Messages in this thread | | | Date | Mon, 2 May 2016 13:45:08 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v5 0/2] ext4: Improve parallel I/O performance on NVDIMM |
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On 05/01/2016 01:28 PM, Christoph Hellwig wrote: > On Fri, Apr 29, 2016 at 12:38:20PM -0400, Waiman Long wrote: >> From my testing, it looked like that parallel overwrites to the same file in >> an ext4 filesystem on DAX can happen in parallel even if their range >> overlaps. It was mainly because the code will drop the i_mutex before the >> write. That means the overlapped blocks can get garbage. I think this is a >> problem, but I am not expert in the ext4 filesystem to say for sure. I would >> like to know your thought on that. > That's another issue with dax I/O pretending to be direct I/O.. Because > it isn't we'll need to synchronize it like buffered I/O and not like > direct I/O in all file systems.
From what I saw in the code, I think filemap_write_and_wait_range() should have prevented concurrent overwrites from stepping on each other for non-DAX I/O. However it is essentially a no-op for DAX I/O and so the protection is gone.
I am planning to send out a patch to disable mutex dropping for DAX overwrite. There is still an issue on the read side. If journal is disabled and the dioread_nolock mount option is used, read will done without locking. Again, the filemap_write_and_wait_range() check on the read side will not protect against write.
Cheers, Longman
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