path: root/Documentation
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authorLinus Torvalds <>2016-05-26 19:34:26 -0700
committerLinus Torvalds <>2016-05-26 19:34:26 -0700
commit315227f6da389f3a560f27f7777080857278e1b4 (patch)
tree11306e1e8d8b66044ab48901b90141b5362c12e3 /Documentation
parenta10c38a4f385f5d7c173a263ff6bb2d36021b3bb (diff)
parent40543f62cbdce42633e3fe10923099feee272e1f (diff)
Merge tag 'dax-misc-for-4.7' of git://
Pull misc DAX updates from Vishal Verma: "DAX error handling for 4.7 - Until now, dax has been disabled if media errors were found on any device. This enables the use of DAX in the presence of these errors by making all sector-aligned zeroing go through the driver. - The driver (already) has the ability to clear errors on writes that are sent through the block layer using 'DSMs' defined in ACPI 6.1. Other misc changes: - When mounting DAX filesystems, check to make sure the partition is page aligned. This is a requirement for DAX, and previously, we allowed such unaligned mounts to succeed, but subsequent reads/writes would fail. - Misc/cleanup fixes from Jan that remove unused code from DAX related to zeroing, writeback, and some size checks" * tag 'dax-misc-for-4.7' of git:// dax: fix a comment in dax_zero_page_range and dax_truncate_page dax: for truncate/hole-punch, do zeroing through the driver if possible dax: export a low-level __dax_zero_page_range helper dax: use sb_issue_zerout instead of calling dax_clear_sectors dax: enable dax in the presence of known media errors (badblocks) dax: fallback from pmd to pte on error block: Update blkdev_dax_capable() for consistency xfs: Add alignment check for DAX mount ext2: Add alignment check for DAX mount ext4: Add alignment check for DAX mount block: Add bdev_dax_supported() for dax mount checks block: Add vfs_msg() interface dax: Remove redundant inode size checks dax: Remove pointless writeback from dax_do_io() dax: Remove zeroing from dax_io() dax: Remove dead zeroing code from fault handlers ext2: Avoid DAX zeroing to corrupt data ext2: Fix block zeroing in ext2_get_blocks() for DAX dax: Remove complete_unwritten argument DAX: move RADIX_DAX_ definitions to dax.c
Diffstat (limited to 'Documentation')
1 files changed, 32 insertions, 0 deletions
diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 7bde64014a89..ce4587d257d2 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -79,6 +79,38 @@ These filesystems may be used for inspiration:
- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt
+Handling Media Errors
+The libnvdimm subsystem stores a record of known media error locations for
+each pmem block device (in gendisk->badblocks). If we fault at such location,
+or one with a latent error not yet discovered, the application can expect
+to receive a SIGBUS. Libnvdimm also allows clearing of these errors by simply
+writing the affected sectors (through the pmem driver, and if the underlying
+NVDIMM supports the clear_poison DSM defined by ACPI).
+Since DAX IO normally doesn't go through the driver/bio path, applications or
+sysadmins have an option to restore the lost data from a prior backup/inbuilt
+redundancy in the following ways:
+1. Delete the affected file, and restore from a backup (sysadmin route):
+ This will free the file system blocks that were being used by the file,
+ and the next time they're allocated, they will be zeroed first, which
+ happens through the driver, and will clear bad sectors.
+2. Truncate or hole-punch the part of the file that has a bad-block (at least
+ an entire aligned sector has to be hole-punched, but not necessarily an
+ entire filesystem block).
+These are the two basic paths that allow DAX filesystems to continue operating
+in the presence of media errors. More robust error recovery mechanisms can be
+built on top of this in the future, for example, involving redundancy/mirroring
+provided at the block layer through DM, or additionally, at the filesystem
+level. These would have to rely on the above two tenets, that error clearing
+can happen either by sending an IO through the driver, or zeroing (also through
+the driver).