On Fri, 24 Aug 2018 17:45:42 +0200 Jan Kara <jack@xxxxxxx> 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?
