On 04/13/2016 11:16 PM, Dave Chinner wrote:
On Tue, Apr 12, 2016 at 02:12:54PM -0400, Waiman Long wrote:
When performing direct I/O, the current ext4 code does
not pass in the DIO_SKIP_DIO_COUNT flag to dax_do_io() or
__blockdev_direct_IO() when inode_dio_begin() has, in fact, been
called. This causes dax_do_io()/__blockdev_direct_IO() to invoke
inode_dio_begin()/inode_dio_end() internally. This doubling of
inode_dio_begin()/inode_dio_end() calls are wasteful.
This patch removes the extra internal inode_dio_begin()/inode_dio_end()
calls when those calls are being issued by the caller directly. For
really fast storage systems like NVDIMM, the removal of the extra
inode_dio_begin()/inode_dio_end() can give a meaningful boost to
I/O performance.
Doesn't this break truncate IO serialisation?
i.e. it appears to me that the ext4 use of inode_dio_begin()/
inode_dio_end() does not cover AIO, where the IO is still in flight
when submission returns. i.e. the inode_dio_end() call
needs to be in IO completion, not in the submitter context. The only
reason it doesn't break right now is that the duplicate accounting
in the DIO code is correct w.r.t. AIO. Hence bypassing the DIO
accounting will cause AIO writes to race with truncate.
Same AIO vs truncate problem occurs with the indirect read case you
modified to skip the direct IO layer accounting.
I don't quite understand how the duplicate accounting is correct wrt
AIO. Both the direct and indirect paths are something like:
inode_dio_begin()
...
inode_dio_begin()
...
inode_dio_end()
...
inode_dio_end()
What the patch does is to eliminate the innermost inode_dio_begin/end
pair. Unless there is a difference between a dio count of 2 vs. 1, I
can't see how the code correctness differ with and without my patch.
Cheers,
Longman
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