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

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Date:   Fri, 30 Mar 2018 22:44:51 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Zhaoyang Huang <huangzhaoyang@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-patch-test@...ts.linaro.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem

On Fri, Mar 30, 2018 at 8:07 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Fri, 30 Mar 2018 19:18:57 -0700
> Matthew Wilcox <willy@...radead.org> wrote:
>
>> Again though, this is the same pattern as vmalloc.  There are any number
>> of places where userspace can cause an arbitrarily large vmalloc to be
>> attempted (grep for kvmalloc_array for a list of promising candidates).
>> I'm pretty sure that just changing your GFP flags to GFP_KERNEL |
>> __GFP_NOWARN will give you the exact behaviour that you want with no
>> need to grub around in the VM to find out if your huge allocation is
>> likely to succeed.
>
> Not sure how this helps. Note, I don't care about consecutive pages, so
> this is not an array. It's a link list of thousands of pages. How do
> you suggest allocating them? The ring buffer is a link list of pages.

Yeah I didn't understand the suggestion either. If I remember
correctly, not using either NO_RETRY or RETRY_MAY_FAIL, and just plain
GFP_KERNEL was precisely causing the buffer_size_kb write to cause an
OOM in my testing. So I think Steven's patch does the right thing in
checking in advance.

thanks,

- Joel