Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

From: Dan Williams
Date: Fri Nov 13 2015 - 21:32:45 EST

On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote:
> On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>> On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler
>> <ross.zwisler@xxxxxxxxxxxxxxx> wrote:
>>> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These
>>> are sent down via blkdev_issue_flush() in response to a fsync() or msync()
>>> and are used by filesystems to order their metadata, among other things.
>>> When we get an msync() or fsync() it is the responsibility of the DAX code
>>> to flush all dirty pages to media. The PMEM driver then just has issue a
>>> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all
>>> the flushed data has been durably stored on the media.
>>> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
>> Hmm, I'm not seeing why we need this patch. If the actual flushing of
>> the cache is done by the core why does the driver need support
>> REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA
>> only makes sense if individual writes can bypass the "drive" cache,
>> but no I/O submitted to the driver proper is ever cached we always
>> flush it through to media.
> If the upper level filesystem gets an error when submitting a flush
> request, then it assumes the underlying hardware is broken and cannot
> be as aggressive in IO submission, but instead has to wait for in-flight
> IO to complete.

Upper level filesystems won't get errors when the driver does not
support flush. Those requests are ended cleanly in
generic_make_request_checks(). Yes, the fs still needs to wait for
outstanding I/O to complete but in the case of pmem all I/O is
synchronous. There's never anything to await when flushing at the
pmem driver level.

> Since FUA/FLUSH is basically a no-op for pmem devices,
> it doesn't make sense _not_ to support this functionality.

Seems to be a nop either way. Given that DAX may lead to dirty data
pending to the device in the cpu cache that a REQ_FLUSH request will
not touch, its better to leave it all to the mm core to handle. I.e.
it doesn't make sense to call the driver just for two instructions
(sfence + pcommit) when the mm core is taking on the cache flushing.
Either handle it all in the mm or the driver, not a mixture.
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