[1/3] btrfs: improve inode's outstanding_extents computation

This issue was revealed by modifying BTRFS_MAX_EXTENT_SIZE(128MB) to 64KB,
When modifying BTRFS_MAX_EXTENT_SIZE(128MB) to 64KB, fsstress test often
gets these warnings from btrfs_destroy_inode():
	WARN_ON(BTRFS_I(inode)->outstanding_extents);
	WARN_ON(BTRFS_I(inode)->reserved_extents);

Simple test program below can reproduce this issue steadily.
Note: you need to modify BTRFS_MAX_EXTENT_SIZE to 64KB to have test,
otherwise there won't be such WARNING.
	#include <string.h>
	#include <unistd.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <fcntl.h>

	int main(void)
	{
		int fd;
		char buf[68 *1024];

		memset(buf, 0, 68 * 1024);
		fd = open("testfile", O_CREAT | O_EXCL | O_RDWR);
		pwrite(fd, buf, 68 * 1024, 64 * 1024);
		return;
	}

When BTRFS_MAX_EXTENT_SIZE is 64KB, and buffered data range is:
64KB						128K		132KB
|-----------------------------------------------|---------------|
                         64 + 4KB

1) for above data range, btrfs_delalloc_reserve_metadata() will reserve
metadata and set BTRFS_I(inode)->outstanding_extents to 2.
(68KB + 64KB - 1) / 64KB == 2

Outstanding_extents: 2

2) then btrfs_dirty_page() will be called to dirty pages and set
EXTENT_DELALLOC flag. In this case, btrfs_set_bit_hook() will be called
twice.
The 1st set_bit_hook() call will set DEALLOC flag for the first 64K.
64KB						128KB
|-----------------------------------------------|
	64KB DELALLOC
Outstanding_extents: 2

Set_bit_hooks() uses FIRST_DELALLOC flag to avoid re-increase
outstanding_extents counter.
So for 1st set_bit_hooks() call, it won't modify outstanding_extents,
it's still 2.

Then FIRST_DELALLOC flag is *CLEARED*.

3) 2nd btrfs_set_bit_hook() call.
Because FIRST_DELALLOC have been cleared by previous set_bit_hook(),
btrfs_set_bit_hook() will increase BTRFS_I(inode)->outstanding_extents by
one, so now BTRFS_I(inode)->outstanding_extents is 3.
64KB                                            128KB            132KB
|-----------------------------------------------|----------------|
	64K DELALLOC				   4K DELALLOC
Outstanding_extents: 3

But the correct outstanding_extents number should be 2, not 3.
The 2nd btrfs_set_bit_hook() call just screwed up this, and leads to the
WARN_ON().

Normally, we can solve it by only increasing outstanding_extents in
set_bit_hook().
But the problem is for delalloc_reserve/release_metadata(), we only have
a 'length' parameter, and calculate in-accurate outstanding_extents.
If we only rely on set_bit_hook() release_metadata() will crew things up
as it will decrease inaccurate number.

So the fix we use is:
1) Increase *INACCURATE* outstanding_extents at delalloc_reserve_meta
   Just as a place holder.
2) Increase *accurate* outstanding_extents at set_bit_hooks()
   This is the real increaser.
3) Decrease *INACCURATE* outstanding_extents before returning
   This makes outstanding_extents to correct value.

For 128M BTRFS_MAX_EXTENT_SIZE, due to limitation of
__btrfs_buffered_write(), each iteration will only handle about 2MB
data.
So btrfs_dirty_pages() won't need to handle cases cross 2 extents.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
---
 fs/btrfs/ctree.h |  2 ++
 fs/btrfs/inode.c | 65 ++++++++++++++++++++++++++++++++++++++++++++++++++------
 fs/btrfs/ioctl.c |  6 ++----
 3 files changed, 62 insertions(+), 11 deletions(-)

[1/3] btrfs: improve inode's outstanding_extents computation

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Patch