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

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Date:   Sat, 7 Mar 2020 15:47:44 -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 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.
> 
> 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.)
> <4>[   68.556470]  [2:  mmap_infinite:12363] mmap: vm_unmapped_area err:-12 total_vm:0xf4c08 flags:0x1 len:0x100000 low:0x8000 high:0xf4583000 mask:0x0 offset:0x0
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h

This patch was messed up by your email client (tabs expanded to spaces).

> @@ -27,6 +27,7 @@
>  #include <linux/memremap.h>
>  #include <linux/overflow.h>
>  #include <linux/sizes.h>
> +#include <linux/ratelimit.h>
>  
>  struct mempolicy;
>  struct anon_vma;
> @@ -2379,10 +2380,20 @@ 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)) {
> +        pr_warn_ratelimited("%s err:%ld total_vm:0x%lx flags:0x%lx len:0x%lx low:0x%lx high:0x%lx mask:0x%lx offset:0x%lx\n",
> +            __func__, addr, current->mm->total_vm, info->flags,
> +            info->length, info->low_limit, info->high_limit,
> +            info->align_mask, info->align_offset);
> +    }
> +    return addr;
>  }

pr_warn_ratelimited() contains static state.  Using it in an inlined
function means that each callsite gets its own copy of that state, so
we're ratelimiting the vm_unmapped_area() output on a per-callsite
basis, not on a kernelwide basis.

Maybe that's what we want, maybe it's not.  But I think
vm_unmapped_area() has become too large to be inlined anyway, so I
suggest making it a regular out-of-line function in mmap.c.  I don't
believe that function needs to be exported to modules.