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