Re: overhaul of direct IO NFS code

[Malahal Naineni <malahal@us.ibm.com>]

Trond, do you happen to have any patches regarding the rewrite you
mention below? We would love to test them or help in anyway we can.

Thanks, Malahal.

>> On Tue, Apr 12, 2011 at 11:49:29AM -0400, Trond Myklebust wrote:
>> 
>> What is the exact plan?  Split the direct I/O into two passes, one
>> to lock down the user pages and then a second one to send the pages
>> over the wire, which is shared with the writeback code?  If that's
>> the case it should naturally allow plugging in a scheme like Badari
>> to send pages from different iovecs in a single on the wire request -
>> after all page cache pages are non-continuous in virtual and physical
>> memory, too.
>
>You can't lock the user pages unfortunately: they may need to be faulted
>in.
>
>What I was thinking of doing is splitting out the code in the RPC
>callbacks that plays around with page flags and puts the pages on the
>inode's dirty list so that they don't get called in the case of
>O_DIRECT.
>I then want to attach the O_DIRECT pages to the nfsi->nfs_page_tree
>radix tree so that they can be tracked by the NFS layer. I'm assuming
>that nobody is going to be silly enough to require simultaneous writes
>via O_DIRECT to the same locations.
>Then we can feed the O_DIRECT pages into nfs_page_async_flush() so that
>they share the existing page cache write coalescing and pnfs code.
>
>The commit code will be easy to reuse too, since the requests are listed
>in the radix tree and so nfs_scan_list() can find and process them in
>the usual fashion.
>
>The main problem that I have yet to figure out is what to do if the
>server flags a reboot and the requests need to be resent. One option I'm
>looking into is using the aio 'kick handler' to resubmit the writes.
>Another may be to just resend directly from the nfsiod work queue.
>
>> When do you plan to release your read/write code re-write?  If it's
>> not anytime soon how is applying Badari's patch going to hurt?  Most
>> of it probably will get reverted with a complete rewrite, but at least
>> the logic to check which direct I/O iovecs can coalesced would stay
>> in the new world order.
>
>I'm hoping that I can do the rewrite fairly quickly once the resend
>problem is solved. It shouldn't be more than a couple of weeks of
>coding.