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vfio: disable filesystem-dax page pinning
Filesystem-DAX is incompatible with 'longterm' page pinning. Without page cache indirection a DAX mapping maps filesystem blocks directly. This means that the filesystem must not modify a file's block map while any page in a mapping is pinned. In order to prevent the situation of userspace holding of filesystem operations indefinitely, disallow 'longterm' Filesystem-DAX mappings. RDMA has the same conflict and the plan there is to add a 'with lease' mechanism to allow the kernel to notify userspace that the mapping is being torn down for block-map maintenance. Perhaps something similar can be put in place for vfio. Note that xfs and ext4 still report: "DAX enabled. Warning: EXPERIMENTAL, use at your own risk" ...at mount time, and resolving the dax-dma-vs-truncate problem is one of the last hurdles to remove that designation. Acked-by: Alex Williamson <[email protected]> Cc: Michal Hocko <[email protected]> Cc: [email protected] Cc: <[email protected]> Reported-by: Haozhong Zhang <[email protected]> Tested-by: Haozhong Zhang <[email protected]> Fixes: d475c63 ("dax,ext2: replace XIP read and write with DAX I/O") Reviewed-by: Christoph Hellwig <[email protected]> Signed-off-by: Dan Williams <[email protected]>
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