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Date: Mon, 7 Sep 2020 11:00:37 +0200
From: Jan Kara <jack@suse.cz>
To: Mikulas Patocka <mpatocka@redhat.com>
Subject: Re: [PATCH 1/2] ext2: don't update mtime on COW faults
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On Sat 05-09-20 08:12:01, Mikulas Patocka wrote:
> 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

Thanks. Good spotting! Linus has already merged this so nothing more to do
here.

								Honza

> 
> ---
>  fs/ext2/file.c |    6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/fs/ext2/file.c
> ===================================================================
> --- linux-2.6.orig/fs/ext2/file.c	2020-09-05 10:01:41.000000000 +0200
> +++ linux-2.6/fs/ext2/file.c	2020-09-05 13:09:50.000000000 +0200
> @@ -93,8 +93,10 @@ static vm_fault_t ext2_dax_fault(struct
>  	struct inode *inode = file_inode(vmf->vma->vm_file);
>  	struct ext2_inode_info *ei = EXT2_I(inode);
>  	vm_fault_t ret;
> +	bool write = (vmf->flags & FAULT_FLAG_WRITE) &&
> +		(vmf->vma->vm_flags & VM_SHARED);
>  
> -	if (vmf->flags & FAULT_FLAG_WRITE) {
> +	if (write) {
>  		sb_start_pagefault(inode->i_sb);
>  		file_update_time(vmf->vma->vm_file);
>  	}
> @@ -103,7 +105,7 @@ static vm_fault_t ext2_dax_fault(struct
>  	ret = dax_iomap_fault(vmf, PE_SIZE_PTE, NULL, NULL, &ext2_iomap_ops);
>  
>  	up_read(&ei->dax_sem);
> -	if (vmf->flags & FAULT_FLAG_WRITE)
> +	if (write)
>  		sb_end_pagefault(inode->i_sb);
>  	return ret;
>  }
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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