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Subject[PATCH RFC 05/10] device-dax: Do not enforce MADV_DONTFORK on mmap()
Date
Currently check_vma() checks for VM_DONTCOPY for a device-dax
when the dax region is not backed by devmap (i.e. PFN_MAP is not set).
VM_DONTCOPY is set through madvise(MADV_DONTFORK) and it only sets it
at an address returned from mmap(). check_vma() is called at devdax
mmap hence checking VM_DONTCOPY prevents a process from mmap-ing the
device.

Let's not enforce MADV_DONTFORK at mmap(), but rather when it actually
gets used (on fault). The assumptions don't change, as it is
expected to still retain/madvise MADV_DONTFORK after mmap.

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
---
drivers/dax/device.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/drivers/dax/device.c b/drivers/dax/device.c
index 1af823b2fe6b..c6a7f5e12c54 100644
--- a/drivers/dax/device.c
+++ b/drivers/dax/device.c
@@ -14,7 +14,7 @@
#include "dax-private.h"
#include "bus.h"

-static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
+static int check_vma_mmap(struct dev_dax *dev_dax, struct vm_area_struct *vma,
const char *func)
{
struct dax_region *dax_region = dev_dax->region;
@@ -41,17 +41,29 @@ static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
return -EINVAL;
}

- if ((dax_region->pfn_flags & (PFN_DEV|PFN_MAP)) == PFN_DEV
- && (vma->vm_flags & VM_DONTCOPY) == 0) {
+ if (!vma_is_dax(vma)) {
dev_info_ratelimited(dev,
- "%s: %s: fail, dax range requires MADV_DONTFORK\n",
+ "%s: %s: fail, vma is not DAX capable\n",
current->comm, func);
return -EINVAL;
}

- if (!vma_is_dax(vma)) {
- dev_info_ratelimited(dev,
- "%s: %s: fail, vma is not DAX capable\n",
+ return 0;
+}
+
+static int check_vma(struct dev_dax *dev_dax, struct vm_area_struct *vma,
+ const char *func)
+{
+ int rc;
+
+ rc = check_vma_mmap(dev_dax, vma, func);
+ if (rc < 0)
+ return rc;
+
+ if ((dev_dax->region->pfn_flags & (PFN_DEV|PFN_MAP)) == PFN_DEV
+ && (vma->vm_flags & VM_DONTCOPY) == 0) {
+ dev_info_ratelimited(&dev_dax->dev,
+ "%s: %s: fail, dax range requires MADV_DONTFORK\n",
current->comm, func);
return -EINVAL;
}
@@ -315,7 +327,7 @@ static int dax_mmap(struct file *filp, struct vm_area_struct *vma)
* fault time.
*/
id = dax_read_lock();
- rc = check_vma(dev_dax, vma, __func__);
+ rc = check_vma_mmap(dev_dax, vma, __func__);
dax_read_unlock(id);
if (rc)
return rc;
--
2.17.1
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