From: Chandan Rajendra <chandan@linux.vnet.ibm.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Cc: linux-btrfs@vger.kernel.org, dsterba@suse.cz
Subject: Re: [PATCH 2/2] btrfs: qgroup: Fix qgroup reserved space underflow by only freeing reserved ranges
Date: Tue, 13 Dec 2016 18:26:59 +0530 [thread overview]
Message-ID: <16662538.OxiIrCRCJe@localhost.localdomain> (raw)
In-Reply-To: <20161202020307.6025-2-quwenruo@cn.fujitsu.com>
On Friday, December 02, 2016 10:03:07 AM Qu Wenruo wrote:
> [BUG]
> For the following case, btrfs can underflow qgroup reserved space
> at error path:
> (Page size 4K, function name without "btrfs_" prefix)
>
> Task A | Task B
> ----------------------------------------------------------------------
> Buffered_write [0, 2K) |
> |- check_data_free_space() |
> | |- qgroup_reserve_data() |
> | Range aligned to page |
> | range [0, 4K) <<< |
> | 4K bytes reserved <<< |
> |- copy pages to page cache |
> | Buffered_write [2K, 4K)
> | |- check_data_free_space()
> | | |- qgroup_reserved_data()
> | | Range alinged to page
> | | range [0, 4K)
> | | Already reserved by A <<<
> | | 0 bytes reserved <<<
> | |- delalloc_reserve_metadata()
> | | And it *FAILED* (Maybe EQUOTA)
> | |- free_reserved_data_space()
> |- qgroup_free_data()
> Range aligned to page range
> [0, 4K)
> Freeing 4K
> (Special thanks to Chandan for the detailed report and analyse)
>
> [CAUSE]
> Above Task B is freeing reserved data range [0, 4K) which is actually
> reserved by Task A.
>
> And at write back time, page dirty by Task A will go through writeback
> routine, which will free 4K reserved data space at file extent insert
> time, causing the qgroup underflow.
>
> [FIX]
> For btrfs_qgroup_free_data(), add @reserved parameter to only free
> data ranges reserved by previous btrfs_qgroup_reserve_data().
> So in above case, Task B will try to free 0 byte, so no underflow.
>
The changes look good to me. Also, I did not notice any regressions when
executing fstests with the patch applied.
Reviewed-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Tested-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
--
chandan
next prev parent reply other threads:[~2016-12-13 12:57 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-02 2:03 [PATCH 1/2] btrfs: qgroup: Introduce extent changeset for qgroup reserve functions Qu Wenruo
2016-12-02 2:03 ` [PATCH 2/2] btrfs: qgroup: Fix qgroup reserved space underflow by only freeing reserved ranges Qu Wenruo
2016-12-13 12:56 ` Chandan Rajendra [this message]
2016-12-08 14:29 ` [PATCH 1/2] btrfs: qgroup: Introduce extent changeset for qgroup reserve functions Chandan Rajendra
2016-12-09 0:39 ` Qu Wenruo
2016-12-13 12:54 ` Chandan Rajendra
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