dax: require 'struct page' by default for filesystem dax
If a dax buffer from a device that does not map pages is passed to read(2) or write(2) as a target for direct-I/O it triggers SIGBUS. If gdb attempts to examine the contents of a dax buffer from a device that does not map pages it triggers SIGBUS. If fork(2) is called on a process with a dax mapping from a device that does not map pages it triggers SIGBUS. 'struct page' is required otherwise several kernel code paths break in surprising ways. Disable filesystem-dax on devices that do not map pages. In addition to needing pfn_to_page() to be valid we also require devmap pages. We need this to detect dax pages in the get_user_pages_fast() path and so that we can stop managing the VM_MIXEDMAP flag. For DAX drivers that have not supported get_user_pages() to date we allow them to opt-in to supporting DAX with the CONFIG_FS_DAX_LIMITED configuration option which requires ->direct_access() to return pfn_t_special() pfns. This leaves DAX support in brd disabled and scheduled for removal. Note that when the initial dax support was being merged a few years back there was concern that struct page was unsuitable for use with next generation persistent memory devices. The theoretical concern was that struct page access, being such a hotly used data structure in the kernel, would lead to media wear out. While that was a reasonable conservative starting position it has not held true in practice. We have long since committed to using devm_memremap_pages() to support higher order kernel functionality that needs get_user_pages() and pfn_to_page(). Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Ross Zwisler <ross.zwisler@linux.intel.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
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@ -297,6 +297,7 @@ config AXON_RAM
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tristate "Axon DDR2 memory device driver"
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depends on PPC_IBM_CELL_BLADE && BLOCK
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select DAX
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select FS_DAX_LIMITED
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default m
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help
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It registers one block device per Axon's DDR2 memory bank found
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@ -15,6 +15,7 @@
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#include <linux/mount.h>
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#include <linux/magic.h>
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#include <linux/genhd.h>
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#include <linux/pfn_t.h>
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#include <linux/cdev.h>
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#include <linux/hash.h>
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#include <linux/slab.h>
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@ -123,6 +124,15 @@ int __bdev_dax_supported(struct super_block *sb, int blocksize)
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return len < 0 ? len : -EIO;
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}
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if ((IS_ENABLED(CONFIG_FS_DAX_LIMITED) && pfn_t_special(pfn))
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|| pfn_t_devmap(pfn))
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/* pass */;
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else {
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pr_debug("VFS (%s): error: dax support not enabled\n",
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sb->s_id);
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return -EOPNOTSUPP;
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}
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return 0;
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}
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EXPORT_SYMBOL_GPL(__bdev_dax_supported);
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@ -16,6 +16,7 @@ config BLK_DEV_XPRAM
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config DCSSBLK
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def_tristate m
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select DAX
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select FS_DAX_LIMITED
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prompt "DCSSBLK support"
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depends on S390 && BLOCK
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help
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@ -58,6 +58,13 @@ config FS_DAX_PMD
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depends on ZONE_DEVICE
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depends on TRANSPARENT_HUGEPAGE
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# Selected by DAX drivers that do not expect filesystem DAX to support
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# get_user_pages() of DAX mappings. I.e. "limited" indicates no support
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# for fork() of processes with MAP_SHARED mappings or support for
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# direct-I/O to a DAX mapping.
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config FS_DAX_LIMITED
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bool
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endif # BLOCK
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# Posix ACL utility routines
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