Re: open(O_DIRECT) on a tmpfs?

[Hugh Dickins]

On Thu, 4 Jan 2007, Michael Tokarev wrote:
> I wonder why open() with O_DIRECT (for example) bit set is
> disallowed on a tmpfs (again, for example) filesystem,
> returning EINVAL.

Because it would be (a very small amount of) work and bloat to
support O_DIRECT on tmpfs; because that work didn't seem useful;
and because the nature of tmpfs (completely in page cache) is at
odds with the nature of O_DIRECT (completely avoiding page cache),
so it would seem misleading to support it.

You have a valid view, that we should not forbid what can easily be
allowed; and a valid (experimental) use for O_DIRECT on tmpfs; and
a valid alternative perception, that the nature of tmpfs is already
direct, so O_DIRECT should be allowed as a no-op upon it.

On the other hand, I'm glad that you've found a good workaround,
using loop, and suspect that it's appropriate that you should have
to use such a workaround: if the app cares so much that it insists
on O_DIRECT succeeding (for the ordering and persistence of its
metadata), would it be right for tmpfs to deceive it?

I'm inclined to stick with the status quo;
but could be persuaded by a chorus behind you.

Hugh

p.s.  You said "O_DIRECT (for example)" - what other open
flag do you think tmpfs should support which it does not?
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