Re: [PATCH] dax, pmem: add support for msync

From: Boaz Harrosh
Date: Thu Sep 03 2015 - 02:32:16 EST

On 09/02/2015 10:04 PM, Ross Zwisler wrote:
> On Tue, Sep 01, 2015 at 03:18:41PM +0300, Boaz Harrosh wrote:
>> Apps expect all these to work:
>> 1. open mmap m-write msync ... close
>> 2. open mmap m-write fsync ... close
>> 3. open mmap m-write unmap ... fsync close
>> 4. open mmap m-write sync ...
> So basically you made close have an implicit fsync? What about the flow that
> looks like this:
> 5. open mmap close m-write

What? no, close means ummap because you need a file* attached to your vma

And you miss-understood me, the vm_opts->close is the *unmap* operation not
the file::close() operation.

I meant memory-cl_flush on unmap before the vma goes away.

> This guy definitely needs an msync/fsync at the end to make sure that the
> m-write becomes durable.

Exactly done at unmap time.

> Also, the CLOSE(2) man page specifically says that a flush does not occur at
> close:
> A successful close does not guarantee that the data has been
> successfully saved to disk, as the kernel defers writes. It
> is not common for a filesystem to flush the buffers when the stream is
> closed. If you need to be sure that the data is physically stored,
> use fsync(2). (It will depend on the disk hardware at this point.)
> I don't think that adding an implicit fsync to close is the right solution -
> we just need to get msync and fsync correctly working.

So above is not relevant, and we are doing that. taking care of cpu-cache flushing.
This is not disk-flushing, on a long memcpy from usermode most of the data is
already durable, is only the leftover margins. Like the dax_io in the kernel
dax implies direct_io always, all we are trying is to have the least
performance hit in memory-cache-flushing.

IS nothing to do with the text above.

>> The first 3 are supported with above, because what happens is that at [3]
>> the fsync actually happens on unmap and fsync is redundant in that case.
>> The only broken scenario is [3]. We do not have a list of "dax-dirty" inodes
>> per sb to iterate on and call inode-sync on. This cause problems mostly in
>> freeze because with actual [3] scenario the file will be eventually closed
>> and persistent, but after the call to sync returns.
>> Its on my TODO to fix [3] based on instructions from Dave.
>> The mmap call will put the inode on the list and the dax_vm_close will
>> remove it. One of the regular dirty list should be used as suggested by
>> Dave.
> I believe in the above two paragraphs you meant [4], so the
> 4. open mmap m-write sync ...
> case needs to be fixed so that we can detect DAX-dirty inodes?

Yes I'll be working on sync (DAX-dirty-i_list) soon but it needs a working
fsync to be in place (eg: dax_fsync(inode))


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