Re: [PATCH] mm: Fix warning in insert_pfn()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 24 Aug 2018 17:45:42 +0200 Jan Kara <jack@suse.cz> wrote:

> In DAX mode a write pagefault can race with write(2) in the following
> way:
> 
> CPU0                            CPU1
>                                 write fault for mapped zero page (hole)
> dax_iomap_rw()
>   iomap_apply()
>     xfs_file_iomap_begin()
>       - allocates blocks
>     dax_iomap_actor()
>       invalidate_inode_pages2_range()
>         - invalidates radix tree entries in given range
>                                 dax_iomap_pte_fault()
>                                   grab_mapping_entry()
>                                     - no entry found, creates empty
>                                   ...
>                                   xfs_file_iomap_begin()
>                                     - finds already allocated block
>                                   ...
>                                   vmf_insert_mixed_mkwrite()
>                                     - WARNs and does nothing because there
>                                       is still zero page mapped in PTE
>         unmap_mapping_pages()
> 
> This race results in WARN_ON from insert_pfn() and is occasionally
> triggered by fstest generic/344. Note that the race is otherwise
> harmless as before write(2) on CPU0 is finished, we will invalidate page
> tables properly and thus user of mmap will see modified data from
> write(2) from that point on. So just restrict the warning only to the
> case when the PFN in PTE is not zero page.
> 
> ...
>
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1787,10 +1787,15 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  			 * in may not match the PFN we have mapped if the
>  			 * mapped PFN is a writeable COW page.  In the mkwrite
>  			 * case we are creating a writable PTE for a shared
> -			 * mapping and we expect the PFNs to match.
> +			 * mapping and we expect the PFNs to match. If they
> +			 * don't match, we are likely racing with block
> +			 * allocation and mapping invalidation so just skip the
> +			 * update.
>  			 */
> -			if (WARN_ON_ONCE(pte_pfn(*pte) != pfn_t_to_pfn(pfn)))
> +			if (pte_pfn(*pte) != pfn_t_to_pfn(pfn)) {
> +				WARN_ON_ONCE(!is_zero_pfn(pte_pfn(*pte)));
>  				goto out_unlock;
> +			}
>  			entry = *pte;

Shouldn't we just remove the warning?  We know it happens and we know
why it happens and we know it's harmless.  What's the point in scaring
people?