From: Jan Kara jack@suse.cz
commit f2c57d91b0d96aa13ccff4e3b178038f17b00658 upstream.
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 Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/memory.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
--- a/mm/memory.c +++ b/mm/memory.c @@ -1787,10 +1787,15 @@ static int insert_pfn(struct vm_area_str * 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