Re: O_DIRECT fails in some kernel and FS

[Chris Wedgwood]

On Sun, Feb 03, 2002 at 09:05:04AM -0600, Stephen Lord wrote:

    I agree is is not a big issue in this case - my interpretation of
    tails was the end of any file could be packed, but if it is only
    small files.....

But you can't mmap (say) a 1k file right now...  so right now this
isn't a problem, but at some point a larger mmap granularity would be
nice --- especially on architectures with small (or untagged) TLBs.
I'm guessing so as not to break backwards compatibility we will have
to support variable page-sizes (creating a plethora of nasties I
imagine).

    They are invalidated at the start of the I/O

Cool.  That much I'd like to see under Linux

    but page faults are not blocked out for the duration of the I/O so
    the coherency is weak.

I was thinking this would also be goof, basically invalidate those
pages and remove them from the VMAs, marking them as unusable pending
IO completion --- the logic her being if you were to fault on an
invalidated page during IO you deserve to block indefinitely until the
IO completes.

    However, if an application is doing a combination of mmapped and
    direct I/O to a file at the same time, then it should generally
    have some form of user space synchronization anyway.

I hadn't considered that.  I imagined an application doing either but
not both, and the kernel enforcing this.  However, in the case when
you want to mmap a large file, you may want to manipulate some pages
using mmap whilst writing others with O_DIRECT.  Although, in such
cases arguably you could using multiple mapping's.

   --cw