Re: [PATCH] mm: extend zero pages to same element pages for zram

[Minchan Kim]

On Tue, Feb 07, 2017 at 10:20:57AM +0800, zhouxianrong wrote:

< snip >

> >>3. the below should be modified.
> >>
> >>static inline bool zram_meta_get(struct zram *zram)
> >>@@ -495,11 +553,17 @@ static void zram_meta_free(struct zram_meta *meta, 
> >>u64 disksize)
> >>
> >>    /* Free all pages that are still in this zram device */
> >>    for (index = 0; index < num_pages; index++) {
> >>-           unsigned long handle = meta->table[index].handle;
> >>+           unsigned long handle;
> >>+
> >>+           bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> >>+           handle = meta->table[index].handle;
> >>
> >>-           if (!handle)
> >>+           if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
> >>+                   bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>                    continue;
> >>+           }
> >>
> >>+           bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >>            zs_free(meta->mem_pool, handle);
> >
> >Could you explain why we need this modification?
> >
> >>    }
> >>
> >>@@ -511,7 +575,7 @@ static void zram_meta_free(struct zram_meta *meta, u64 
> >>disksize)
> >> static struct zram_meta *zram_meta_alloc(char *pool_name, u64 disksize)
> >> {
> >>    size_t num_pages;
> >>-   struct zram_meta *meta = kmalloc(sizeof(*meta), GFP_KERNEL);
> >>+   struct zram_meta *meta = kzalloc(sizeof(*meta), GFP_KERNEL);
> >
> >Ditto
> >
> >>
> >>
> >
> >.
> >
> 
> because of union of handle and element, i think a non-zero element (other 
> than handle) is prevented from freeing.
> if zram_meta_get was modified, zram_meta_alloc did so.

Right. Thanks but I don't see why we need the locking in there and modification 
of
zram_meta_alloc.

Isn't it enough with this?

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index c20b05a84f21..a25d34a8af19 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -425,8 +425,11 @@ static void zram_meta_free(struct zram_meta *meta, u64 
disksize)
        /* Free all pages that are still in this zram device */
        for (index = 0; index < num_pages; index++) {
                unsigned long handle = meta->table[index].handle;
-
-               if (!handle)
+               /*
+                * No memory is allocated for same element filled pages.
+                * Simply clear same page flag.
+                */
+               if (!handle || zram_test_flag(meta, index, ZRAM_SAME))
                        continue;
 
                zs_free(meta->mem_pool, handle);