Re: [PATCH 2/2] xfs: don't update mtime on COW faults

[Darrick J. Wong]

On Sat, Sep 05, 2020 at 08:13:02AM -0400, Mikulas Patocka wrote:
> 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@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> 
> ---
>  fs/xfs/xfs_file.c |   11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/fs/xfs/xfs_file.c
> ===================================================================
> --- linux-2.6.orig/fs/xfs/xfs_file.c	2020-09-05 10:01:42.000000000 +0200
> +++ linux-2.6/fs/xfs/xfs_file.c	2020-09-05 13:59:12.000000000 +0200
> @@ -1223,6 +1223,13 @@ __xfs_filemap_fault(
>  	return ret;
>  }
>  
> +static bool
> +xfs_is_write_fault(

Call this xfs_is_shared_dax_write_fault, and throw in the IS_DAX() test?

You might as well make it a static inline.

> +	struct vm_fault		*vmf)
> +{
> +	return vmf->flags & FAULT_FLAG_WRITE && vmf->vma->vm_flags & VM_SHARED;

Also, is "shortcutting the normal fault path" the reason for ext2 and
xfs both being broken?

/me puzzles over why write_fault is always true for page_mkwrite and
pfn_mkwrite, but not for fault and huge_fault...

Also: Can you please turn this (checking for timestamp update behavior
wrt shared and private mapping write faults) into an fstest so we don't
mess this up again?

--D

> +}
> +
>  static vm_fault_t
>  xfs_filemap_fault(
>  	struct vm_fault		*vmf)
> @@ -1230,7 +1237,7 @@ xfs_filemap_fault(
>  	/* 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 vm_fault_t
> @@ -1243,7 +1250,7 @@ xfs_filemap_huge_fault(
>  
>  	/* 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 vm_fault_t
>