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 change avoids journal writes for faults that are expected to COW.
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>
---
This version is simplified as suggested by Ross; all fault sizes and fallbacks
are handled by dax_iomap_fault.
fs/ext4/file.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 0d7cf0cc9b87..dc1e1fb6b54c 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -279,7 +279,20 @@ 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;
+
+ /*
+ * We have to distinguish real writes from writes which will result in a
+ * COW page; COW writes should *not* poke the journal (the file will not
+ * be changed). Doing so would cause unintended failures when mounted
+ * read-only.
+ *
+ * We check for VM_SHARED rather than vmf->cow_page since the latter is
+ * unset for pe_size != PE_SIZE_PTE (i.e. only in do_cow_fault); for
+ * other sizes, dax_iomap_fault will handle splitting / fallback so that
+ * we eventually come back with a COW page.
+ */
+ bool write = (vmf->flags & FAULT_FLAG_WRITE) &&
+ (vmf->vma->vm_flags & VM_SHARED);
if (write) {
sb_start_pagefault(sb);
--
2.14.1.342.g6490525c54-goog
