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

[Michael Kerrisk <mtk.manpages@googlemail.com>]

Hi Jens,

On Thu, Jun 3, 2010 at 9:48 AM, Michael Kerrisk
<mtk.manpages@googlemail.com> wrote:
> On Thu, Jun 3, 2010 at 9:05 AM, Michael Kerrisk
> <mtk.manpages@googlemail.com> wrote:
>> On Thu, Jun 3, 2010 at 9:01 AM, Jens Axboe <jaxboe@fusionio.com> wrote:
>>> On Thu, Jun 03 2010, Michael Kerrisk wrote:
>>>> Hi Jens,
>>>>
>>>> On Thu, Jun 3, 2010 at 8:10 AM, Jens Axboe <jaxboe@fusionio.com> wrote:
>>>> > On Wed, Jun 02 2010, Michael Kerrisk wrote:
>>>> >> On Tue, Jun 1, 2010 at 9:45 AM, Jens Axboe <axboe@kernel.dk> wrote:
>>>> >> > On Thu, May 27 2010, Michael Kerrisk wrote:
>>>> >> >> Jens,
>>>> >> >>
>>>> >> >> On Mon, May 24, 2010 at 7:56 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
>>>> >> >> > On Mon, May 24 2010, Michael Kerrisk wrote:
>>>> >> >> >> On Mon, May 24, 2010 at 7:35 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
>>>> >> >> >> > On Mon, May 24 2010, Michael Kerrisk wrote:
>>>> >> >> >> >> > Right, that looks like a thinko.
>>>> >> >> >> >> >
>>>> >> >> >> >> > I'll submit a patch changing it to bytes and the agreed API and fix this
>>>> >> >> >> >> > -Eerror. Thanks for your comments and suggestions!
>>>> >> >> >> >>
>>>> >> >> >> >> Thanks. And of course you are welcome. (Please CC linux-api@vger on
>>>> >> >> >> >> this patche (and all patches that change the API/ABI.)
>>>> >> >> >> >
>>>> >> >> >> > The first change is this:
>>>> >> >> >> >
>>>> >> >> >> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=0191f8697bbdfefcd36e7b8dc3eeddfe82893e4b
>>>> >> >> >> >
>>>> >> >> >> > and the one dealing with the pages vs bytes API is this:
>>>> >> >> >> >
>>>> >> >> >> > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=b9598db3401282bb27b4aef77e3eee12015f7f29
>>>> >> >> >> >
>>>> >> >> >> > Not tested yet, will do so before sending in of course.
>>>> >> >> >>
>>>> >> >> >> Eyeballing it quickly, these changes look right.
>>>> >> >> >
>>>> >> >> > Good, thanks.
>>>> >> >> >
>>>> >> >> >> Do you have some test programs you can make available?
>>>> >> >> >
>>>> >> >> > Actually I don't, I test it by modifying fio's splice engine to set/get
>>>> >> >> > the pipe size and test the resulting transfers.
>>>> >> >>
>>>> >> >> An afterthought. Do there not also need to be fixes to the /proc
>>>> >> >> interfaces. I don't think they were included in your revised patches.
>>>> >> >
>>>> >> > I think the proc part can be sanely left in pages, since it's just a
>>>> >> > memory limiter.
>>>> >>
>>>> >> I can't see any advantage to using two different units for these
>>>> >> closely related APIs, and it does seem like it could be a source of
>>>> >> confusion. Similar APIs that I can think of like RLIMIT_MEMLOCK and
>>>> >> shmget() SHMMAX that impose per-process memory-related limits use
>>>> >> bytes. Best to be consistent, don't you think?
>>>> >
>>>> > But they are different interfaces.  I think the 'pass in required size,
>>>> > return actual size' where actual size is >= required size makes sense
>>>> > for the syscall part, but for an "admin" interface it is more logical to
>>>> > deal in pages. Perhaps that's just me and the average admin does not
>>>> > agree. So while it's just detail, it's also an interface so has some
>>>> > importance. And if there's consensus that bytes is a cleaner interface
>>>> > on the proc side as well, then lets change it.
>>>>
>>>> I'll add one more datapoint to those that I already mentioned.
>>>> RLIMIT_STACK and RLIMIT_DATA (getrlimit()) is also expressed in bytes.
>>>>
>>>> There was only one vaguely related limit that I could find that
>>>> measured things in pages. Consider these two System V shared memory
>>>> limits:
>>>>
>>>> SHMMAX
>>>> This is the maximum size (in bytes) of a shared memory segment.
>>>>
>>>> SHMALL
>>>> This is a system-wide limit on the total number of pages of shared memory.
>>>>
>>>> But in a way this almost confirms my point. SHMMAX is a limit the
>>>> governs the behavior of individual processes (like your /proc file),
>>>> while SHMALL is a limit that governs the behavior of the system as a
>>>> whole. There is a (sort of) logic to using bytes for one and pages for
>>>> the other.
>>>>
>>>> I think that I've said all I need to say on the topic. I'm inclined to
>>>> think yours /proc file should use bytes, since it seems consistent
>>>> with other simialr APIs. Others may confirm, or someone else mught
>>>> have a different insight.
>>>
>>> I'll commit a patch to change it to bytes.
>>
>> Thanks Jens.
>
> Since I'm going to document the /proc file, it occurred to me... What
> are you going to call this file now? "pipe_max_pages"  no longer makes
> sense. "pipe_size_ceiling" may be more expressive than simply
> "pipe_max".

So, I'm looking at this interface still more closely now. How about
using CAP_SYS_RESOURCE, rather than the hugely overloaded
CAP_SYS_ADMIN as the governor for the capability check? Again, it's
about consistency. Here's what CAP_SYS_RESOURCE currently governs:

       CAP_SYS_RESOURCE
              * Use reserved space on ext2 file systems;
              * make ioctl(2) calls controlling ext3 journaling;
              * override disk quota limits;
              * increase resource limits (see setrlimit(2));
              * override RLIMIT_NPROC resource limit;
              * raise msg_qbytes limit for a System V message queue
above the limit
                in /proc/sys/kernel/msgmnb (see msgop(2) and msgctl(2)).

Including the pipe size limit in this list makes sense.

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Author of "The Linux Programming Interface" http://blog.man7.org/