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: Jan Kara <[email protected]> Cc: Jeff Moyer <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Ross Zwisler <[email protected]> Cc: Benjamin Herrenschmidt <[email protected]> Cc: Paul Mackerras <[email protected]> Cc: Michael Ellerman <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Heiko Carstens <[email protected]> Cc: Gerald Schaefer <[email protected]> Signed-off-by: Dan Williams <[email protected]> --- arch/powerpc/platforms/Kconfig | 1 + arch/powerpc/sysdev/axonram.c | 1 + drivers/dax/super.c | 10 ++++++++++ drivers/s390/block/Kconfig | 1 + drivers/s390/block/dcssblk.c | 1 + fs/Kconfig | 7 +++++++ 6 files changed, 21 insertions(+) diff --git a/arch/powerpc/platforms/Kconfig b/arch/powerpc/platforms/Kconfig index 4fd64d3f5c44..031313968f9a 100644 --- a/arch/powerpc/platforms/Kconfig +++ b/arch/powerpc/platforms/Kconfig @@ -296,6 +296,7 @@ config AXON_RAM 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/arch/powerpc/sysdev/axonram.c b/arch/powerpc/sysdev/axonram.c index aaf540efb92c..c1abd443836f 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..66bcdf42c413 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 31f014b57bfc..594ae5fc8e9d 100644 --- a/drivers/s390/block/Kconfig +++ b/drivers/s390/block/Kconfig @@ -15,6 +15,7 @@ config BLK_DEV_XPRAM config DCSSBLK def_tristate m select DAX + select FS_DAX_LIMITED prompt "DCSSBLK support" depends on S390 && BLOCK help diff --git a/drivers/s390/block/dcssblk.c b/drivers/s390/block/dcssblk.c index 87756e28c29b..dbe07ab71e32 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, }; diff --git a/fs/Kconfig b/fs/Kconfig index 7aee6d699fd6..b40128bf6d1a 100644 --- a/fs/Kconfig +++ b/fs/Kconfig @@ -58,6 +58,13 @@ config FS_DAX_PMD 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
