linux-kernel - Correct commit mask for page data size

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Date:   Mon, 1 Apr 2019 06:49:07 -0700
From:   Jason Behmer <jbehmer@...gle.com>
To:     rostedt@...dmis.org
Cc:     Craig Barabas <craigbarabas@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Correct commit mask for page data size

Hi Steven,
We're wondering what the correct number of bits to take from the
commit field is when determining the size of the page data.  The
format file shows the bottom 56 bits not overlapping with anything:

        field: local_t commit;  offset:8;       size:8; signed:1;
        field: int overwrite;   offset:8;       size:1; signed:1;

We first naively interpreted this as the size, but eventually ran into
cases where this gave back a nonsense result.  But then in our
investigation of what the correct thing to do is, we found conflicting
answers.

In the kernel we see that commit is often updated to write, which is
masked against RB_WRITE_MASK.  So it seems taking the bottom 20 bits
is correct.  However, in trace-cmd, a fairly authoritative parser, we
see that COMMIT_MASK is set to take the bottom 27 bits and set that to
the page data size.

Could you provide some guidance?

Thanks,
Jason