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On 3/30/15 3:29 PM, Filipe David Manana wrote:
> On Mon, Mar 30, 2015 at 8:12 PM, Jeff Mahoney <je...@suse.com>
> wrote:
>> The block group removal patch adds an alternate path to forget
>> extents other than btrfs_finish_extent_commit. As a result, any
>> extents that would be freed when the block group is removed
>> aren't discarded. In my test run, with a large copy of mixed
>> sized files followed by removal, it left nearly 2/3 of extents
>> undiscarded.
>>
>> As part of the extent removal review, I also added the discard
>> call to btrfs_destroy_pinned_extent.
>>
>> Signed-off-by: Jeff Mahoney <je...@suse.com> ---
>> fs/btrfs/disk-io.c | 7 +++++++ fs/btrfs/extent-tree.c | 9
>> +++++++++ 2 files changed, 16 insertions(+)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index
>> 639f266..f7dfcf8 100644 --- a/fs/btrfs/disk-io.c +++
>> b/fs/btrfs/disk-io.c @@ -4167,6 +4167,13 @@ again: if (ret)
>> break;
>>
>> + if (btrfs_test_opt(root, DISCARD)) { + ret =
>> btrfs_discard_extent(root, start, + end + 1 - start, NULL); +
>> if (ret) + break; + } +
>
> Jeff we can't do a discard here, specially if unpin ==
> fs_info->pinned_extents, as explained in:
>
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit
/?id=678886bdc6378c1cbd5072da2c5a3035000214e3
>
>
>
>
>
>> clear_extent_dirty(unpin, start, end, GFP_NOFS);
>> btrfs_error_unpin_extent_range(root, start, end); cond_resched();
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index 0bf45b8..6a7ff46 100644 --- a/fs/btrfs/extent-tree.c +++
>> b/fs/btrfs/extent-tree.c @@ -9630,6 +9630,15 @@ void
>> btrfs_delete_unused_bgs(struct btrfs_fs_info *fs_info) * a
>> BUG_ON() at btrfs_unpin_extent_range(). */
>> mutex_lock(&fs_info->unused_bg_unpin_mutex); + if
>> (btrfs_test_opt(root, DISCARD)) { + ret =
>> btrfs_discard_extent(root, start, end + 1 - start, + NULL); +
>> if (ret) { + mutex_unlock(&fs_info->unused_bg_unpin_mutex); +
>> btrfs_set_block_group_rw(root, block_group); + goto end_trans; +
>> } + }
>
> This isn't safe either for the similar reason as above. For
> example, transaction N running, metadata bg X becomes empty and
> it's added to fs_info->unused_bgs, cleaner kthread running and
> gets bg X from fs_info->unused_bgs (running
> btrfs_delete_unused_bgs), does the discard, power failure, reboot,
> superblock points to metadata extents in the physical location of
> bg X that are now full of zeroes.
>
> thanks
>
>> ret = clear_extent_bits(&fs_info->freed_extents[0], start, end,
>> EXTENT_DIRTY, GFP_NOFS); if (ret) {
Thanks for the review, Filipe. I think I understand what you mean.
btrfs_finish_extent_commit is effectively the only safe place to
perform the discards, since we know the transaction is finished. I'll
need to rework block group deletions a bit since the clearing of the
dirty bits for the whole block group mean that
btrfs_finish_extent_commit never sees those blocks as eligible for
discard.
- -Jeff
- --
Jeff Mahoney
SUSE Labs
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