dax, iomap: Add support for synchronous faults

Add a flag to iomap interface informing the caller that inode needs
fdstasync(2) for returned extent to become persistent and use it in DAX
fault code so that we don't map such extents into page tables
immediately. Instead we propagate the information that fdatasync(2) is
necessary from dax_iomap_fault() with a new VM_FAULT_NEEDDSYNC flag.
Filesystem fault handler is then responsible for calling fdatasync(2)
and inserting pfn into page tables.

Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5411cb7442de..f57e55782d7d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1182,6 +1182,9 @@ static inline void clear_page_pfmemalloc(struct page *page)
 #define VM_FAULT_RETRY  0x0400  /* ->fault blocked, must retry */
 #define VM_FAULT_FALLBACK 0x0800        /* huge page fault failed, fall back to small */
 #define VM_FAULT_DONE_COW   0x1000      /* ->fault has fully handled COW */
+#define VM_FAULT_NEEDDSYNC  0x2000      /* ->fault did not modify page tables
+                                         * and needs fsync() to complete (for
+                                         * synchronous page faults in DAX) */
 
 #define VM_FAULT_ERROR  (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | \
                          VM_FAULT_HWPOISON | VM_FAULT_HWPOISON_LARGE | \
@@ -1199,7 +1202,8 @@ static inline void clear_page_pfmemalloc(struct page *page)
         { VM_FAULT_LOCKED,              "LOCKED" }, \
         { VM_FAULT_RETRY,               "RETRY" }, \
         { VM_FAULT_FALLBACK,            "FALLBACK" }, \
-        { VM_FAULT_DONE_COW,            "DONE_COW" }
+        { VM_FAULT_DONE_COW,            "DONE_COW" }, \
+        { VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" }
 
 /* Encode hstate index for a hwpoisoned large page */
 #define VM_FAULT_SET_HINDEX(x) ((x) << 12)