Re: [PATCH v4 3/3] fs, xfs: introduce MAP_DIRECT for creating block-map-sealed file ranges

[Kirill A. Shutemov]

On Mon, Aug 14, 2017 at 11:12:22PM -0700, Dan Williams wrote:
> MAP_DIRECT is an mmap(2) flag with the following semantics:
> 
>   MAP_DIRECT
>   In addition to this mapping having MAP_SHARED semantics, successful
>   faults in this range may assume that the block map (logical-file-offset
>   to physical memory address) is pinned for the lifetime of the mapping.
>   Successful MAP_DIRECT faults establish mappings that bypass any kernel
>   indirections like the page-cache. All updates are carried directly
>   through to the underlying file physical blocks (modulo cpu cache
>   effects).
> 
>   ETXTBSY is returned on attempts to change the block map (allocate blocks
>   / convert unwritten extents / break shared extents) in the mapped range.
>   Some filesystems may extend these same restrictions outside the mapped
>   range and return ETXTBSY to any file operations that might mutate the
>   block map. MAP_DIRECT faults may fail with a SIGSEGV if the filesystem
>   needs to write the block map to satisfy the fault. For example, if the
>   mapping was established over a hole in a sparse file.

We had issues before with user-imposed ETXTBSY. See MAP_DENYWRITE.

Are we sure it won't a source of denial-of-service attacks?

>   The kernel ignores attempts to mark a MAP_DIRECT mapping MAP_PRIVATE and
>   will silently fall back to MAP_SHARED semantics.

Hm.. Any reason for this strage behaviour? Looks just broken to me.

> 
>   ERRORS
>   EACCES A MAP_DIRECT mapping was requested and PROT_WRITE was not set.
> 
>   EINVAL MAP_ANONYMOUS was specified with MAP_DIRECT.
> 
>   EOPNOTSUPP The filesystem explicitly does not support the flag
> 
>   SIGSEGV Attempted to write a MAP_DIRECT mapping at a file offset that
>           might require block-map updates.

I think it should be SIGBUS.

-- 
 Kirill A. Shutemov