[PATCH v4 8/7] Documentation: add error handling information to dax.txt

[Vishal Verma]

This just provides information of the basic paths that can be used to
deal with (i.e. clear) media errors from the file system point-of-view.

Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Signed-off-by: Vishal Verma <vishal.l.verma@xxxxxxxxx>
---

While this isn't a design document for new mechanisms for adding
error recovery/redundancy at the block/fs layers, this attempts to
explain the bare essentials required for anything operating above
the pmem block driver in the stack.

 Documentation/filesystems/dax.txt | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/Documentation/filesystems/dax.txt b/Documentation/filesystems/dax.txt
index 7bde640..71cd8fa 100644
--- a/Documentation/filesystems/dax.txt
+++ b/Documentation/filesystems/dax.txt
@@ -79,6 +79,40 @@ 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 two 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. Open the file with O_DIRECT, and restore a sector's worth of data at the
+   bad location (application route):
+   We allow O_DIRECT writes to go through the normal O_DIRECT path that sends
+   bios down through the driver. If an application is able to restore its own
+   data, it can use this path to clear errors.
+
+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).
+
+
 Shortcomings
 ------------
 
-- 
2.5.5