Re: [PATCH v3 00/13] dax: fix dma vs truncate and remove 'page-less' support

[Dave Chinner]

On Thu, Oct 26, 2017 at 11:51:04PM +0000, Williams, Dan J wrote:
> On Thu, 2017-10-26 at 12:58 +0200, Jan Kara wrote:
> > On Fri 20-10-17 11:31:48, Christoph Hellwig wrote:
> > > On Fri, Oct 20, 2017 at 09:47:50AM +0200, Christoph Hellwig wrote:
> > > > I'd like to brainstorm how we can do something better.
> > > > 
> > > > How about:
> > > > 
> > > > If we hit a page with an elevated refcount in truncate / hole puch
> > > > etc for a DAX file system we do not free the blocks in the file system,
> > > > but add it to the extent busy list.  We mark the page as delayed
> > > > free (e.g. page flag?) so that when it finally hits refcount zero we
> > > > call back into the file system to remove it from the busy list.
> > > 
> > > Brainstorming some more:
> > > 
> > > Given that on a DAX file there shouldn't be any long-term page
> > > references after we unmap it from the page table and don't allow
> > > get_user_pages calls why not wait for the references for all
> > > DAX pages to go away first?  E.g. if we find a DAX page in
> > > truncate_inode_pages_range that has an elevated refcount we set
> > > a new flag to prevent new references from showing up, and then
> > > simply wait for it to go away.  Instead of a busy way we can
> > > do this through a few hashed waitqueued in dev_pagemap.  And in
> > > fact put_zone_device_page already gets called when putting the
> > > last page so we can handle the wakeup from there.
> > > 
> > > In fact if we can't find a page flag for the stop new callers
> > > things we could probably come up with a way to do that through
> > > dev_pagemap somehow, but I'm not sure how efficient that would
> > > be.
> > 
> > We were talking about this yesterday with Dan so some more brainstorming
> > from us. We can implement the solution with extent busy list in ext4
> > relatively easily - we already have such list currently similarly to XFS.
> > There would be some modifications needed but nothing too complex. The
> > biggest downside of this solution I see is that it requires per-filesystem
> > solution for busy extents - ext4 and XFS are reasonably fine, however btrfs
> > may have problems and ext2 definitely will need some modifications.
> > Invisible used blocks may be surprising to users at times although given
> > page refs should be relatively short term, that should not be a big issue.
> > But are we guaranteed page refs are short term? E.g. if someone creates
> > v4l2 videobuf in MAP_SHARED mapping of a file on DAX filesystem, page refs
> > can be rather long-term similarly as in RDMA case. Also freeing of blocks
> > on page reference drop is another async entry point into the filesystem
> > which could unpleasantly surprise us but I guess workqueues would solve
> > that reasonably fine.
> > 
> > WRT waiting for page refs to be dropped before proceeding with truncate (or
> > punch hole for that matter - that case is even nastier since we don't have
> > i_size to guard us). What I like about this solution is that it is very
> > visible there's something unusual going on with the file being truncated /
> > punched and so problems are easier to diagnose / fix from the admin side.
> > So far we have guarded hole punching from concurrent faults (and
> > get_user_pages() does fault once you do unmap_mapping_range()) with
> > I_MMAP_LOCK (or its equivalent in ext4). We cannot easily wait for page
> > refs to be dropped under I_MMAP_LOCK as that could deadlock - the most
> > obvious case Dan came up with is when GUP obtains ref to page A, then hole
> > punch comes grabbing I_MMAP_LOCK and waiting for page ref on A to be
> > dropped, and then GUP blocks on trying to fault in another page.
> > 
> > I think we cannot easily prevent new page references to be grabbed as you
> > write above since nobody expects stuff like get_page() to fail. But I 
> > think that unmapping relevant pages and then preventing them to be faulted
> > in again is workable and stops GUP as well. The problem with that is though
> > what to do with page faults to such pages - you cannot just fail them for
> > hole punch, and you cannot easily allocate new blocks either. So we are
> > back at a situation where we need to detach blocks from the inode and then
> > wait for page refs to be dropped - so some form of busy extents. Am I
> > missing something?
> > 
> 
> No, that's a good summary of what we talked about. However, I did go
> back and give the new lock approach a try and was able to get my test
> to pass. The new locking is not pretty especially since you need to
> drop and reacquire the lock so that get_user_pages() can finish
> grabbing all the pages it needs. Here are the two primary patches in
> the series, do you think the extent-busy approach would be cleaner?

The XFS_DAXDMA.... 

$DEITY that patch is so ugly I can't even bring myself to type it.

-Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx