ext4: fix fault handling when mounted with -o dax,ro

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@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 86ea1d92839aac834e51b2e29d943c6f6abc2867..197653ea60417aebac8284e69060a6e3e072ca69 100644 (file)
--- 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);