Re: O_DIRECT, ext3fs, kernel 2.4.32... again

[DervishD <lkml@dervishd.net>]

    Hi Marcelo :)

 * Marcelo Tosatti <marcelo@kvack.org> dixit:
> >     Shouldn't ext3fs return an error when the O_DIRECT flag is
> > used in the open call? Is the open call userspace only and thus
> > only libc can return such error? Am I misunderstanding the entire
> > issue and this is a perfectly legal behaviour (allowing the open,
> > failing in the read operation)?
> 
> Your interpretation is correct. It would be nicer for open() to
> fail on fs'es which don't support O_DIRECT, but v2.4 makes such
> check later at read/write unfortunately ;(

    Oops :(
 
> And its too late for changing that IMO...

    Probably. Anyway, since an userspace app shouldn't bother about
which underlying filesystem a file is under, ext3 should:

    - fail in the open() call: OK, it's too late for that.
    - don't check while in read()/write(): I'm not sure about this.

    The problem I see is that I can't tell if (given that probably
the bug cannot be fixed right now) it's better to let the userspace
app believe that O_DIRECT is honored but silently ignore it, or let
the userspace believe that O_DIRECT was honored in the open() call
and make all subsequent calls to read()/write() fail.

    Myself, I would prefer to be deceived and have successful calls
even if the O_DIRECT flag was ignored instead of having successful
calls to open(O_DIRECT) but failures on subsequent read()'s, but I
must confess that I don't know what kind of scenarios need the use of
O_DIRECT and I don't know if having O_DIRECT accepted but ignored is a
good thing :(

    I'm not familiar with the ext3 code, so I don't know if it's easy
to modify it so it will reject an open if O_DIRECT is specified :(((

    Thanks for your answer, Marcelo :)

    Raúl Núñez de Arenas Coronado

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