From: Zhang Yi <yi.zhang@huaweicloud.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>,
linux-ext4@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, tytso@mit.edu,
adilger.kernel@dilger.ca, jack@suse.cz, hch@infradead.org,
djwong@kernel.org, david@fromorbit.com, willy@infradead.org,
zokeefe@google.com, yi.zhang@huawei.com, chengzhihao1@huawei.com,
yukuai3@huawei.com, wangkefeng.wang@huawei.com
Subject: Re: [PATCH v4 02/34] ext4: check the extent status again before inserting delalloc block
Date: Sun, 28 Apr 2024 11:00:36 +0800 [thread overview]
Message-ID: <3243c67d-e783-4ec5-998f-0b6170f36e35@huaweicloud.com> (raw)
In-Reply-To: <87cyqcyt6t.fsf@gmail.com>
On 2024/4/27 0:39, Ritesh Harjani (IBM) wrote:
> Zhang Yi <yi.zhang@huaweicloud.com> writes:
>
>> On 2024/4/26 20:57, Ritesh Harjani (IBM) wrote:
>>> Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes:
>>>
>>>> Zhang Yi <yi.zhang@huaweicloud.com> writes:
>>>>
>>>>> From: Zhang Yi <yi.zhang@huawei.com>
>>>>>
>>>>> Now we lookup extent status entry without holding the i_data_sem before
>>>>> inserting delalloc block, it works fine in buffered write path and
>>>>> because it holds i_rwsem and folio lock, and the mmap path holds folio
>>>>> lock, so the found extent locklessly couldn't be modified concurrently.
>>>>> But it could be raced by fallocate since it allocate block whitout
>>>>> holding i_rwsem and folio lock.
>>>>>
>>>>> ext4_page_mkwrite() ext4_fallocate()
>>>>> block_page_mkwrite()
>>>>> ext4_da_map_blocks()
>>>>> //find hole in extent status tree
>>>>> ext4_alloc_file_blocks()
>>>>> ext4_map_blocks()
>>>>> //allocate block and unwritten extent
>>>>> ext4_insert_delayed_block()
>>>>> ext4_da_reserve_space()
>>>>> //reserve one more block
>>>>> ext4_es_insert_delayed_block()
>>>>> //drop unwritten extent and add delayed extent by mistake
>>>>>
>>>>> Then, the delalloc extent is wrong until writeback, the one more
>>>>> reserved block can't be release any more and trigger below warning:
>>>>>
>>>>> EXT4-fs (pmem2): Inode 13 (00000000bbbd4d23): i_reserved_data_blocks(1) not cleared!
>>>>>
>>>>> Hold i_data_sem in write mode directly can fix the problem, but it's
>>>>> expansive, we should keep the lockless check and check the extent again
>>>>> once we need to add an new delalloc block.
>>>>
>>>> Hi Zhang,
>>>>
>>>> It's a nice finding. I was wondering if this was caught in any of the
>>>> xfstests?
>>>>
>>
>> Hi, Ritesh
>>
>> I caught this issue when I tested my iomap series in generic/344 and
>> generic/346. It's easy to reproduce because the iomap's buffered write path
>> doesn't hold folio lock while inserting delalloc blocks, so it could be raced
>> by the mmap page fault path. But the buffer_head's buffered write path can't
>> trigger this problem,
>
> ya right! That's the difference between how ->map_blocks() is called
> between buffer_head v/s iomap path. In iomap the ->map_blocks() call
> happens first to map a large extent and then it iterate over all the
> locked folios covering the mapped extent for doing writes.
> Whereas in buffer_head while iterating, we first instantiate/lock the
> folio and then call ->map_blocks() to map an extent for the given folio.
>
> ... So this opens up this window for a race between iomap buffered write
> path v/s page mkwrite path for inserting delalloc blocks entries.
>
>> the race between buffered write path and fallocate path
>> was discovered while I was analyzing the code, so I'm not sure if it could
>> be caught by xfstests now, at least I haven't noticed this problem so far.
>>
>
> Did you mean the race between page fault path and fallocate path here?
> Because buffered write path and fallocate path should not have any race
> since both takes the inode_lock. I guess you meant page fault path and
> fallocate path for which you wrote this patch too :)
Yep.
>
> I am surprised, why we cannot see the this race between page mkwrite and
> fallocate in fstests for inserting da entries to extent status cache.
> Because the race you identified looks like a legitimate race and is
> mostly happening since ext4_da_map_blocks() was not doing the right
> thing.
> ... looking at the src/holetest, it doesn't really excercise this path.
> So maybe we can writing such fstest to trigger this race.
>
I guess the stress tests and smoke tests in fstests have caught it,
e.g. generic/476. Since there is only one error message in ext4_destroy_inode()
when the race issue happened, we can't detect it unless we go and check the logs
manually.
I suppose we need to add more warnings, something like this, how does it sound?
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index c8b691e605f1..4b6fd9b63b12 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1255,6 +1255,8 @@ static void ext4_percpu_param_destroy(struct ext4_sb_info *sbi)
percpu_counter_destroy(&sbi->s_freeclusters_counter);
percpu_counter_destroy(&sbi->s_freeinodes_counter);
percpu_counter_destroy(&sbi->s_dirs_counter);
+ WARN_ON_ONCE(!ext4_forced_shutdown(sbi->s_sb) &&
+ percpu_counter_sum(&sbi->s_dirtyclusters_counter));
percpu_counter_destroy(&sbi->s_dirtyclusters_counter);
percpu_counter_destroy(&sbi->s_sra_exceeded_retry_limit);
percpu_free_rwsem(&sbi->s_writepages_rwsem);
@@ -1476,7 +1478,8 @@ static void ext4_destroy_inode(struct inode *inode)
dump_stack();
}
- if (EXT4_I(inode)->i_reserved_data_blocks)
+ if (!ext4_forced_shutdown(inode->i_sb) &&
+ WARN_ON_ONCE(EXT4_I(inode)->i_reserved_data_blocks))
ext4_msg(inode->i_sb, KERN_ERR,
"Inode %lu (%p): i_reserved_data_blocks (%u) not cleared!",
inode->i_ino, EXT4_I(inode),
Thanks,
Yi.
>
>>>> I have reworded some of the commit message, feel free to use it if you
>>>> think this version is better. The use of which path uses which locks was
>>>> a bit confusing in the original commit message.
>>>>
>>
>> Thanks for the message improvement, it looks more clear then mine, I will
>> use it.
>>
>
> Glad, it was helpful.
>
> -ritesh
>
next prev parent reply other threads:[~2024-04-28 3:00 UTC|newest]
Thread overview: 67+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-10 14:29 [RESEND RFC PATCH v4 00/34] ext4: use iomap for regular file's buffered IO path and enable large folio Zhang Yi
2024-04-10 14:29 ` [PATCH v4 01/34] ext4: factor out a common helper to query extent map Zhang Yi
2024-04-26 11:55 ` Ritesh Harjani
2024-04-10 14:29 ` [PATCH v4 02/34] ext4: check the extent status again before inserting delalloc block Zhang Yi
2024-04-26 12:31 ` Ritesh Harjani
2024-04-26 12:57 ` Ritesh Harjani
2024-04-26 13:19 ` Zhang Yi
2024-04-26 16:39 ` Ritesh Harjani
2024-04-28 3:00 ` Zhang Yi [this message]
2024-04-29 14:59 ` Ritesh Harjani
2024-05-07 3:15 ` Zhang Yi
2024-05-01 7:47 ` Dave Chinner
2024-05-01 6:51 ` Dave Chinner
2024-05-01 12:19 ` Ritesh Harjani
2024-05-01 22:49 ` Dave Chinner
2024-05-02 4:11 ` Ritesh Harjani
2024-05-06 3:49 ` Zhang Yi
2024-04-10 14:29 ` [PATCH v4 03/34] ext4: trim delalloc extent Zhang Yi
2024-05-01 14:31 ` Ritesh Harjani
2024-05-06 6:15 ` Zhang Yi
2024-04-10 14:29 ` [PATCH v4 04/34] ext4: drop iblock parameter Zhang Yi
2024-05-01 14:41 ` Ritesh Harjani
2024-04-10 14:29 ` [PATCH v4 05/34] ext4: make ext4_es_insert_delayed_block() insert multi-blocks Zhang Yi
2024-04-10 14:29 ` [PATCH v4 06/34] ext4: make ext4_da_reserve_space() reserve multi-clusters Zhang Yi
2024-04-10 14:29 ` [PATCH v4 07/34] ext4: factor out check for whether a cluster is allocated Zhang Yi
2024-04-10 14:29 ` [PATCH v4 08/34] ext4: make ext4_insert_delayed_block() insert multi-blocks Zhang Yi
2024-04-10 14:29 ` [PATCH v4 09/34] ext4: make ext4_da_map_blocks() buffer_head unaware Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 10/34] ext4: factor out ext4_map_create_blocks() to allocate new blocks Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 11/34] ext4: optimize the EXT4_GET_BLOCKS_DELALLOC_RESERVE flag set Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 12/34] ext4: don't set EXTENT_STATUS_DELAYED on allocated blocks Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 13/34] ext4: let __revise_pending() return newly inserted pendings Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 14/34] ext4: count removed reserved blocks for delalloc only extent entry Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 15/34] ext4: update delalloc data reserve spcae in ext4_es_insert_extent() Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 16/34] ext4: drop ext4_es_delayed_clu() Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 17/34] ext4: use ext4_map_query_blocks() in ext4_map_blocks() Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 18/34] ext4: drop ext4_es_is_delonly() Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 19/34] ext4: drop all delonly descriptions Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 20/34] ext4: use reserved metadata blocks when splitting extent on endio Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 21/34] ext4: introduce seq counter for the extent status entry Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 22/34] ext4: add a new iomap aops for regular file's buffered IO path Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 23/34] ext4: implement buffered read iomap path Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 24/34] ext4: implement buffered write " Zhang Yi
2024-05-01 8:11 ` Dave Chinner
2024-05-01 8:33 ` Dave Chinner
2024-05-06 11:44 ` Zhang Yi
2024-05-06 23:19 ` Dave Chinner
2024-05-07 5:10 ` Zhang Yi
2024-05-06 11:21 ` Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 25/34] ext4: implement writeback " Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 26/34] ext4: implement mmap " Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 27/34] ext4: implement zero_range " Zhang Yi
2024-05-01 9:40 ` Dave Chinner
2024-05-06 12:33 ` Zhang Yi
2024-04-10 14:29 ` [RFC PATCH v4 28/34] ext4: writeback partial blocks before zeroing out range Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 29/34] ext4: fall back to buffer_head path for defrag Zhang Yi
2024-05-01 9:32 ` Dave Chinner
2024-05-06 13:05 ` Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 30/34] ext4: partial enable iomap for regular file's buffered IO path Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 31/34] filemap: support disable large folios on active inode Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 32/34] ext4: enable large folio for regular file with iomap buffered IO path Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 33/34] ext4: don't mark IOMAP_F_DIRTY for buffer write Zhang Yi
2024-05-01 9:27 ` Dave Chinner
2024-05-06 14:02 ` Zhang Yi
2024-04-10 15:03 ` [RFC PATCH v4 34/34] ext4: add mount option for buffered IO iomap path Zhang Yi
2024-04-11 1:12 ` [RESEND RFC PATCH v4 00/34] ext4: use iomap for regular file's buffered IO path and enable large folio Zhang Yi
2024-04-24 8:12 ` Zhang Yi
-- strict thread matches above, loose matches on Subject: below --
2024-04-10 13:27 [RFC " Zhang Yi
2024-04-10 13:27 ` [PATCH v4 02/34] ext4: check the extent status again before inserting delalloc block Zhang Yi
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