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From: Michal Hocko <mhocko@kernel.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Zhaoyang Huang <huangzhaoyang@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	linux-kernel@vger.kernel.org, kernel-patch-test@lists.linaro.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Joel Fernandes <joelaf@google.com>,
	linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem
Date: Tue, 3 Apr 2018 14:35:14 +0200
Message-ID: <20180403123514.GX5501@dhcp22.suse.cz> (raw)
In-Reply-To: <20180403082348.28cd3c1c@gandalf.local.home>

On Tue 03-04-18 08:23:48, Steven Rostedt wrote:
> On Tue, 3 Apr 2018 14:16:14 +0200
> Michal Hocko <mhocko@kernel.org> wrote:
> 
> > > This came up because there's scripts or programs that set the size of
> > > the ring buffer. The complaint was that the application would just set
> > > the size to something bigger than what was available and cause an OOM
> > > killing other applications. The final solution is to simply check the
> > > available memory before allocating the ring buffer:
> > > 
> > > 	/* Check if the available memory is there first */
> > > 	i = si_mem_available();
> > > 	if (i < nr_pages)
> > > 		return -ENOMEM;
> > > 
> > > And it works well.  
> > 
> > Except that it doesn't work. si_mem_available is not really suitable for
> > any allocation estimations. Its only purpose is to provide a very rough
> > estimation for userspace. Any other use is basically abuse. The
> > situation can change really quickly. Really it is really hard to be
> > clever here with the volatility the memory allocations can cause.
> 
> OK, then what do you suggest? Because currently, it appears to work. A
> rough estimate may be good enough.
> 
> If we use NORETRY, then we have those that complain that we do not try
> hard enough to reclaim memory. If we use RETRY_MAYFAIL we have this
> issue of taking up all memory before we get what we want.

Just try to do what admin asks for and trust it will not try to shoot
his foot? I mean there are other ways admin can shoot the machine down.
Being clever is OK if it doesn't add a tricky code. And relying on
si_mem_available is definitely tricky and obscure.

> Perhaps I should try to allocate a large group of pages with
> RETRY_MAYFAIL, and if that fails go back to NORETRY, with the thinking
> that the large allocation may reclaim some memory that would allow the
> NORETRY to succeed with smaller allocations (one page at a time)?

That again relies on a subtle dependencies of the current
implementation. So I would rather ask whether this is something that
really deserves special treatment. If admin asks for a buffer of a
certain size then try to do so. If we get OOM then bad luck you cannot
get large memory buffers for free...
-- 
Michal Hocko
SUSE Labs

  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-29 10:41 Zhaoyang Huang
2018-03-29 16:05 ` Steven Rostedt
2018-03-30  3:32   ` Zhaoyang Huang
2018-03-30 14:07     ` Steven Rostedt
2018-03-30  6:53 ` [Kernel-patch-test] " kbuild test robot
2018-03-30  6:54 ` kbuild test robot
2018-03-30 14:20 ` Steven Rostedt
2018-03-30 16:37   ` Joel Fernandes
2018-03-30 19:10     ` Steven Rostedt
2018-03-30 20:37       ` Joel Fernandes
2018-03-30 20:53   ` Matthew Wilcox
2018-03-30 21:30     ` Steven Rostedt
2018-03-30 21:42       ` Steven Rostedt
2018-03-30 23:38         ` Joel Fernandes
2018-03-31  1:41           ` Steven Rostedt
2018-03-31  2:18             ` Matthew Wilcox
2018-03-31  3:07               ` Steven Rostedt
2018-03-31  5:44                 ` Joel Fernandes
2018-04-02  0:52         ` Zhaoyang Huang
2018-04-03 11:06   ` Michal Hocko
2018-04-03 11:51     ` Steven Rostedt
2018-04-03 12:16       ` Michal Hocko
2018-04-03 12:23         ` Steven Rostedt
2018-04-03 12:35           ` Michal Hocko [this message]
2018-04-03 13:32             ` Steven Rostedt
2018-04-03 13:56               ` Michal Hocko
2018-04-03 14:17                 ` Steven Rostedt
2018-04-03 16:11                   ` Michal Hocko
2018-04-03 16:59                     ` Steven Rostedt
2018-04-03 22:56                     ` Steven Rostedt
2018-04-04  6:20                       ` Michal Hocko
2018-04-04 12:21                         ` Joel Fernandes
2018-04-04 12:59                         ` Steven Rostedt
2018-04-04 14:10                           ` Michal Hocko
2018-04-04 14:25                             ` Steven Rostedt
2018-04-04 14:42                               ` Michal Hocko
2018-04-04 15:04                                 ` Steven Rostedt
2018-04-04 15:27                                   ` Michal Hocko
2018-04-04 15:38                                     ` Steven Rostedt
2018-04-04  2:58                 ` Zhaoyang Huang
2018-04-04  6:23                   ` Michal Hocko
2018-04-04  9:29                     ` Zhaoyang Huang
2018-04-04 14:11                     ` Steven Rostedt
2018-04-04 14:23                       ` Michal Hocko
2018-04-04 14:31                         ` Steven Rostedt
2018-04-04 14:47                           ` Michal Hocko
2018-04-04 15:47                         ` Steven Rostedt
2018-04-05  2:58                           ` Matthew Wilcox
2018-04-05  4:12                             ` Joel Fernandes
2018-04-05 14:22                               ` Matthew Wilcox
2018-04-05 14:27                                 ` Michal Hocko
2018-04-05 14:34                                   ` Steven Rostedt
2018-04-05 15:13                                   ` Matthew Wilcox
2018-04-05 15:32                                     ` Michal Hocko
2018-04-05 16:15                                       ` Matthew Wilcox
2018-04-05 18:54                                         ` Michal Hocko
2018-04-05 20:15                                           ` __GFP_LOW Matthew Wilcox
2018-04-06  6:09                                             ` __GFP_LOW Michal Hocko
2018-04-08  4:27                                               ` __GFP_LOW Matthew Wilcox
2018-04-09  7:34                                                 ` __GFP_LOW Michal Hocko
2018-04-09 15:51                                                   ` __GFP_LOW Matthew Wilcox
2018-04-09 18:14                                                     ` __GFP_LOW Michal Hocko
     [not found]                                                       ` <CA+JonM0HG9kWb6-0iyDQ8UMxTeR-f=+ZL89t5DvvDULDC8Sfyw@mail.gmail.com>
2018-04-10 12:19                                                         ` __GFP_LOW Matthew Wilcox
2018-04-05 14:30                                 ` [PATCH v1] kernel/trace:check the val against the available mem Steven Rostedt

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