Re: [PATCH v14.4 01/15] btrfs: improve inode's outstanding_extents computation

[Josef Bacik <josef@toxicpanda.com>]

On Wed, Jul 12, 2017 at 04:49:48PM +0800, Lu Fengqi wrote:
> From: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
> 
> 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.
> 

NACK on this, it just makes a crappy problem harder to understand.  We need to
stop using outstanding_extents as both a reservation thing _and_ an accurate
count.  I'll rework this and fix the other issues related to over-reservation
with compression.  Thanks,

Josef