Re: [LTP] [PATCH v4 3/3] syscalls/pipe2_03: Add new test for pipe2 O_DIRECT flag

[Yang Xu]

Hi Linus


> On Sun, Apr 26, 2020 at 4:59 AM Li Wang <liwang@xxxxxxxxxx> wrote:
> >  From kernel code seems you are right. The pipe indeed takes use of PAGE_SIZE(ppc64le: 64kB) to split the writes data in the packetized mode (marked by O_DIRECT). But in the manual page, O_DIRECT indicates us the PIPE_BUF is the correct atomic unit.
>
> The manual is correct.
>
> PIPE_BUF is the size we _guarantee_ can be used atomically.
>
> The fact that in practice we do have bigger buffers on some platforms
> is an implementation detail.
>
> Yes, that implementation detail can be visible, but basically any test
> code that tries to test for "what if we use a bigger bug that
> PIPE_BUF" is buggy. It's simply not guaranteed to work any more.
>
> O_DIRECT is kind of immaterial, except it's just one of those things
> where the atomic size is slightly more visible. But basically,
> packetized pipes with bigger packets than PIPE_BUF is random behavior.
> It may work. It may not.
Thanks for your explanation. I am more curious about the user scene of 
this flag.

@Li, so how to design this test? In this test, we don't have complex 
scene to test this automic unit.

Best Regards
Yang Xu
>                  Linus