From: Chao Yu <chao@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>, Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Theodore Ts'o <tytso@mit.edu>,
linux-f2fs-devel@lists.sourceforge.net,
linux-fsdevel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] f2fs: remove broken support for allocating DIO writes
Date: Fri, 20 Aug 2021 17:35:21 +0800 [thread overview]
Message-ID: <c4e5c71d-1652-7174-fa36-674fab4e61df@kernel.org> (raw)
In-Reply-To: <YRsY6dyHyaChkQ6n@gmail.com>
On 2021/8/17 10:03, Eric Biggers wrote:
> On Mon, Aug 02, 2021 at 06:34:48PM -0700, Jaegeuk Kim wrote:
>> On 08/03, Chao Yu wrote:
>>> On 2021/8/3 2:23, Jaegeuk Kim wrote:
>>>> On 08/02, Chao Yu wrote:
>>>>> On 2021/8/2 12:39, Eric Biggers wrote:
>>>>>> On Fri, Jul 30, 2021 at 10:46:16PM -0400, Theodore Ts'o wrote:
>>>>>>> On Fri, Jul 30, 2021 at 12:17:26PM -0700, Eric Biggers wrote:
>>>>>>>>> Currently, non-overwrite DIO writes are fundamentally unsafe on f2fs as
>>>>>>>>> they require preallocating blocks, but f2fs doesn't support unwritten
>>>>>>>>> blocks and therefore has to preallocate the blocks as regular blocks.
>>>>>>>>> f2fs has no way to reliably roll back such preallocations, so as a
>>>>>>>>> result, f2fs will leak uninitialized blocks to users if a DIO write
>>>>>>>>> doesn't fully complete.
>>>>>>>
>>>>>>> There's another way of solving this problem which doesn't require
>>>>>>> supporting unwritten blocks. What a file system *could* do is to
>>>>>>> allocate the blocks, but *not* update the on-disk data structures ---
>>>>>>> so the allocation happens in memory only, so you know that the
>>>>>>> physical blocks won't get used for another files, and then issue the
>>>>>>> data block writes. On the block I/O completion, trigger a workqueue
>>>>>>> function which updates the on-disk metadata to assign physical blocks
>>>>>>> to the inode.
>>>>>>>
>>>>>>> That way if you crash before the data I/O has a chance to complete,
>>>>>>> the on-disk logical block -> physical block map hasn't been updated
>>>>>>> yet, and so you don't need to worry about leaking uninitialized blocks.
>>>>>
>>>>> Thanks for your suggestion, I think it makes sense.
>>>>>
>>>>>>>
>>>>>>> Cheers,
>>>>>>>
>>>>>>> - Ted
>>>>>>
>>>>>> Jaegeuk and Chao, any idea how feasible it would be for f2fs to do this?
>>>>>
>>>>> Firstly, let's notice that below metadata will be touched during DIO
>>>>> preallocation flow:
>>>>> - log header
>>>>> - sit bitmap/count
>>>>> - free seg/sec bitmap/count
>>>>> - dirty seg/sec bitmap/count
>>>>>
>>>>> And there is one case we need to concern about is: checkpoint() can be
>>>>> triggered randomly in between dio_preallocate() and dio_end_io(), we should
>>>>> not persist any DIO preallocation related metadata during checkpoint(),
>>>>> otherwise, sudden power-cut after the checkpoint will corrupt filesytem.
>>>>>
>>>>> So it needs to well separate two kinds of metadata update:
>>>>> a) belong to dio preallocation
>>>>> b) the left one
>>>>>
>>>>> After that, it will simply checkpoint() flow to just flush metadata b), for
>>>>> other flow, like GC, data/node allocation, it needs to query/update metadata
>>>>> after we combine metadata a) and b).
>>>>>
>>>>> In addition, there is an existing in-memory log header framework in f2fs,
>>>>> based on this fwk, it's very easy for us to add a new in-memory log header
>>>>> for DIO preallocation.
>>>>>
>>>>> So it seems feasible for me until now...
>>>>>
>>>>> Jaegeuk, any other concerns about the implementation details?
>>>>
>>>> Hmm, I'm still trying to deal with this as a corner case where the writes
>>>> haven't completed due to an error. How about keeping the preallocated block
>>>> offsets and releasing them if we get an error? Do we need to handle EIO right?
>>>
>>> What about the case that CP + SPO following DIO preallocation? User will
>>> encounter uninitialized block after recovery.
>>
>> I think buffered writes as a workaround can expose the last unwritten block as
>> well, if SPO happens right after block allocation. We may need to compromise
>> at certain level?
>>
>
> Freeing preallocated blocks on error would be better than nothing, although note
> that the preallocated blocks may have filled an arbitrary sequence of holes --
> so simply truncating past EOF would *not* be sufficient.
>
> But really filesystems need to be designed to never expose uninitialized data,
> even if I/O errors or a sudden power failure occurs. It is unfortunate that
> f2fs apparently wasn't designed with that goal in mind.
>
> In any case, I don't think we can proceed with any other f2fs direct I/O
> improvements until this data leakage bug can be solved one way or another. If
> my patch to remove support for allocating writes isn't acceptable and the
> desired solution is going to require some more invasive f2fs surgery, are you or
> Chao going to work on it? I'm not sure there's much I can do here.
I may have time to take look into the implementation as I proposed above, maybe
just enabling this in FSYNC_MODE_STRICT mode if user concerns unwritten data?
thoughts?
>
> - Eric
>
WARNING: multiple messages have this Message-ID (diff)
From: Chao Yu <chao@kernel.org>
To: Eric Biggers <ebiggers@kernel.org>, Jaegeuk Kim <jaegeuk@kernel.org>
Cc: linux-fsdevel@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>,
stable@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: remove broken support for allocating DIO writes
Date: Fri, 20 Aug 2021 17:35:21 +0800 [thread overview]
Message-ID: <c4e5c71d-1652-7174-fa36-674fab4e61df@kernel.org> (raw)
In-Reply-To: <YRsY6dyHyaChkQ6n@gmail.com>
On 2021/8/17 10:03, Eric Biggers wrote:
> On Mon, Aug 02, 2021 at 06:34:48PM -0700, Jaegeuk Kim wrote:
>> On 08/03, Chao Yu wrote:
>>> On 2021/8/3 2:23, Jaegeuk Kim wrote:
>>>> On 08/02, Chao Yu wrote:
>>>>> On 2021/8/2 12:39, Eric Biggers wrote:
>>>>>> On Fri, Jul 30, 2021 at 10:46:16PM -0400, Theodore Ts'o wrote:
>>>>>>> On Fri, Jul 30, 2021 at 12:17:26PM -0700, Eric Biggers wrote:
>>>>>>>>> Currently, non-overwrite DIO writes are fundamentally unsafe on f2fs as
>>>>>>>>> they require preallocating blocks, but f2fs doesn't support unwritten
>>>>>>>>> blocks and therefore has to preallocate the blocks as regular blocks.
>>>>>>>>> f2fs has no way to reliably roll back such preallocations, so as a
>>>>>>>>> result, f2fs will leak uninitialized blocks to users if a DIO write
>>>>>>>>> doesn't fully complete.
>>>>>>>
>>>>>>> There's another way of solving this problem which doesn't require
>>>>>>> supporting unwritten blocks. What a file system *could* do is to
>>>>>>> allocate the blocks, but *not* update the on-disk data structures ---
>>>>>>> so the allocation happens in memory only, so you know that the
>>>>>>> physical blocks won't get used for another files, and then issue the
>>>>>>> data block writes. On the block I/O completion, trigger a workqueue
>>>>>>> function which updates the on-disk metadata to assign physical blocks
>>>>>>> to the inode.
>>>>>>>
>>>>>>> That way if you crash before the data I/O has a chance to complete,
>>>>>>> the on-disk logical block -> physical block map hasn't been updated
>>>>>>> yet, and so you don't need to worry about leaking uninitialized blocks.
>>>>>
>>>>> Thanks for your suggestion, I think it makes sense.
>>>>>
>>>>>>>
>>>>>>> Cheers,
>>>>>>>
>>>>>>> - Ted
>>>>>>
>>>>>> Jaegeuk and Chao, any idea how feasible it would be for f2fs to do this?
>>>>>
>>>>> Firstly, let's notice that below metadata will be touched during DIO
>>>>> preallocation flow:
>>>>> - log header
>>>>> - sit bitmap/count
>>>>> - free seg/sec bitmap/count
>>>>> - dirty seg/sec bitmap/count
>>>>>
>>>>> And there is one case we need to concern about is: checkpoint() can be
>>>>> triggered randomly in between dio_preallocate() and dio_end_io(), we should
>>>>> not persist any DIO preallocation related metadata during checkpoint(),
>>>>> otherwise, sudden power-cut after the checkpoint will corrupt filesytem.
>>>>>
>>>>> So it needs to well separate two kinds of metadata update:
>>>>> a) belong to dio preallocation
>>>>> b) the left one
>>>>>
>>>>> After that, it will simply checkpoint() flow to just flush metadata b), for
>>>>> other flow, like GC, data/node allocation, it needs to query/update metadata
>>>>> after we combine metadata a) and b).
>>>>>
>>>>> In addition, there is an existing in-memory log header framework in f2fs,
>>>>> based on this fwk, it's very easy for us to add a new in-memory log header
>>>>> for DIO preallocation.
>>>>>
>>>>> So it seems feasible for me until now...
>>>>>
>>>>> Jaegeuk, any other concerns about the implementation details?
>>>>
>>>> Hmm, I'm still trying to deal with this as a corner case where the writes
>>>> haven't completed due to an error. How about keeping the preallocated block
>>>> offsets and releasing them if we get an error? Do we need to handle EIO right?
>>>
>>> What about the case that CP + SPO following DIO preallocation? User will
>>> encounter uninitialized block after recovery.
>>
>> I think buffered writes as a workaround can expose the last unwritten block as
>> well, if SPO happens right after block allocation. We may need to compromise
>> at certain level?
>>
>
> Freeing preallocated blocks on error would be better than nothing, although note
> that the preallocated blocks may have filled an arbitrary sequence of holes --
> so simply truncating past EOF would *not* be sufficient.
>
> But really filesystems need to be designed to never expose uninitialized data,
> even if I/O errors or a sudden power failure occurs. It is unfortunate that
> f2fs apparently wasn't designed with that goal in mind.
>
> In any case, I don't think we can proceed with any other f2fs direct I/O
> improvements until this data leakage bug can be solved one way or another. If
> my patch to remove support for allocating writes isn't acceptable and the
> desired solution is going to require some more invasive f2fs surgery, are you or
> Chao going to work on it? I'm not sure there's much I can do here.
I may have time to take look into the implementation as I proposed above, maybe
just enabling this in FSYNC_MODE_STRICT mode if user concerns unwritten data?
thoughts?
>
> - Eric
>
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next prev parent reply other threads:[~2021-08-20 9:35 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-28 1:51 [f2fs-dev] [PATCH] f2fs: remove broken support for allocating DIO writes Eric Biggers
2021-07-28 1:51 ` Eric Biggers
2021-07-30 19:17 ` [f2fs-dev] " Eric Biggers
2021-07-30 19:17 ` Eric Biggers
2021-07-30 22:12 ` [f2fs-dev] " Jaegeuk Kim
2021-07-30 22:12 ` Jaegeuk Kim
2021-07-30 22:19 ` Eric Biggers
2021-07-30 22:19 ` [f2fs-dev] " Eric Biggers
2021-07-31 1:05 ` Jaegeuk Kim
2021-07-31 1:05 ` [f2fs-dev] " Jaegeuk Kim
2021-07-31 1:18 ` Eric Biggers
2021-07-31 1:18 ` Eric Biggers
2021-07-31 2:46 ` Theodore Ts'o
2021-07-31 2:46 ` [f2fs-dev] " Theodore Ts'o
2021-08-02 4:39 ` Eric Biggers
2021-08-02 4:39 ` Eric Biggers
2021-08-02 9:00 ` Chao Yu
2021-08-02 9:00 ` [f2fs-dev] " Chao Yu
2021-08-02 18:23 ` Jaegeuk Kim
2021-08-02 18:23 ` Jaegeuk Kim
2021-08-03 1:19 ` [f2fs-dev] " Chao Yu
2021-08-03 1:19 ` Chao Yu
2021-08-03 1:34 ` [f2fs-dev] " Jaegeuk Kim
2021-08-03 1:34 ` Jaegeuk Kim
2021-08-17 2:03 ` [f2fs-dev] " Eric Biggers
2021-08-17 2:03 ` Eric Biggers
2021-08-17 5:42 ` [f2fs-dev] " Christoph Hellwig
2021-08-17 5:42 ` Christoph Hellwig
2021-08-17 18:57 ` Jaegeuk Kim
2021-08-17 18:57 ` [f2fs-dev] " Jaegeuk Kim
2021-08-17 20:27 ` Eric Biggers
2021-08-17 20:27 ` [f2fs-dev] " Eric Biggers
2021-08-17 21:33 ` Jaegeuk Kim
2021-08-17 21:33 ` Jaegeuk Kim
2021-08-18 0:06 ` [f2fs-dev] " Eric Biggers
2021-08-18 0:06 ` Eric Biggers
2021-08-20 9:35 ` Chao Yu [this message]
2021-08-20 9:35 ` [f2fs-dev] " Chao Yu
2021-08-20 18:11 ` Eric Biggers
2021-08-20 18:11 ` Eric Biggers
2021-08-20 22:01 ` [f2fs-dev] " Chao Yu
2021-08-20 22:01 ` Chao Yu
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