linux-kernel - Re: [PATCH v1] kernel/trace:check the val against the available mem

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Date:   Wed, 4 Apr 2018 16:23:29 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Zhaoyang Huang <huangzhaoyang@...il.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-patch-test@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joel Fernandes <joelaf@...gle.com>, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem

On Wed 04-04-18 10:11:49, Steven Rostedt wrote:
> On Wed, 4 Apr 2018 08:23:40 +0200
> Michal Hocko <mhocko@...nel.org> wrote:
> 
> > If you are afraid of that then you can have a look at {set,clear}_current_oom_origin()
> > which will automatically select the current process as an oom victim and
> > kill it.
> 
> Would it even receive the signal? Does alloc_pages_node() even respond
> to signals? Because the OOM happens while the allocation loop is
> running.

Well, you would need to do something like:

> 
> I tried it out, I did the following:
> 
> 	set_current_oom_origin();
> 	for (i = 0; i < nr_pages; i++) {
> 		struct page *page;
> 		/*
> 		 * __GFP_RETRY_MAYFAIL flag makes sure that the allocation fails
> 		 * gracefully without invoking oom-killer and the system is not
> 		 * destabilized.
> 		 */
> 		bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
> 				    GFP_KERNEL | __GFP_RETRY_MAYFAIL,
> 				    cpu_to_node(cpu));
> 		if (!bpage)
> 			goto free_pages;
> 
> 		list_add(&bpage->list, pages);
> 
> 		page = alloc_pages_node(cpu_to_node(cpu),
> 					GFP_KERNEL | __GFP_RETRY_MAYFAIL, 0);
> 		if (!page)
> 			goto free_pages;

		if (fatal_signal_pending())
			fgoto free_pages;

> 		bpage->page = page_address(page);
> 		rb_init_page(bpage->page);
> 	}
> 	clear_current_oom_origin();

If you use __GFP_RETRY_MAYFAIL it would have to be somedy else to
trigger the OOM killer and this user context would get killed. If you
drop __GFP_RETRY_MAYFAIL it would be this context to trigger the OOM but
it would still be the selected victim.
-- 
Michal Hocko
SUSE Labs