Patch "device-dax: fix pmd/pte fault fallback handling" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

device-dax: fix pmd/pte fault fallback handling

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
device-dax-fix-pmd-pte-fault-fallback-handling.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dave Jiang <[email protected]>
Date: Fri, 10 Mar 2017 13:24:22 -0700
Subject: device-dax: fix pmd/pte fault fallback handling

From: Dave Jiang <[email protected]>

commit 0134ed4fb9e78672ee9f7b18007114404c81e63f upstream.

Jeff Moyer reports:

With a device dax alignment of 4KB or 2MB, I get sigbus when running the attached fio job file for the current kernel (4.11.0-rc1+). If I specify an alignment of 1GB, it works.

I turned on debug output, and saw that it was failing in the huge fault code.

dax dax1.0: dax_open
dax dax1.0: dax_mmap
dax dax1.0: dax_dev_huge_fault: fio: write (0x7f08f0a00000 - dax dax1.0: __dax_dev_pud_fault: phys_to_pgoff(0xffffffffcf60 dax dax1.0: dax_release

fio config for reproduce:
[global]
ioengine=dev-dax
direct=0
filename=/dev/dax0.0
bs=2m

[write]
rw=write

[read]
stonewall
rw=read

The driver fails to fallback when taking a fault that is larger than the device alignment, or handling a larger fault when a smaller mapping is already established. While we could support larger mappings for a device with a smaller alignment, that change is too large for the immediate fix. The simplest change is to force fallback until the fault size matches the alignment.

---
 drivers/dax/dax.c |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

@@ -334,6 +334,7 @@ static int __dax_dev_fault(struct dax_de
 	int rc = VM_FAULT_SIGBUS;
 	phys_addr_t phys;
 	pfn_t pfn;
+	unsigned int fault_size = PAGE_SIZE;
 
 	if (check_vma(dax_dev, vma, __func__))
 		return VM_FAULT_SIGBUS;

@@ -344,6 +345,9 @@ static int __dax_dev_fault(struct dax_de
 		return VM_FAULT_SIGBUS;
 	}
 
+	if (fault_size != dax_region->align)
+		return VM_FAULT_SIGBUS;
+
 	phys = pgoff_to_phys(dax_dev, vmf->pgoff, PAGE_SIZE);
 	if (phys == -1) {
 		dev_dbg(dev, "%s: phys_to_pgoff(%#lx) failed\n", __func__,

@@ -389,6 +393,7 @@ static int __dax_dev_pmd_fault(struct da
 	phys_addr_t phys;
 	pgoff_t pgoff;
 	pfn_t pfn;
+	unsigned int fault_size = PMD_SIZE;
 
 	if (check_vma(dax_dev, vma, __func__))
 		return VM_FAULT_SIGBUS;

@@ -405,6 +410,16 @@ static int __dax_dev_pmd_fault(struct da
 		return VM_FAULT_SIGBUS;
 	}
 
+	if (fault_size < dax_region->align)
+		return VM_FAULT_SIGBUS;
+	else if (fault_size > dax_region->align)
+		return VM_FAULT_FALLBACK;
+
+	/* if we are outside of the VMA */
+	if (pmd_addr < vma->vm_start ||
+			(pmd_addr + PMD_SIZE) > vma->vm_end)
+		return VM_FAULT_SIGBUS;
+
 	pgoff = linear_page_index(vma, pmd_addr);
 	phys = pgoff_to_phys(dax_dev, pgoff, PMD_SIZE);
 	if (phys == -1) {

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/device-dax-fix-pmd-pte-fault-fallback-handling.patch