From: Miklos Szeredi page_mkclean() doesn't re-protect ptes for non-linear mappings, so a later re-dirty through such a mapping will not generate a fault, PG_dirty will not reflect the dirty state and the dirty count will be skewed. This implies that msync() is also currently broken for nonlinear mappings. The easiest solution is to emulate remap_file_pages on non-linear mappings with simple mmap() for non ram-backed filesystems. Applications continue to work (albeit slower), as long as the number of remappings remain below the maximum vma count. However all currently known real uses of non-linear mappings are for ram backed filesystems, which this patch doesn't affect. Signed-off-by: Miklos Szeredi Acked-by: Peter Zijlstra Cc: William Lee Irwin III Cc: Nick Piggin Cc: Ingo Molnar Signed-off-by: Andrew Morton --- mm/fremap.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff -puN mm/fremap.c~only-allow-nonlinear-vmas-for-ram-backed-filesystems mm/fremap.c --- a/mm/fremap.c~only-allow-nonlinear-vmas-for-ram-backed-filesystems +++ a/mm/fremap.c @@ -181,6 +181,24 @@ asmlinkage long sys_remap_file_pages(uns goto retry; } mapping = vma->vm_file->f_mapping; + /* + * page_mkclean doesn't work on nonlinear vmas, so if dirty + * pages need to be accounted, emulate with linear vmas. + */ + if (mapping_cap_account_dirty(mapping)) { + unsigned long addr; + + flags &= MAP_NONBLOCK; + addr = mmap_region(vma->vm_file, start, size, flags, + vma->vm_flags, pgoff, 1); + if (IS_ERR_VALUE(addr)) + err = addr; + else { + BUG_ON(addr != start); + err = 0; + } + goto out; + } spin_lock(&mapping->i_mmap_lock); flush_dcache_mmap_lock(mapping); vma->vm_flags |= VM_NONLINEAR; _