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When rolling back a dataset, ZFS has to purge file data resident in the system page cache. To do this, it loops over all vnodes for the mountpoint and calls vn_pages_remove() to purge pages associated with the vnode's VM object. Each page is thus exclusively busied while the dataset's teardown write lock is held. When handling a page fault on a mapped ZFS file, FreeBSD's page fault handler busies newly allocated pages and then uses VOP_GETPAGES to fill them. The ZFS getpages VOP acquires the teardown read lock with vnode pages already busied. This represents a lock order reversal which can lead to deadlock. To break the deadlock, observe that zfs_rezget() need only purge those pages marked valid, and that pages busied by the page fault handler are, by definition, invalid. Furthermore, ZFS pages always transition from invalid to valid with the teardown lock held, and ZFS never creates partially valid pages. Thus, zfs_rezget() can use the new vn_pages_remove_valid() to skip over pages busied by the fault handler. PR: 258208 Tested by: pho Reviewed by: avg, sef, kib MFC after: 2 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D32931 Reviewed-by: Tony Hutter <hutter2@llnl.gov> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Alexander Motin <mav@FreeBSD.org> Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org> Closes #12828 |
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