Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20221024150922.129814-1-gautammenghani201@gmail.com> <20221024111621.3ba17e2c@gandalf.local.home>
In-Reply-To: <20221024111621.3ba17e2c@gandalf.local.home>
From:   "Zach O'Keefe" <zokeefe@google.com>
Date:   Mon, 24 Oct 2022 09:44:16 -0700
Message-ID: <CAAa6QmSKtj6T2dW1tkg5_HVj2+rXj5inOLdEzr0MkJzQxxcPXQ@mail.gmail.com>
Subject: Re: [PATCH] mm/khugepaged: add tracepoint to collapse_file()
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Gautam Menghani <gautammenghani201@gmail.com>,
        akpm@linux-foundation.org, mhiramat@kernel.org,
        shy828301@gmail.com, vbabka@suse.cz, david@redhat.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Thanks for your mail, Gautam.

> I try to keep dereferences out of the calling path as much as possible
> (adds to I$ at the call site).

This was probably due to the way I handled
trace_mm_khugepaged_scan_file(). Perhaps that can be cleaned up at the
same time as this patch, for consistency.

Also, no qualms about adding this tracepoint; there are a few scan
result codes that overlap between hpage_collapse_scan_file() and those
possibly returned in collapse_file() such that, if we only have the
one tracepoint in hpage_collapse_scan_file(), it could be ambiguous
what callsite the error path stemmed from. Luckily this hasn't been an
issue thus far.

Lastly, a few other items we might care about capturing:

- is_shmem (perhaps the filename is enough to know this - but I know
at least once during development I was caught off-guard b/c a mount I
thought to be file-backed turned out to be tmpfs (and something I
didn't think to question until I had wasted some time on other
paths)).
- index

Best,
Zach


> Could you just pass in file, and then have:
>
>         __string(filename, file->f_path.dentry->d_iname)
>
>         [..]
>
>         __assign_string(filename, file->f_path.dentry->d_iname);
>
>
> If you are paranoid, you can have the above also be:
>
>                         file ? file->f_path.dentry ? file->f_path.dentry->d_iname : "(null)" : "(null)")
>
>
> -- Steve
>
>
> > +                                   nr, result);
> >       return result;