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From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Alexander Potapenko <glider@google.com>,
	Fengguang Wu <fengguang.wu@intel.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v6 1/3] printk: Add line-buffered printk() API.
Date: Wed, 7 Nov 2018 19:52:53 +0900	[thread overview]
Message-ID: <42f33aae-a1d1-197f-a1d5-8c5ec88e88d1@i-love.sakura.ne.jp> (raw)
In-Reply-To: <20181106143502.GA32748@tigerII.localdomain>

On 2018/11/06 23:35, Sergey Senozhatsky wrote:
>> Since we want to remove "struct cont" eventually, we will try to remove
>> both "implicit printk() users who are expecting KERN_CONT behavior" and
>> "explicit pr_cont()/printk(KERN_CONT) users". Therefore, converting to
>> this API is recommended.
> 
> - The printk-fallback sounds like a hint that the existing 'cont' handling
>   better stay in the kernel. I don't see how the existing 'cont' is
>   significantly worse than
> 		bpr_warn(NULL, ...)->printk() // no 'cont' support
>   I don't see why would we want to do it, sorry. I don't see "it takes 16
>   printk-buffers to make a thing go right" as a sure thing.

Existing 'cont' handling will stay for a while. After majority of
pr_cont()/KERN_CONT users are converted, 'cont' support will be removed
(e.g. KERN_CONT becomes "").

> 
> A question.
> 
> How bad would it actually be to:
> 
> - Allocate seq_buf 512-bytes buffer (GFP_ATOMIC) just-in-time, when we
>   need it.
>     // How often systems cannot allocate a 512-byte buffer? //

It is a very bad thing to do GFP_ATOMIC without __GFP_NOWARN. See
"[PATCH 2/3] mm: Use line-buffered printk() for show_free_areas()."
which helps exactly when GFP_ATOMIC without __GFP_NOWARN failed.
Without __GFP_NOWARN, GFP_ATOMIC for printk() can trigger infinite
recursion and kernel stack overflow.

Even without recursion, doing kmalloc(GFP_ATOMIC | __GFP_NOWARN) temporarily
consumes some kernel stack. I don't know the exact amount needed for
kmalloc(GFP_ATOMIC | __GFP_NOWARN), but it might still emit memory allocation
fault injection messages. What GFP_ATOMIC can guarantee is nothing but
"it does not sleep". Not suitable for printk() which might be called from
critically dangerous situations.

> 
> - OK, assuming that systems around the world are so badly OOM like all the
>   time and even kmalloc(512) is absolutely impossible, then have a fallback
>   to the existing 'cont' handling; it just looks to me better than a plain
>   printk()-fallback with removed 'cont' support.

Since I want to eventually remove 'cont' support inside printk(),
I dropped KERN_CONT in patch [2/3] and [3/3].

> 
> - Do not allocate seq_buf if we are in printk-safe or in printk-nmi mode.
>   To avoid "buffering for the sake of buffering". IOW, when in printk-safe
>   use printk-safe.

Why? Since printk_safe_flush_buffer() forcibly flushes the partial line,
calling printk_safe_log_store() after line buffering can reduce possibility of
flushing partial lines, can't it?


  parent reply	other threads:[~2018-11-07 10:52 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-02 13:31 Tetsuo Handa
2018-11-02 13:31 ` [PATCH 2/3] mm: Use line-buffered printk() for show_free_areas() Tetsuo Handa
2018-11-07 14:07   ` Petr Mladek
2018-11-02 13:31 ` [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages Tetsuo Handa
2018-11-02 13:36   ` Peter Zijlstra
2018-11-03  2:00     ` Tetsuo Handa
2018-11-06  8:38     ` Petr Mladek
2018-11-06  9:05       ` Sergey Senozhatsky
2018-11-06 12:57         ` Petr Mladek
2018-11-06  9:56       ` Tetsuo Handa
2018-11-07 15:19   ` Petr Mladek
2018-11-08  4:45     ` Sergey Senozhatsky
2018-11-08 11:37       ` Tetsuo Handa
2018-11-09  6:12         ` Sergey Senozhatsky
2018-11-09  9:55           ` Tetsuo Handa
2018-11-09 15:43             ` Petr Mladek
2018-11-10  2:42               ` Tetsuo Handa
2018-11-23 12:46                 ` Petr Mladek
2018-11-23 13:12                   ` Tetsuo Handa
2018-11-23 15:56                   ` Steven Rostedt
2018-11-24  0:24                     ` Tetsuo Handa
2018-11-26  4:34                       ` Sergey Senozhatsky
2018-11-28 13:29                     ` David Laight
2018-11-29 10:09                       ` Tetsuo Handa
2018-11-30 16:01                         ` Petr Mladek
2018-11-10  8:52               ` Tetsuo Handa
2018-11-23 12:52                 ` Petr Mladek
2018-11-09 14:08           ` Linus Torvalds
2018-11-09 14:42             ` Steven Rostedt
2018-11-08 11:53       ` Petr Mladek
2018-11-08 12:44         ` Sergey Senozhatsky
2018-11-08 14:21           ` Sergey Senozhatsky
2018-11-09  9:54     ` Tetsuo Handa
2018-11-02 14:40 ` [PATCH v6 1/3] printk: Add line-buffered printk() API Matthew Wilcox
2018-11-03  1:55   ` Tetsuo Handa
2018-11-02 18:12 ` kbuild test robot
2018-11-02 18:12   ` kbuild test robot
2018-11-06 14:35 ` Sergey Senozhatsky
2018-11-07 10:21   ` Petr Mladek
2018-11-07 12:54     ` Tetsuo Handa
2018-11-08  2:21     ` Sergey Senozhatsky
2018-11-08 11:24       ` Petr Mladek
2018-11-08 11:46         ` Tetsuo Handa
2018-11-08 12:30         ` Sergey Senozhatsky
2018-11-09 14:10           ` Petr Mladek
2018-11-12  7:59             ` Sergey Senozhatsky
2018-11-12 10:42               ` Tetsuo Handa
2018-11-17 10:14               ` Tetsuo Handa
2018-11-07 10:52   ` Tetsuo Handa [this message]
2018-11-07 11:01     ` David Laight
2018-11-07 12:00       ` Petr Mladek
2018-11-07 11:45     ` Petr Mladek
2018-11-08  2:30     ` Sergey Senozhatsky
2018-11-07 13:41 ` Petr Mladek
2018-11-07 14:06 ` Petr Mladek

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