overhaul of direct IO NFS code

Quoted 9 lines · Trond Myklebust

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.

Quoted 5 lines

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.

Quoted 9 lines · Trond Myklebust

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.

Quoted 5 lines

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.