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

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Message-ID: <20170207025426.GA1528@bbox>
Date:   Tue, 7 Feb 2017 11:54:26 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     zhouxianrong <zhouxianrong@...wei.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <sergey.senozhatsky@...il.com>,
        <willy@...radead.org>, <iamjoonsoo.kim@....com>,
        <ngupta@...are.org>, <Mi.Sophia.Wang@...wei.com>,
        <zhouxiyu@...wei.com>, <weidu.du@...wei.com>,
        <zhangshiming5@...wei.com>, <won.ho.park@...wei.com>
Subject: Re: [PATCH] mm: extend zero pages to same element pages for zram

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);