Re: [PATCH mmotm 1/5] nilfs2: fix problems of memory allocation in ioctl

[Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>]

This is the revised patch for fixing the following problems of a
memory copy function in nilfs2 ioctl.

(1) It tries to allocate 128KB size of memory even for small objects.

(2) Though the function repeatedly tries large memory allocations
    while reducing the size, GFP_NOWAIT flag is not specified.
    This increases the possibility of system memory shortage.

(3) During the retries of (2), verbose warnings are printed
    because _GFP_NOWARN flag is not used for the kmalloc calls.

The first patch was still doing large allocations by kmalloc which are
repeatedly tried while reducing the size.

Andi Kleen has pointed out that just using vmalloc would be
easy/faster/more reliable, and he also told me that using
copy_from_user for large memory is not good from the viewpoint of
preempt latency:

 On Fri, 12 Dec 2008 21:24:11 +0100, Andi Kleen <andi@firstfloor.org> wrote:
 > > In the current interface, each data item is copied twice: one is to
 > > the allocated memory from user space (via copy_from_user), and another
 >
 > For such large copies it is better to use multiple smaller (e.g. 4K)
 > copy user, that gives better real time preempt latencies. Each cfu has a
 > cond_resched(), but only one, not multiple times in the inner loop.

For the function in question, the size of buffer memory can be reduced
since the buffer is repeatedly used for a number of small objects.  On
the other hand, it may incur large preempt latencies for larger buffer
because a copy_from_user (and a copy_to_user) was applied only once
each cycle.

So, this revision avoids the latency issue as well as fixes the
original problems merely by reducing allocation size of the buffer.

Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
---
 fs/nilfs2/ioctl.c |   12 ++++--------
 1 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/fs/nilfs2/ioctl.c b/fs/nilfs2/ioctl.c
index 35ba60e..23378c3 100644
--- a/fs/nilfs2/ioctl.c
+++ b/fs/nilfs2/ioctl.c
@@ -34,8 +34,7 @@
 #include "dat.h"
 
 
-#define KMALLOC_SIZE_MIN	4096	/* 4KB */
-#define KMALLOC_SIZE_MAX	131072	/* 128 KB */
+#define NILFS_IOCTL_KMALLOC_SIZE	8192	/* 8KB */
 
 static int nilfs_ioctl_wrap_copy(struct the_nilfs *nilfs,
 				 struct nilfs_argv *argv, int dir,
@@ -51,12 +50,9 @@ static int nilfs_ioctl_wrap_copy(struct the_nilfs *nilfs,
 	if (argv->v_nmembs == 0)
 		return 0;
 
-	for (ksize = KMALLOC_SIZE_MAX; ksize >= KMALLOC_SIZE_MIN; ksize /= 2) {
-		buf = kmalloc(ksize, GFP_NOFS);
-		if (buf != NULL)
-			break;
-	}
-	if (ksize < KMALLOC_SIZE_MIN)
+	ksize = max_t(size_t, NILFS_IOCTL_KMALLOC_SIZE, argv->v_size);
+	buf = kmalloc(ksize, GFP_NOFS);
+	if (unlikely(!buf))
 		return -ENOMEM;
 	maxmembs = ksize / argv->v_size;
 
-- 
1.5.6.5