From: Zhang Yi <yi.zhang@huaweicloud.com>
To: "Ritesh Harjani (IBM)" <ritesh.list@gmail.com>,
Dave Chinner <david@fromorbit.com>
Cc: linux-ext4@vger.kernel.org, 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, 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: Mon, 6 May 2024 11:49:52 +0800 [thread overview]
Message-ID: <7fa1a8da-f335-b8b1-bfb6-fae88f20d598@huaweicloud.com> (raw)
In-Reply-To: <87a5l8am4k.fsf@gmail.com>
On 2024/5/2 12:11, Ritesh Harjani (IBM) wrote:
> Dave Chinner <david@fromorbit.com> writes:
>
>> On Wed, May 01, 2024 at 05:49:50PM +0530, Ritesh Harjani wrote:
>>> Dave Chinner <david@fromorbit.com> writes:
>>>
>>>> On Wed, Apr 10, 2024 at 10:29:16PM +0800, Zhang Yi wrote:
>>>>> 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
>>>>
>>>> Shouldn't this be serialised by the file invalidation lock? Hole
>>>> punching via fallocate must do this to avoid data use-after-free
>>>> bugs w.r.t racing page faults and all the other fallocate ops need
>>>> to serialise page faults to avoid page cache level data corruption.
>>>> Yet here we see a problem resulting from a fallocate operation
>>>> racing with a page fault....
>>>
>>> IIUC, fallocate operations which invalidates the page cache contents needs
>>> to take th invalidate_lock in exclusive mode to prevent page fault
>>> operations from loading pages for stale mappings (blocks which were
>>> marked free might get reused). This can cause stale data exposure.
>>>
>>> Here the fallocate operation require allocation of unwritten extents and
>>> does not require truncate of pagecache range. So I guess, it is not
>>> strictly necessary to hold the invalidate lock here.
>>
>> True, but you can make exactly the same argument for write() vs
>> fallocate(). Yet this path in ext4_fallocate() locks out
>> concurrent write()s and waits for DIOs in flight to drain. What
>> makes buffered writes triggered by page faults special?
>>
>> i.e. if you are going to say "we don't need serialisation between
>> writes and fallocate() allocating unwritten extents", then why is it
>> still explicitly serialising against both buffered and direct IO and
>> not just truncate and other fallocate() operations?
>>
>>> But I see XFS does take IOLOCK_EXCL AND MMAPLOCK_EXCL even for this operation.
>>
>> Yes, that's the behaviour preallocation has had in XFS since we
>> introduced the MMAPLOCK almost a decade ago. This was long before
>> the file_invalidation_lock() was even a glimmer in Jan's eye.
>>
>> btrfs does the same thing, for the same reasons. COW support makes
>> extent tree manipulations excitingly complex at times...
>>
>>> I guess we could use the invalidate lock for fallocate operation in ext4
>>> too. However, I think we still require the current patch. The reason is
>>> ext4_da_map_blocks() call here first tries to lookup the extent status
>>> cache w/o any i_data_sem lock in the fastpath. If it finds a hole, it
>>> takes the i_data_sem in write mode and just inserts an entry into extent
>>> status cache w/o re-checking for the same under the exclusive lock.
>>> ...So I believe we still should have this patch which re-verify under
>>> the write lock if whether any other operation has inserted any entry
>>> already or not.
>>
>> Yup, I never said the code in the patch is wrong or unnecessary; I'm
>> commenting on the high level race condition that lead to the bug
>> beting triggered. i.e. that racing data modification operations with
>> low level extent manipulations is often dangerous and a potential
>> source of very subtle, hard to trigger, reproduce and debug issues
>> like the one reported...
>>
>
> Yes, thanks for explaining and commenting on the high level design.
> It was indeed helpful. And I agree with your comment on, we can refactor
> out the common operations from fallocate path and use invalidate lock to
> protect against data modification (page fault) and extent manipulation
> path (fallocate operations).
>
Yeah, thanks for explanation and suggestion, too. After looking at your
discussion, I also suppose we could refactor a common helper and use the
file invalidation lock for the whole ext4 fallocate path, current code is
too scattered.
Thanks,
Yi.
next prev parent reply other threads:[~2024-05-06 3:49 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
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 [this message]
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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