[PATCHv2] mm/page_owner: use kvmalloc instead of kmalloc

[PATCHv2] mm/page_owner: use kvmalloc instead of kmalloc

[miles.chen]

From: Miles Chen <miles.chen@mediatek.com>

The kbuf used by page owner is allocated by kmalloc(), which means it
can use only normal memory and there might be a "out of memory"
issue when we're out of normal memory.

Use kvmalloc() so we can also allocate kbuf from
normal/hihghmem on 32bit kernel.

Clamp the kbuf size to PAGE_SIZE.

Change since v1:
  - use kvmalloc()
  - clamp buffer size to PAGE_SIZE

Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Cc: Joe Perches <joe@perches.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
---
 mm/page_owner.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/page_owner.c b/mm/page_owner.c
index d80adfe..a064cd0 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -1,7 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/debugfs.h>
 #include <linux/mm.h>
-#include <linux/slab.h>
 #include <linux/uaccess.h>
 #include <linux/bootmem.h>
 #include <linux/stacktrace.h>
@@ -351,7 +350,8 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
 		.skip = 0
 	};
 
-	kbuf = kmalloc(count, GFP_KERNEL);
+	count = count > PAGE_SIZE ? PAGE_SIZE : count;
+	kbuf = kvmalloc(count, GFP_KERNEL);
 	if (!kbuf)
 		return -ENOMEM;
 
@@ -397,11 +397,11 @@ void pagetypeinfo_showmixedcount_print(struct seq_file *m,
 	if (copy_to_user(buf, kbuf, ret))
 		ret = -EFAULT;
 
-	kfree(kbuf);
+	kvfree(kbuf);
 	return ret;
 
 err:
-	kfree(kbuf);
+	kvfree(kbuf);
 	return -ENOMEM;
 }
 
-- 
1.9.1

Re: [PATCHv2] mm/page_owner: use kvmalloc instead of kmalloc

[Matthew Wilcox]

On Mon, Oct 29, 2018 at 10:16:43AM +0800, miles.chen@mediatek.com wrote:
> The kbuf used by page owner is allocated by kmalloc(), which means it
> can use only normal memory and there might be a "out of memory"
> issue when we're out of normal memory.
> 
> Use kvmalloc() so we can also allocate kbuf from
> normal/hihghmem on 32bit kernel.

That's a misconception:

        ret = kmalloc_node(size, kmalloc_flags, node);

        /*
         * It doesn't really make sense to fallback to vmalloc for sub page
         * requests
         */
        if (ret || size <= PAGE_SIZE)
                return ret;

Now, maybe this is an opportunity for us to improve kvmalloc.  Maybe like
this ...

diff --git a/mm/util.c b/mm/util.c
index 8bf08b5b5760..fdf5b34d2c28 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -416,10 +416,10 @@ void *kvmalloc_node(size_t size, gfp_t flags, int node)
 	ret = kmalloc_node(size, kmalloc_flags, node);
 
 	/*
-	 * It doesn't really make sense to fallback to vmalloc for sub page
-	 * requests
+	 * It only makes sense to fallback to vmalloc for sub page
+	 * requests if we might be able to allocate highmem pages.
 	 */
-	if (ret || size <= PAGE_SIZE)
+	if (ret || (!IS_ENABLED(CONFIG_HIGHMEM) && size <= PAGE_SIZE))
 		return ret;
 
 	return __vmalloc_node_flags_caller(size, node, flags,