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

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Message-ID: <20170207045722.GA2215@bbox>
Date:   Tue, 7 Feb 2017 13:57:22 +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 11:24:40AM +0800, zhouxianrong wrote:
> 
> 
> On 2017/2/7 10:54, Minchan Kim wrote:
> >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?
> 
> i am afraid someone do reset_store, so did lock.

reset_store is protected by zram->claim and and init_done so I don't
think so.