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

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Date:   Thu, 5 Mar 2020 20:24:43 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Jaewon Kim <jaewon31.kim@...sung.com>
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 Thu, 5 Mar 2020 10:35:05 +0900 Jaewon Kim <jaewon31.kim@...sung.com> wrote:

> Hello
> 
> sorry for build warning.
> I've changed %d to %ld reported by kbuild.
> Let me attach full patch again below.
> --------------------------------------------------
> 
> 
> 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.
> 
> The vm_unmapped_area can be called to by some drivers or other kernel
> core system like filesystem. It can be hard to know which code layer
> returns the -ENOMEM.
> 
> Print error log of vm_unmapped_area with rate limited. Without rate
> limited, soft lockup ocurrs on infinite mmap sytem call.
> 
> i.e.)
> <3>[  576.024088]  [6:  mmap_infinite:14251] mmap: vm_unmapped_area err:-12 total_vm:0xfee08 flags:0x1 len:0xa00000 low:0x8000 high:0xf3f63000
> 

hm, I suppose that could be useful.  Although the choice of which info
to display could be a source of dispute.  Why did you choose this info
and omit other things?  Perhaps a stack trace could also be useful?

> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2379,10 +2379,19 @@ extern unsigned long unmapped_area_topdown(struct vm_unmapped_area_info *info);
>  static inline unsigned long
>  vm_unmapped_area(struct vm_unmapped_area_info *info)
>  {
> +    unsigned long addr;
> +
>      if (info->flags & VM_UNMAPPED_AREA_TOPDOWN)
> -        return unmapped_area_topdown(info);
> +        addr = unmapped_area_topdown(info);
>      else
> -        return unmapped_area(info);
> +        addr = unmapped_area(info);
> +
> +    if (IS_ERR_VALUE(addr) && printk_ratelimit()) {

Please avoid using printk_ratelimit().  See the comment at the
printk_ratelimit() definition site:

/*
 * Please don't use printk_ratelimit(), because it shares ratelimiting state
 * with all other unrelated printk_ratelimit() callsites.  Instead use
 * printk_ratelimited() or plain old __ratelimit().
 */

> +        pr_err("%s err:%ld total_vm:0x%lx flags:0x%lx len:0x%lx low:0x%lx high:0x%lx\n",
> +            __func__, addr, current->mm->total_vm, info->flags,
> +            info->length, info->low_limit, info->high_limit);
> +    }
> +    return addr;
>  }