Re: [PATCH] md/bitmap: avoid read out of the disk

[NeilBrown <neilb@suse.com>]

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On Mon, Oct 16 2017, Song Liu wrote:

>> On Oct 13, 2017, at 12:51 PM, Shaohua Li <shli@kernel.org> wrote:
>> 
>> On Fri, Oct 13, 2017 at 04:16:33PM +1100, Neil Brown wrote:
>>> On Thu, Oct 12 2017, Song Liu wrote:
>>> 
>>>>> On Oct 12, 2017, at 10:30 AM, Shaohua Li <shli@kernel.org> wrote:
>>>>> 
>>>>> On Thu, Oct 12, 2017 at 02:09:21PM +1100, Neil Brown wrote:
>>>>>> On Tue, Oct 10 2017, Shaohua Li wrote:
>>>>>> 
>>>>>>> From: Shaohua Li <shli@fb.com>
>>>>>>> 
>>>>>>> If PAGE_SIZE is bigger than 4k, we could read out of the disk boundary. Limit
>>>>>>> the read size to the end of disk. Write path already has similar limitation.
>>>>>>> 
>>>>>>> Fix: 8031c3ddc70a(md/bitmap: copy correct data for bitmap super)
>>>>>>> Reported-by: Joshua Kinard <kumba@gentoo.org>
>>>>>>> Tested-by: Joshua Kinard <kumba@gentoo.org>
>>>>>>> Cc: Song Liu <songliubraving@fb.com>
>>>>>>> Signed-off-by: Shaohua Li <shli@fb.com>
>>>>>> 
>>>>>> Given that this bug was introduced by
>>>>>> Commit: 8031c3ddc70a ("md/bitmap: copy correct data for bitmap super")
>>>>>> 
>>>>>> and that patch is markted:
>>>>>> 
>>>>>>   Cc: stable@vger.kernel.org (4.10+)
>>>>>> 
>>>>>> I think this patch should be tagged "CC: stable" too.
>>>>> 
>>>>> I thought the Fix tag is enough, but I'll add the stable 
>>>>>> However ... that earlier patch looks strange to me.
>>>>>> Why is it that "raid5 cache could write bitmap superblock before bitmap superblock is
>>>>>> initialized."  Can we just get raid5 cache *not* to write the bitmap
>>>>>> superblock too early?
>>>>>> I think that would better than breaking code that previously worked.
>>>>> 
>>>>> That's the log reply code, which must update superblock and hence bitmap
>>>>> superblock, because reply happens very earlier. I agree the reply might still
>>>>> have problem with bitmap. We'd better defer reply after the raid is fully
>>>>> initialized. Song, any idea?
>>>>> 
>>>> 
>>>> With write back cache, there are two different types of stripes in recovery:
>>>> data-parity, and data-only. For data-parity stripes, we can simply replay data
>>>> from the journal. But for data-only stripes, we need to do rcw or rmw to update
>>>> parities. Currently, the writes are handled with raid5 state machine. Therefore,
>>>> we wake up mddev->thread in r5l_recovery_log(). It is necessary to finish these 
>>>> stripes before we fully initialize the array, because these stripes need to be 
>>>> handled with write back state machine; while we we always start the array with 
>>>> write through journal_mode. 
>>>> 
>>>> Maybe we can fix this by change the order of initialization in md_run(), 
>>>> specifically, moving bitmap_create() before pers->run(). 
>>> 
>>> I've looked at some of the details here now.
>>> 
>>> I think I would like raid5-cache to not perform any recovery until we
>>> reach
>>> 
>>> 
>>> 	md_wakeup_thread(mddev->thread);
>>> 	md_wakeup_thread(mddev->sync_thread); /* possibly kick off a reshape */
>>> 
>>> 
>>> in do_md_run().  Before that point it is possible to fail and abort -
>>> e.g. if bitmap_load() fails.
>>> 
>>> Possibly we could insert another personality call here "->start()" ??
>>> That could then do whatever is needed before
>>> 
>>> 	set_capacity(mddev->gendisk, mddev->array_sectors);
>>> 	revalidate_disk(mddev->gendisk);
>>> 
>>> makes the array accessible.
>>> 
>>> Might that be reasonable?
>> 
>> Looks good. I think we should call the ->start before
>> md_wakeup_thread(mddev->thread); because we don't want to start recovery before
>> log is recovered.
>
> I also like this idea. In the coming month, I won't have much bandwidth to 
> implement this. Please let me know if you want to make the change. Otherwise, 
> I will do it later (in December, I guess). 

It isn't something we should rush so take your time.
However I think we need to clean up the patches that have gone to
-stable.
I think
 Commit: 8031c3ddc70a ("md/bitmap: copy correct data for bitmap super")
should be reverted (in -stable too) and possibly be replaced by a patch
which refuses any attempt to combine a bitmap with a journal.
As Shaohua pointed that, that shouldn't really be needed anyway.
That would address the issue that 8031c3ddc70a was meant to fix.
I can write that patch if necessary.

Thanks,
NeilBrown

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