From b17164e258e3888d376a7434415013175d637377 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@redhat.com>
Date: Sat, 5 Sep 2020 08:13:02 -0400
Subject: [PATCH] xfs: don't update mtime on COW faults
Git-commit: b17164e258e3888d376a7434415013175d637377
Patch-mainline: v5.9-rc4
References: bsc#1167030

When running in a dax mode, if the user maps a page with MAP_PRIVATE and
PROT_WRITE, the xfs filesystem would incorrectly update ctime and mtime
when the user hits a COW fault.

This breaks building of the Linux kernel.  How to reproduce:

 1. extract the Linux kernel tree on dax-mounted xfs filesystem
 2. run make clean
 3. run make -j12
 4. run make -j12

at step 4, make would incorrectly rebuild the whole kernel (although it
was already built in step 3).

The reason for the breakage is that almost all object files depend on
objtool.  When we run objtool, it takes COW page fault on its .data
section, and these faults will incorrectly update the timestamp of the
objtool binary.  The updated timestamp causes make to rebuild the whole
tree.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Anthony Iliopoulos <ailiop@suse.com>

---
 fs/xfs/xfs_file.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1109,6 +1109,14 @@
 	return ret;
 }
 
+static inline bool
+xfs_is_write_fault(
+	struct vm_fault		*vmf)
+{
+	return (vmf->flags & FAULT_FLAG_WRITE) &&
+	       (vmf->vma->vm_flags & VM_SHARED);
+}
+
 static int
 xfs_filemap_fault(
 	struct vm_fault		*vmf)
@@ -1116,7 +1124,7 @@
 	/* DAX can shortcut the normal fault path on write faults! */
 	return __xfs_filemap_fault(vmf, PE_SIZE_PTE,
 			IS_DAX(file_inode(vmf->vma->vm_file)) &&
-			(vmf->flags & FAULT_FLAG_WRITE));
+			xfs_is_write_fault(vmf));
 }
 
 static int
@@ -1129,7 +1137,7 @@
 
 	/* DAX can shortcut the normal fault path on write faults! */
 	return __xfs_filemap_fault(vmf, pe_size,
-			(vmf->flags & FAULT_FLAG_WRITE));
+			xfs_is_write_fault(vmf));
 }
 
 static int