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

[Jens Axboe]

On Sun, May 23 2010, Michael Kerrisk wrote:
> On Sun, May 23, 2010 at 4:38 AM, Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Sun, 23 May 2010 07:30:01 +0200 Michael Kerrisk <mtk.manpages@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> 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.
> 
> Quite. All of the other memory-related APIs that I can think of
> require the user to express the info in bytes. (mlock(),
> remap_file_pages(), mmap(), mremap(), mprotect(), shmget(), and so
> on). Not doing the same for this interface is needlessly inconsistent.
> And while there will be the silly users you mention above, smart users
> will know how to do the right thing with a consistently designed
> interface.

We can easily make F_GETPIPE_SZ return bytes, but I don't think passing
in bytes to F_SETPIPE_SZ makes a lot of sense. The pipe array must be a
power of 2 in pages. So the question is if that makes the API cleaner,
passing in number of pages but returning bytes? Or pass in bytes all
around, but have F_SETPIPE_SZ round to the nearest multiple of pow2 in
pages if need be. Then it would return a size at least what was passed
in, or error.

-- 
Jens Axboe

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