Commit f2c57d91 authored Oct 30, 2018 by Jan Kara Committed by Linus Torvalds Oct 31, 2018

mm: Fix warning in insert_pfn()

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.

Link: http://lkml.kernel.org/r/20180824154542.26872-1-jack@suse.cz


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent dee6da22

mm/memory.c

+7 −2

Original line number	Diff line number	Diff line
		@@ -1537,10 +1537,15 @@ static vm_fault_t 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;
		goto out_mkwrite;
		} else

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