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ext2: don't update mtime on COW faults

When running in a dax mode, if the user maps a page with MAP_PRIVATE and
PROT_WRITE, the ext2 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 ext2 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>

authored by

Mikulas Patocka and committed by
Linus Torvalds 1ef6ea0e c70672d8

+4 -2
+4 -2
fs/ext2/file.c
··· 93 93 struct inode *inode = file_inode(vmf->vma->vm_file); 94 94 struct ext2_inode_info *ei = EXT2_I(inode); 95 95 vm_fault_t ret; 96 + bool write = (vmf->flags & FAULT_FLAG_WRITE) && 97 + (vmf->vma->vm_flags & VM_SHARED); 96 98 97 - if (vmf->flags & FAULT_FLAG_WRITE) { 99 + if (write) { 98 100 sb_start_pagefault(inode->i_sb); 99 101 file_update_time(vmf->vma->vm_file); 100 102 } ··· 105 103 ret = dax_iomap_fault(vmf, PE_SIZE_PTE, NULL, NULL, &ext2_iomap_ops); 106 104 107 105 up_read(&ei->dax_sem); 108 - if (vmf->flags & FAULT_FLAG_WRITE) 106 + if (write) 109 107 sb_end_pagefault(inode->i_sb); 110 108 return ret; 111 109 }