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Subject: Re: [LTP] [PATCH v4 3/3] syscalls/pipe2_03: Add new test for pipe2
 O_DIRECT flag
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Li Wang <liwang@redhat.com>
CC:     LTP List <ltp@lists.linux.it>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>
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From:   Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Message-ID: <36bd1e20-414b-ec24-f7e3-16ef7e2395d9@cn.fujitsu.com>
Date:   Thu, 30 Apr 2020 11:23:34 +0800
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Hi Linus


> On Sun, Apr 26, 2020 at 4:59 AM Li Wang <liwang@redhat.com> 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
> 
>