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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sun, 26 Apr 2020 10:45:15 -0700
Message-ID: <CAHk-=wg7u4AjZQd49VjrZXikDMfK-HchOKHYqoUFwcXjsbJ3XQ@mail.gmail.com>
Subject: Re: [LTP] [PATCH v4 3/3] syscalls/pipe2_03: Add new test for pipe2
 O_DIRECT flag
To:     Li Wang <liwang@redhat.com>
Cc:     Yang Xu <xuyang2018.jy@cn.fujitsu.com>,
        LTP List <ltp@lists.linux.it>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>
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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.

                Linus