On 29.10.18 г. 9:51 ч., Nikolay Borisov wrote:
>
>
> On 29.10.18 г. 7:53 ч., Qu Wenruo wrote:
>> [snip]
>>>> The cause sounds valid, however would you please explain more about how
>>>> such cleaning on unrelated delalloc range happens?
>>>
>>> So in my case the following happened - 2 block groups were created as
>>> delalloc ranges in the - 0-1m and 1m-128m. Their respective pages were
>>> dirtied, so when page 0 - 0-4k when into writepage_delalloc,
>>> find_lock_delalloc_range would return the range 0-1m. So the call to
>>> fill_delalloc instantiates OE 0-1m and writeback continues as expected.
>>>
>>> Now, when the 2nd page from range 0-1m is again set for writeback and
>>> find_lock_delalloc_range is called with delalloc_start == 4096 it will
>>> actually return the range 1m-128m.
>>
>> IMHO this looks strange and may need extra investigation.
>>
>> Normally I would expect it returns the range 0~1M or 4K~1M.
>
> It cannot return 4k-1m since the writeback for the first page has
> already dealt with this range. Also take a look in writepage_delalloc
> how find_lock_delalloc_range is called : for 'start' it's passed the
> page offset, calculated in __extent_writepage. And when
> find_delalloc_range is called it just searches for an extent which ends
> after passed start value. In find_delalloc_range first tree_search
> is called which returns the 1m-128m range, then we go in the while(1)
> loop on the first itertaion found is 0 so *end is populated with 128m ,
> found is set to 1 and *start is set to 1m.
>
> On the second iteration the check if (found && (state->start !=
> cur_start || (state->state & EXTENT_BOUNDARY)))
>
> is triggered since the next extent found will have EXTENT_BOUNDARY since
> it will be the next block group from relocation. EXTENT_BOUNDARY will be
> set from relocate_file_extent_cluster' main loop:
>
> if (nr < cluster->nr &&
>
> page_start + offset == cluster->boundary[nr]) {
>
> set_extent_bits(&BTRFS_I(inode)->io_tree,
>
> page_start, page_end,
>
> EXTENT_BOUNDARY);
>
> nr++;
>
> }
So it seems I was wrong w.r.t to sequence of events that result in the extra
extent being returned.
Here is what I got after further investigation. First let's look at the
relocation side:
btrfs-4018 [001] .... 186.783244: relocate_file_extent_cluster: Setting
EXTENT_BOUNDARY for page: 0000000074cc47c4 page_offset: 0 end: 4095 <- first
page of range 0-1m
btrfs-4018 [001] .... 186.783248: relocate_file_extent_cluster: Setting
DELALLOC for page: 0000000074cc47c4 page_offset: 0 block group:1104150528
btrfs-4018 [001] .... 186.783286: relocate_file_extent_cluster: Setting
DELALLOC for page: 000000004a28475a page_offset: 4096 block group:1104150528 <-
2nd page of range 0-1m
btrfs-4018 [001] .... 186.784855: relocate_file_extent_cluster: Setting
EXTENT_BOUNDARY for page: 00000000f58f50dc page_offset: 1048576 end: 1052671 <
- 1st page of range 1m-128m
Mind the addresses of the given pages, they are all predicated on btrfs_ino ==
260, which is the ino for relocation inode.
So the assumption is that when writing back page 000000004a28475a we cannot
really be processing range >1m since it will
be outside of the range for the page, but this is not the case. Now on the
writeback side:
kworker/u12:1-68 [002] .... 188.100471: find_lock_delalloc_range:
Processing delalloc range: 0 - 1048575 for page: 0000000074cc47c4 < - writeback
for first page of range 0-1m happens
so we naturally instantiate this range.
kworker/u12:1-68 [002] ...1 188.106523: find_delalloc_range.constprop.25:
1213015261107184058: Got start: 1048576 end 1052671 <- This is output from
find_delalloc_range when start (the offset of the passed page to
find_lock_delalloc_range is 4096 i.e it's the 2nd page for range 0-1m). So we
find 1m - 1m + 4k on the first iteration of the loop in find_delalloc_range, at
this point *start = 1048576 and *end = 1052671 and cached_state = the extent
_state representing this first 4k range of the larger 1m-128m range.
kworker/u12:1-68 [002] ...1 188.106526: find_delalloc_range.constprop.25:
1213015261107184058: Got start: 1052672 end 135266303 - We loop for the second
time, this time we find the 1m+4k - 128m range and actually trigger the
(total_bytes >= max_bytes) check
kworker/u12:1-68 [002] ...1 188.106526: find_delalloc_range.constprop.25:
1213015261107184058: returning from total_bytes >= max_bytes
kworker/u12:1-68 [002] .... 188.106528: find_delalloc_range.constprop.25:
1213015261107184058: Returning cached_state->start: 1048576 cached_state->end:
1052671 *start = 1048576 *end = 135266303 <--- this is what is returned from
find_delalloc_range - cached_state and *start and *end differ, since *end was
set to 128m right before we hit the total_bytes check.
kworker/u12:1-68 [002] .... 188.106529: find_lock_delalloc_range:
Processing delalloc range: 1048576 - 135266303 for page: 000000004a28475a <---
after we return from find_delalloc_range this is what
fin_lock_delalloc_range is processing - the next delalloc range 1m-128m but the
page passed is indeed the 2nd page for the previous range as evident from the
address 000000004a28475a
kworker/u12:1-68 [002] .... 188.113870: run_delalloc_nocow:
btrfs_reloc_clone_csums err for OE[1048576 134217728] page->offset: 4096 end:
8191 <--- here we just create the OE in run_delalloc_nocow.
The only thing which I'm now struggling to rationalize is why
find_delalloc_range finds an extent_state in the range 1048576-1052671 and then
finds another range 1052672 - 135266303, shouldn't in fact it
find just 1048576 - 135266303. Perhaps it's due to the search being performed
in the io_extent tree and not the extent_tree.
Here is the data_reloc tree :
item 2 key (260 INODE_ITEM 0) itemoff 15951 itemsize 160
generation 1 transid 30 size 940572672 nbytes 940572672
block group 0 mode 100600 links 0 uid 0 gid 0 rdev 0
sequence 8 flags 0x18(NOCOMPRESS|PREALLOC)
atime 0.0 (1970-01-01 00:00:00)
ctime 1540814915.362521123 (2018-10-29 12:08:35)
mtime 0.0 (1970-01-01 00:00:00)
otime 0.0 (1970-01-01 00:00:00)
item 3 key (260 EXTENT_DATA 0) itemoff 15898 itemsize 53
generation 28 type 2 (prealloc)
prealloc data disk byte 3872391168 nr 1048576
prealloc data offset 0 nr 1048576
item 4 key (260 EXTENT_DATA 1048576) itemoff 15845 itemsize 53
generation 28 type 2 (prealloc)
prealloc data disk byte 3873439744 nr 134217728
prealloc data offset 0 nr 134217728
as you can see we have a 0- 1048576 extent and 1048576-134217728. Those
correspond to the following data block group being relocated:
item 139 key (1104150528 EXTENT_ITEM 1048576) itemoff 11661 itemsize 53
refs 1 gen 7 flags DATA
extent data backref root FS_TREE objectid 257 offset 0 count 1
item 140 key (1104150528 BLOCK_GROUP_ITEM 1073741824) itemoff 11637 itemsize 24
block group used 940572672 chunk_objectid 256 flags DATA|RAID1
item 141 key (1105199104 EXTENT_ITEM 134217728) itemoff 11584 itemsize 53
refs 1 gen 9 flags DATA
extent data backref root FS_TREE objectid 258 offset 0 count 1
<snip>
