Re: [PATCH] 2.6.10 - direct-io async short read bug

From: Joel Becker
Date: Tue Mar 08 2005 - 23:19:34 EST

On Tue, Mar 08, 2005 at 03:54:04PM -0800, Badari Pulavarty wrote:
> > 1. return EINVAL if the DIO goes past EOF.
> >
> > 2. truncate the request to file size (which is what your patch does)
> > and if it works, it works.
> >
> > 3. truncate the request to a size that actually works - like a multiple
> > of 512.
> >
> > 4. Do the full i/o since the user buffer is big enough, truncate the
> > result returned to file size (and clear out the user buffer where it
> > read past EOF).
> >
> > Number 4 would make it easy on the user-level code, but AIO DIO might be
> > a bit tricky and might be a security hole since the data would be dma'ed
> > there and then cleared. I need to look at the code some more.

Solaris, which does forcedirectio as a mount option, actually
will do buffered I/O on the trailing part. Consider it like a bounce
buffer. That way they don't DMA the trailing data and succeed the I/O.
The I/O returns actual bytes till EOF, just like read(2) is supposed to.
Either this or a fully DMA'd number 4 is really what we should
do. If security can only be solved via a bounce buffer, who cares? If
the user created themselves a non-aligned file to open O_DIRECT, that's
their problem if the last part-sector is negligably slower.



Life's Little Instruction Book #3

"Watch a sunrise at least once a year."

Joel Becker
Senior Member of Technical Staff
E-mail: joel.becker@xxxxxxxxxx
Phone: (650) 506-8127
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