesses would mean reading data from disk, i.e. thrashing.

>>  	 * low watermark worth of cache, needs to stay.
>> +	 * But if CONFIG_SWAP is N or SWAP is off, do not consider it.
>>  	 */
>>  	pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
>> -	pagecache -= min(pagecache / 2, wmark_low);
>> +#ifdef CONFIG_SWAP
>> +	if (i.totalswap > 0)
>> +		pagecache -= min(pagecache / 2, wmark_low);
>> +#endif
>>  	available += pagecache;
>>  
>>  	/*
>> -- 
>> 2.15.2
>>