Re: [PATCH] mm: mmap: show vm_unmapped_area error log

[Jaewon Kim <jaewon31.kim@samsung.com>]

On 2020년 03월 08일 08:47, Andrew Morton wrote:
> On Fri, 6 Mar 2020 15:16:22 +0900 Jaewon Kim <jaewon31.kim@samsung.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).
Sorry for this. Let me fix when I resubmit.
>> @@ -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.
Thank you for your comment.
Though, on v5.6-rc4, I just found couple of code which calls to vm_unmapped_area,
I may be able to move this to out-of-line function on next patch version.

By the way, I need to discuss userspace triggered printk with Matthew Wilcox.
If possible, I'd like to hear your opinion for this.

Thank you
>
>
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