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In particular: - suspend the mount around vflush() to avoid new writes come after the vnode is processed; - flush pending metadata updates (mostly node times); - remap all rw mappings of files from the mount into ro. It is not clear to me how to handle writeable mappings on rw->ro for tmpfs best. Other filesystems, which use vnode vm object, call vgone() on vnodes with writers, which sets the vm object type to OBJT_DEAD, and keep the resident pages and installed ptes as is. In particular, the existing mappings continue to work as far as application only accesses resident pages, but changes are not flushed to file. For tmpfs the vm object of VREG vnodes also serves as the data pages container, giving single copy of the mapped pages, so it cannot be set to OBJT_DEAD. Alternatives for making rw mappings ro could be either invalidating them at all, or marking as CoW. Tested by: pho Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential revision: https://reviews.freebsd.org/D19737
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