On Fri, Oct 20, 2017 at 9:32 AM, Christoph Hellwig <hch@xxxxxx> wrote: > On Fri, Oct 20, 2017 at 08:42:00AM -0700, Dan Williams wrote: >> I agree, but it needs quite a bit more thought and restructuring of >> the truncate path. I also wonder how we reclaim those stranded >> filesystem blocks, but a first approximation is wait for the >> administrator to delete them or auto-delete them at the next mount. >> XFS seems well prepared to reflink-swap these DMA blocks around, but >> I'm not sure about EXT4. > > reflink still is an optional and experimental feature in XFS. That > being said we should not need to swap block pointers around on disk. > We just need to prevent the block allocator from reusing the blocks > for new allocations, and we have code for that, both for transactions > that haven't been committed to disk yet, and for deleted blocks > undergoing discard operations. > > But as mentioned in my second mail from this morning I'm not even > sure we need that. For short-term elevated page counts like normal > get_user_pages users I think we can just wait for the page count > to reach zero, while for abuses of get_user_pages for long term > pinning memory (not sure if anyone but rdma is doing that) we'll need > something like FL_LAYOUT leases to release the mapping. I'll take a look at hooking this up through a page-idle callback. Can I get some breadcrumbs to grep for from XFS folks on how to set/clear the busy state of extents?
