This commit, which has not yet made it upstream but is in the -mm tree:
dax: Fix race between colliding PMD & PTE entries
fixed a pair of race conditions where racing DAX PTE and PMD faults could
corrupt page tables. This fix had two shortcomings which are addressed by
this patch:
1) In the PTE fault handler we only checked for a collision using
pmd_devmap(). The pmd_devmap() check will trigger when we have raced with
a PMD that has real DAX storage, but to account for the case where we
collide with a huge zero page entry we also need to check for
pmd_trans_huge().
2) In the PMD fault handler we only continued with the fault if no PMD at
all was present (pmd_none()). This is the case when we are faulting in a
PMD for the first time, but there are two other cases to consider. The
first is that we are servicing a write fault over a PMD huge zero page,
which we detect with pmd_trans_huge(). The second is that we are servicing
a write fault over a DAX PMD with real storage, which we address with
pmd_devmap().
Fix both of these, and instead of manually triggering a fallback in the PMD
collision case instead be consistent with the other collision detection
code in the fault handlers and just retry.
Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
For both the -mm tree and for stable, feel free to squash this with the
original commit if you think that is appropriate.
This has passed targeted testing and an xfstests run.
---
fs/dax.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index fc62f36..2a6889b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1160,7 +1160,7 @@ static int dax_iomap_pte_fault(struct vm_fault *vmf,
* the PTE we need to set up. If so just return and the fault will be
* retried.
*/
- if (pmd_devmap(*vmf->pmd)) {
+ if (pmd_trans_huge(*vmf->pmd) || pmd_devmap(*vmf->pmd)) {
vmf_ret = VM_FAULT_NOPAGE;
goto unlock_entry;
}
@@ -1411,11 +1411,14 @@ static int dax_iomap_pmd_fault(struct vm_fault *vmf,
/*
* It is possible, particularly with mixed reads & writes to private
* mappings, that we have raced with a PTE fault that overlaps with
- * the PMD we need to set up. If so we just fall back to a PTE fault
- * ourselves.
+ * the PMD we need to set up. If so just return and the fault will be
+ * retried.
*/
- if (!pmd_none(*vmf->pmd))
+ if (!pmd_none(*vmf->pmd) && !pmd_trans_huge(*vmf->pmd) &&
+ !pmd_devmap(*vmf->pmd)) {
+ result = 0;
goto unlock_entry;
+ }
/*
* Note that we don't use iomap_apply here. We aren't doing I/O, only
--
2.9.4
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