[PATCH v3] dax: enable dax fault handler to report VM_FAULT_HWPOISON

[Jane Chu]

When multiple processes mmap() a dax file, then at some point,
a process issues a 'load' and consumes a hwpoison, the process
receives a SIGBUS with si_code = BUS_MCEERR_AR and with si_lsb
set for the poison scope. Soon after, any other process issues
a 'load' to the poisoned page (that is unmapped from the kernel
side by memory_failure), it receives a SIGBUS with
si_code = BUS_ADRERR and without valid si_lsb.

This is confusing to user, and is different from page fault due
to poison in RAM memory, also some helpful information is lost.

Channel dax backend driver's poison detection to the filesystem
such that instead of reporting VM_FAULT_SIGBUS, it could report
VM_FAULT_HWPOISON.

Change from v2:
  Convert -EHWPOISON to -EIO to prevent EHWPOISON errno from leaking
out to block read(2) - suggested by Matthew.

Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
---
 drivers/nvdimm/pmem.c | 2 +-
 fs/dax.c              | 4 ++--
 include/linux/mm.h    | 2 ++
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
index ceea55f621cc..46e094e56159 100644
--- a/drivers/nvdimm/pmem.c
+++ b/drivers/nvdimm/pmem.c
@@ -260,7 +260,7 @@ __weak long __pmem_direct_access(struct pmem_device *pmem, pgoff_t pgoff,
 		long actual_nr;
 
 		if (mode != DAX_RECOVERY_WRITE)
-			return -EIO;
+			return -EHWPOISON;
 
 		/*
 		 * Set the recovery stride is set to kernel page size because
diff --git a/fs/dax.c b/fs/dax.c
index 2ababb89918d..18f9598951f1 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1498,7 +1498,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
 
 		map_len = dax_direct_access(dax_dev, pgoff, PHYS_PFN(size),
 				DAX_ACCESS, &kaddr, NULL);
-		if (map_len == -EIO && iov_iter_rw(iter) == WRITE) {
+		if (map_len == -EHWPOISON && iov_iter_rw(iter) == WRITE) {
 			map_len = dax_direct_access(dax_dev, pgoff,
 					PHYS_PFN(size), DAX_RECOVERY_WRITE,
 					&kaddr, NULL);
@@ -1506,7 +1506,7 @@ static loff_t dax_iomap_iter(const struct iomap_iter *iomi,
 				recovery = true;
 		}
 		if (map_len < 0) {
-			ret = map_len;
+			ret = (map_len == -EHWPOISON) ? -EIO : map_len;
 			break;
 		}
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 1f79667824eb..e4c974587659 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3217,6 +3217,8 @@ static inline vm_fault_t vmf_error(int err)
 {
 	if (err == -ENOMEM)
 		return VM_FAULT_OOM;
+	else if (err == -EHWPOISON)
+		return VM_FAULT_HWPOISON;
 	return VM_FAULT_SIGBUS;
 }
 
-- 
2.18.4