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
