On Friday 16 October 2020 12:15:45 AM IST Darrick J. Wong wrote: > On Thu, Oct 15, 2020 at 03:31:26PM +0530, Chandan Babu R wrote: > > On Thursday 15 October 2020 2:09:45 PM IST Christoph Hellwig wrote: > > > This patch demonstrates very well why I think having these magic > > > defines and the comments in a header makes no sense. > > > > > > > +/* > > > > + * Remapping an extent involves unmapping the existing extent and mapping in the > > > > + * new extent. > > > > + * > > > > + * When unmapping, an extent containing the entire unmap range can be split into > > > > + * two extents, > > > > + * i.e. | Old extent | hole | Old extent | > > > > + * Hence extent count increases by 1. > > > > + * > > > > + * Mapping in the new extent into the destination file can increase the extent > > > > + * count by 1. > > > > + */ > > > > +#define XFS_IEXT_REFLINK_REMAP_CNT(smap_real, dmap_written) \ > > > > + (((smap_real) ? 1 : 0) + ((dmap_written) ? 1 : 0)) > > > > + > > > > /* > > > > * Fork handling. > > > > */ > > > > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c > > > > index 4f0198f636ad..c9f9ff68b5bb 100644 > > > > --- a/fs/xfs/xfs_reflink.c > > > > +++ b/fs/xfs/xfs_reflink.c > > > > @@ -1099,6 +1099,11 @@ xfs_reflink_remap_extent( > > > > goto out_cancel; > > > > } > > > > > > > > + error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK, > > > > + XFS_IEXT_REFLINK_REMAP_CNT(smap_real, dmap_written)); > > > > + if (error) > > > > + goto out_cancel; > > > > + > > > > > > This is a completely mess. > > > > > > If OTOH xfs_reflink_remap_extent had a local variable for the potential > > > max number of extents, which is incremented near the initialization > > > of smap_real and dmap_written, with a nice comment near to each > > > increment it would make complete sense to the reader. > > > > > > > How about following the traits of XFS_IEXT_WRITE_UNWRITTEN_CNT (writing > > to unwritten extent) and XFS_IEXT_REFLINK_END_COW_CNT (moving an extent > > from cow fork to data fork) and setting XFS_IEXT_REFLINK_REMAP_CNT to a > > worst case value of 2? A write spanning the entirety of an unwritten extent > > does not change the extent count. Similarly, If there are no extents in the > > data fork spanning the file range mapped by an extent in the cow > > fork, moving the extent from cow fork to data fork increases the extent count > > by just 1 and not by the worst case count of 2. > > Probably not a huge deal, since at worst we bail out of reflink early > and userspace can just decide to fall back to a pagecache copy or > whatever. It'd be harder to deal with if this was the cow path where we > long ago returned from write() and even writeback... > > ...though now that I think about it, what /does/ happens if > _reflink_end_cow trips over this? I wonder if inodes need to be able to > reserve extent count for later, but ... hgnghghg that seems hard to > reason about. I am not sure if I am following you. Looks like you are asking about what happens if xfs_reflink_end_cow_extent() trips over XFS_IEXT_REFLINK_END_COW_CNT limit. If that occurs, then the end_io path would set AS_EIO on mapping->flags which would in turn cause the upper layers of the kernel to return -EIO to the corresponding fsync() call made by a userspace program. -- chandan
