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Date: Wed, 8 Jan 2025 04:51:35 +0000
From: Eric Wong <e@80x24.org>
To: Alyssa Ross <hi@alyssa.is>
Cc: meta@public-inbox.org
Subject: Re: [PATCH 2/2] treewide: lose F_SETPIPE_SZ page size assumptions
Message-ID: <20250108045136.M43020@dcvr>
References: <20250107174624.1504268-1-hi@alyssa.is>
 <20250107174624.1504268-3-hi@alyssa.is>
 <20250107222656.M989952@dcvr>
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Eric Wong <e@80x24.org> wrote:
> Alyssa Ross <hi@alyssa.is> wrote:
> > fcntl(2) is documented as rounding up sizes to an integer multiple of
> > the page size.  When a pipe buffer as small as possible is desired,
> > it's cleaner to pass 0 and let the kernel set the pipe buffer to
> > exactly one page than it is to assume that pages are 4096 bytes.
> 
> OK.  I suppose the Linux kernel has enough testers that it's
> chances of it mishandling someday 0 would be caught before any
> release.

Well, that was only thinking about potential future bugs...

Looking backwards, older Linux appeared to handle 0 improperly before
d3f14c485867 (pipe: avoid round_pipe_size() nr_pages overflow on 32-bit, 2017-11-17)
https://yhbt.net/lore/lkml/d3f14c485867cfb2e0c48aa88c41d0ef4bf5209c/s/

I haven't tried an ancient kernel to verify and my arithmetic is awful
(d3f14c485867 also got backported to older distro kernels).