Re: another nfs puzzle

[Peter Staubach]

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
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