Re: another nfs puzzle

[Peter Staubach <staubach@redhat.com>]

Kenny Simpson wrote:

>Hi again,
>  I am seeing some odd behavior with O_DIRECT.  If a file opened with O_DIRECT has a page mmap'd,
>and the file is extended via pwrite, then the mmap'd region seems to get lost - i.e. it neither
>takes up system memory, nor does it get written out.
>  
>

I don't think that I understand why or how the kernel allows a file,
which was opened with O_DIRECT, to be mmap'd.  The use of O_DIRECT
implies no caching and mmap implies the use of caching.

I do understand that the kernel can flush and invalidate pages in
the ranges of the i/o operations done, but does it really guarantee
that a pagein operation won't happen on a page within the range of
a region of the file being written to using direct i/o?  If it does
not, then any consistency guarantees are gone and data corruption
can occur.

    Thanx...

       ps