dax/fsdev: fix multi-range offset in memory_failure handler

Fix memory_failure offset calculation for multi-range devices. The old
code subtracted ranges[0].range.start from the faulting PFN's physical
address, which produces an incorrect (inflated) logical offset when the
PFN falls in ranges[1] or beyond due to physical gaps between ranges.
Add fsdev_pfn_to_offset() to walk the range list and compute the correct
device-linear byte offset relative to ranges[0].start (the device data
start) -- the base the holder (xfs, famfs) maps from -- for both static
and dynamic devices.

V5 walked the pagemap's immutable pgmap->ranges[] instead, to avoid
reading the mutable dev_dax->ranges[] from this callback. That had a
different problem: it regressed static devices, where pgmap->ranges[0].start
can sit data_offset below the data start, so the reported offset came out
data_offset too high and the holder would act on the wrong blocks. For
dynamic devices the two arrays are identical, so pgmap->ranges[] only ever
helped the dynamic case while breaking the static one. Walk
dev_dax->ranges[] instead. (Richard Cheng spotted the static regression.)

Reading dev_dax->ranges[] here may race a concurrent krealloc() of the
range array via sysfs (mapping_store(), under dax_region_rwsem, which
this ->memory_failure callback does not hold). That exposure is
pre-existing -- the original single-range code read dev_dax->ranges[0]
locklessly as well -- so this patch does not make it worse; a proper fix
(locking or snapshotting) belongs in a separate change.

Fixes: d5406bd458b0a ("dax: add fsdev.c driver for fs-dax on character dax")

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Signed-off-by: John Groves <john@groves.net>
Reviewed-by: Richard Cheng <icheng@nvidia.com>
Link: https://patch.msgid.link/0100019ecc08d74f-ec0d09b8-11e9-4e5b-af48-8c6d382af486-000000@email.amazonses.com
Signed-off-by: Alison Schofield <alison.schofield@intel.com>

0 files changed, 0 insertions, 0 deletions