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authorRandy Dodgen <dodgen@google.com>2017-08-24 22:26:01 +0300
committerTheodore Ts'o <tytso@mit.edu>2017-08-24 22:26:01 +0300
commitfd96b8da68d32a9403726db09b229f4b5ac849c7 (patch)
treed5085d7a2e0433e05456740c80ebaea83261480e /fs/ext4/file.c
parent95f1fda47c9d8738f858c3861add7bf0a36a7c0b (diff)
downloadlinux-fd96b8da68d32a9403726db09b229f4b5ac849c7.tar.xz
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>
Diffstat (limited to 'fs/ext4/file.c')
-rw-r--r--fs/ext4/file.c15
1 files changed, 14 insertions, 1 deletions
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 86ea1d92839a..197653ea6041 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);