linux-kernel - Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

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Message-ID: <20250226171645.GH8995@redhat.com>
Date: Wed, 26 Feb 2025 18:16:46 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: "Sapkal, Swapnil" <swapnil.sapkal@....com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Brauner <brauner@...nel.org>,
	David Howells <dhowells@...hat.com>,
	WangYuli <wangyuli@...ontech.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Shenoy, Gautham Ranjal" <gautham.shenoy@....com>,
	Neeraj.Upadhyay@....com
Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still
 full

On 02/26, Mateusz Guzik wrote:
>
> On Wed, Feb 26, 2025 at 2:19 PM Mateusz Guzik <mjguzik@...il.com> wrote:
> >
> Now that I sent the e-mail, I realized the page would have unread data
> after some offset, so there is no room to *append* to it, unless one
> wants to memmove everythiing back.

Yes, but... even "memmove everything back" won't help if
pipe->ring_size > 1 (PIPE_DEF_BUFFERS == 16 by default).

> However, the suggestion below stands:

Agreed, any additional info can help.

Oleg.

> > As for the bug, I don't see anything obvious myself.
> >
> > However, I think there are 2 avenues which warrant checking.
> >
> > Sapkal, if you have time, can you please boot up the kernel which is
> > more likely to run into the problem and then run hackbench as follows:
> >
> > 1. with 1 fd instead of 20:
> >
> > /usr/bin/hackbench -g 16 -f 1 --threads --pipe -l 100000 -s 100
> >
> > 2. with a size which divides 4096 evenly (e.g., 128):
> >
> > /usr/bin/hackbench -g 1 -f 20 --threads --pipe -l 100000 -s 128
> 
> 
> 
> -- 
> Mateusz Guzik <mjguzik gmail.com>
>