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>
diff --git a/arch/powerpc/platforms/Kconfig b/arch/powerpc/platforms/Kconfig
index 5a96a27..2ce89b4 100644
--- a/arch/powerpc/platforms/Kconfig
+++ b/arch/powerpc/platforms/Kconfig
@@ -297,6 +297,7 @@
tristate "Axon DDR2 memory device driver"
depends on PPC_IBM_CELL_BLADE && BLOCK
select DAX
+ select FS_DAX_LIMITED
default m
help
It registers one block device per Axon's DDR2 memory bank found
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index 3ec8046..473af69 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -15,6 +15,7 @@
#include <linux/mount.h>
#include <linux/magic.h>
#include <linux/genhd.h>
+#include <linux/pfn_t.h>
#include <linux/cdev.h>
#include <linux/hash.h>
#include <linux/slab.h>
@@ -123,6 +124,15 @@ int __bdev_dax_supported(struct super_block *sb, int blocksize)
return len < 0 ? len : -EIO;
}
+ if ((IS_ENABLED(CONFIG_FS_DAX_LIMITED) && pfn_t_special(pfn))
+ || pfn_t_devmap(pfn))
+ /* pass */;
+ else {
+ pr_debug("VFS (%s): error: dax support not enabled\n",
+ sb->s_id);
+ return -EOPNOTSUPP;
+ }
+
return 0;
}
EXPORT_SYMBOL_GPL(__bdev_dax_supported);
diff --git a/drivers/s390/block/Kconfig b/drivers/s390/block/Kconfig
index bc27d71..1444333 100644
--- a/drivers/s390/block/Kconfig
+++ b/drivers/s390/block/Kconfig
@@ -16,6 +16,7 @@
config DCSSBLK
def_tristate m
select DAX
+ select FS_DAX_LIMITED
prompt "DCSSBLK support"
depends on S390 && BLOCK
help
diff --git a/fs/Kconfig b/fs/Kconfig
index 7aee6d6..b40128b 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -58,6 +58,13 @@
depends on ZONE_DEVICE
depends on TRANSPARENT_HUGEPAGE
+# Selected by DAX drivers that do not expect filesystem DAX to support
+# get_user_pages() of DAX mappings. I.e. "limited" indicates no support
+# for fork() of processes with MAP_SHARED mappings or support for
+# direct-I/O to a DAX mapping.
+config FS_DAX_LIMITED
+ bool
+
endif # BLOCK
# Posix ACL utility routines
