From: Randy Dodgen <dodgen@xxxxxxxxxx>
If an ext4 filesystem is mounted with both the DAX and read-only
options, executables on that filesystem will fail to start (claiming
'Segmentation fault') due to the fault handler returning
VM_FAULT_SIGBUS.
This is due to the DAX fault handler (see ext4_dax_huge_fault)
attempting to write to the journal when FAULT_FLAG_WRITE is set. This is
the wrong behavior for write faults which will lead to a COW page; in
particular, this fails for readonly mounts.
This changes replicates some check from dax_iomap_fault to more
precisely reason about when a journal-write is needed.
It might be the case that this could be better handled in
ext4_iomap_begin / ext4_iomap_end (called via iomap_ops inside
dax_iomap_fault). These is some overlap already (e.g. grabbing journal
handles).
Signed-off-by: Randy Dodgen <dodgen@xxxxxxxxxx>
---
I'm resending for some DMARC-proofing (thanks Ted for the explanation), a
missing Signed-off-by, and some extra cc's. Oops!
fs/ext4/file.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0d7cf0cc9b87..d512fb85a3e3 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -279,7 +279,31 @@ static int ext4_dax_huge_fault(struct vm_fault *vmf,
handle_t *handle = NULL;
struct inode *inode = file_inode(vmf->vma->vm_file);
struct super_block *sb = inode->i_sb;
- bool write = vmf->flags & FAULT_FLAG_WRITE;
+ bool write;
+
+ /*
+ * We have to distinguish real writes from writes which will result in a
+ * COW page
+ * - COW writes need to fall-back to installing PTEs. See
+ * dax_iomap_pmd_fault.
+ * - COW writes should *not* poke the journal (the file will not be
+ * changed). Doing so would cause unintended failures when mounted
+ * read-only.
+ */
+ if (pe_size == PE_SIZE_PTE) {
+ /* See dax_iomap_pte_fault. */
+ write = (vmf->flags & FAULT_FLAG_WRITE) && !vmf->cow_page;
+ } else if (pe_size == PE_SIZE_PMD) {
+ /* See dax_iomap_pmd_fault. */
+ write = vmf->flags & FAULT_FLAG_WRITE;
+ if (write && !(vmf->vma->vm_flags & VM_SHARED)) {
+ split_huge_pmd(vmf->vma, vmf->pmd, vmf->address);
+ count_vm_event(THP_FAULT_FALLBACK);
+ return VM_FAULT_FALLBACK;
+ }
+ } else {
+ return VM_FAULT_FALLBACK;
+ }
if (write) {
sb_start_pagefault(sb);
--
2.14.1.480.gb18f417b89-goog
