[PATCH] zram: keep the exact overcommited value in mem_used_max

[PATCH] zram: keep the exact overcommited value in mem_used_max

[Sergey Senozhatsky]

From: Sergey SENOZHATSKY <sergey.senozhatsky@gmail.com>

`mem_used_max' is designed to store the max amount of memory zram
consumed to store the data. However, it does not represent the actual
'overcommited' (max) value. The existing code goes to -ENOMEM
overcommited case before it updates `->stats.max_used_pages', which
hides the reason we went to -ENOMEM in the first place -- we actually
used more memory than `->limit_pages':

        alloced_pages = zs_get_total_pages(meta->mem_pool);
        if (zram->limit_pages && alloced_pages > zram->limit_pages) {
                zs_free(meta->mem_pool, handle);
                ret = -ENOMEM;
                goto out;
        }

        update_used_max(zram, alloced_pages);

Which is misleading. User will see -ENOMEM, check `->limit_pages',
check `->stats.max_used_pages', which will keep the value BEFORE zram
passed `->limit_pages', and see:
	`->stats.max_used_pages' < `->limit_pages'

Move update_used_max() before we do `->limit_pages' check, so that
user will see:
	`->stats.max_used_pages' > `->limit_pages'
should the overcommit and -ENOMEM happen.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 drivers/block/zram/zram_drv.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index cb478ca..6f04fb2 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -738,14 +738,14 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 	}
 
 	alloced_pages = zs_get_total_pages(meta->mem_pool);
+	update_used_max(zram, alloced_pages);
+
 	if (zram->limit_pages && alloced_pages > zram->limit_pages) {
 		zs_free(meta->mem_pool, handle);
 		ret = -ENOMEM;
 		goto out;
 	}
 
-	update_used_max(zram, alloced_pages);
-
 	cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_WO);
 
 	if ((clen == PAGE_SIZE) && !is_partial_io(bvec)) {
-- 
2.5.3

Re: [PATCH] zram: keep the exact overcommited value in mem_used_max

[Minchan Kim]

On Thu, Sep 24, 2015 at 06:51:52PM +0900, Sergey Senozhatsky wrote:
> From: Sergey SENOZHATSKY <sergey.senozhatsky@gmail.com>
> 
> `mem_used_max' is designed to store the max amount of memory zram
> consumed to store the data. However, it does not represent the actual
> 'overcommited' (max) value. The existing code goes to -ENOMEM
> overcommited case before it updates `->stats.max_used_pages', which
> hides the reason we went to -ENOMEM in the first place -- we actually
> used more memory than `->limit_pages':
> 
>         alloced_pages = zs_get_total_pages(meta->mem_pool);
>         if (zram->limit_pages && alloced_pages > zram->limit_pages) {
>                 zs_free(meta->mem_pool, handle);
>                 ret = -ENOMEM;
>                 goto out;
>         }
> 
>         update_used_max(zram, alloced_pages);
> 
> Which is misleading. User will see -ENOMEM, check `->limit_pages',
> check `->stats.max_used_pages', which will keep the value BEFORE zram
> passed `->limit_pages', and see:
> 	`->stats.max_used_pages' < `->limit_pages'
> 
> Move update_used_max() before we do `->limit_pages' check, so that
> user will see:
> 	`->stats.max_used_pages' > `->limit_pages'
> should the overcommit and -ENOMEM happen.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>

It was intent.

IOW, I want that max_mem_used is never higher than mem_limit but
I realized you are right because user doesn't have way to detect
where -ENOMEM is coming from.
For example, it could come from mem_limit or real memory alloc fail.

Thanks, Sergey.

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