linux-kernel - Re: [PATCH] mm: mmap: show vm_unmapped

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Date:   Sun, 8 Mar 2020 18:58:47 +0900
From:   Jaewon Kim <jaewon31.kim@...sung.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     walken@...gle.com, bp@...e.de, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, jaewon31.kim@...il.com
Subject: Re: [PATCH] mm: mmap: show vm_unmapped_area error log

On 2020년 03월 08일 10:58, Matthew Wilcox wrote:
> On Sat, Mar 07, 2020 at 03:47:44PM -0800, Andrew Morton wrote:
>> On Fri, 6 Mar 2020 15:16:22 +0900 Jaewon Kim <jaewon31.kim@...sung.com> wrote:
>>> Even on 64 bit kernel, the mmap failure can happen for a 32 bit task.
>>> Virtual memory space shortage of a task on mmap is reported to userspace
>>> as -ENOMEM. It can be confused as physical memory shortage of overall
>>> system.
> But userspace can trigger this printk.  We don't usually allow printks
> under those circumstances, even ratelimited.
Hello thank you your comment.

Yes, userspace can trigger printk, but this was useful for to know why
a userspace task was crashed. There seems to be still many userspace
applications which did not check error of mmap and access invalid address.

Additionally in my AARCH64 Android environment, display driver tries to
get userspace address to map its display memory. The display driver
report -ENOMEM from vm_unmapped_area and but also from GPU related
address space.

Please let me know your comment again if this debug is now allowed
in that userspace triggered perspective.

I may change to pr_debug or drop this change.

Thank you
>
>