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. This impacts
the dax drivers that do not use dev_memremap_pages(): brd, dcssblk, and
axonram.
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: Jan Kara <jack@xxxxxxx>
Cc: Jeff Moyer <jmoyer@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
Cc: Paul Mackerras <paulus@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
Cc: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
arch/powerpc/sysdev/axonram.c | 1 +
drivers/dax/super.c | 7 +++++++
drivers/s390/block/dcssblk.c | 1 +
3 files changed, 9 insertions(+)
diff --git a/arch/powerpc/sysdev/axonram.c b/arch/powerpc/sysdev/axonram.c
index c60e84e4558d..9da64d95e6f1 100644
--- a/arch/powerpc/sysdev/axonram.c
+++ b/arch/powerpc/sysdev/axonram.c
@@ -172,6 +172,7 @@ static size_t axon_ram_copy_from_iter(struct dax_device *dax_dev, pgoff_t pgoff,
static const struct dax_operations axon_ram_dax_ops = {
.direct_access = axon_ram_dax_direct_access,
+
.copy_from_iter = axon_ram_copy_from_iter,
};
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index b0cc8117eebe..26c324a5aef4 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 +124,12 @@ int __bdev_dax_supported(struct super_block *sb, int blocksize)
return len < 0 ? len : -EIO;
}
+ if (!pfn_t_devmap(pfn)) {
+ 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/dcssblk.c b/drivers/s390/block/dcssblk.c
index 7abb240847c0..e7e5db07e339 100644
--- a/drivers/s390/block/dcssblk.c
+++ b/drivers/s390/block/dcssblk.c
@@ -52,6 +52,7 @@ static size_t dcssblk_dax_copy_from_iter(struct dax_device *dax_dev,
static const struct dax_operations dcssblk_dax_ops = {
.direct_access = dcssblk_dax_direct_access,
+
.copy_from_iter = dcssblk_dax_copy_from_iter,
};
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