Re: [patch] pipe: add support for shrinking and growing pipes

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 23 May 2010 07:30:01 +0200 Michael Kerrisk <mtk.manpages@gmail.com> wrote:

> Hi all,
> 
> I see that this patch has hit Linus's git, so some questions
> 
> On Wed, May 19, 2010 at 6:49 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> >
> > On Wed, 19 May 2010, Miklos Szeredi wrote:
> >>
> >> One issue I see is that it's possible to grow pipes indefinitely.
> >> Should this be restricted to privileged users?
> >
> > Yes. But perhaps only if it grows past the default (or perhaps "default*2"
> > or similar). That way a normal user could shrink the pipe buffers, and
> > then grow them again if he wants to.
> >
> > Oh, and I think you need to also require that there be at least two
> > buffers. Otherwise we can't guarantee POSIX behavior, I think.
> 
> Is there any documentation (e.g., a man-pages patch) for these changes?
> 
> The argument of the fcntl() operations is expressed in pages. I take
> it that this means that the semantics of the argument will very
> depending on the system page size? So for example, 2 on x86 will mean
> 8192 bytes, but will mean 32768 of ia64? That seems very weird. (And
> what about architectures where the page size is switchable?) Such
> changes in semantics should not be silent for the use, IMO.

Well, there is getpagesize().  But I agree - this interface is just
asking (x86) people to write non-portable code.

otoh, if the arg was in bytes, they'd just hard-code "8192".  They're
clever like that.

But we have gone to some lengths to avoid exposing things like
PAGE_SIZE and HZ in procfs, so it makes sense to take the same approach
to syscalls.