linux-kernel - Re: [PATCH v2 1/8] lib/printbuf: New data structure for heap-allocated strings

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Message-ID: <20220422182052.4994525d@gandalf.local.home>
Date:   Fri, 22 Apr 2022 18:20:52 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kent Overstreet <kent.overstreet@...il.com>
Cc:     Christoph Hellwig <hch@....de>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        hannes@...xchg.org, akpm@...ux-foundation.org,
        linux-clk@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-input@...r.kernel.org, roman.gushchin@...ux.dev
Subject: Re: [PATCH v2 1/8] lib/printbuf: New data structure for
 heap-allocated strings

On Fri, 22 Apr 2022 17:51:46 -0400
Kent Overstreet <kent.overstreet@...il.com> wrote:

> 
> But it's definitely not an unreasonable idea - I can try it out and see how it
> turns out. Would you have any objections to making some changes to seq_buf?

No I don't mind, and that's why I want the coupled, as enhancements or bug
fixes would happen to both.

> 
>  - You've got size and len as size_t, I've got them as unsigned. Given that we
>    need to be checking for overflow anyways for correctens, I like having them
>    as u32s.

I had it as size_t as I had planned (and still plan to) make seq_file use
seq_buf, and seq_file uses size_t. Who knows, perhaps in the future, we may
have strings that are more than 4GBs. ;-)

>  - seq_buf->readpos - it looks like this is only used by seq_buf_to_user(), does
>    it need to be in seq_buf?

Perhaps.

>  - in printbufs, I make sure the buffer is always nul-terminated - seems
>    simplest, given that we need to make sure there's always room for the
>    terminating nul anyways.

I'm not against that. It was an optimization, but I never actually
benchmarked it. But I'm not sure how many fast paths it is used in to
warrant that kind of optimization over the complexity it can bring for
users.


> 
> A downside of having printbuf on top of seq_buf is that now we've got two apis
> that functions can output to - vs. if we modified printbuf by adding a flag for
> "this is an external buffer, don't reallocate it". That approach would be less
> code overall, for sure.
> 
> Could I get you to look over printbuf and share your thoughts on the different
> approaches?

Sure, but will have to wait till next week.

-- Steve