linux-kernel - Re: [RFC PATCH] Take mmap_min_addr into account while choosing unmapped address for x86-64.

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Date:   Tue, 27 Feb 2018 16:27:29 +0300
From:   Ilya Smith <blackzert@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, kirill.shutemov@...ux.intel.com,
        dsafonov@...tuozzo.com, hughd@...gle.com,
        gregkh@...uxfoundation.org, craigb@...gle.com, oleg@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] Take mmap_min_addr into account while choosing
 unmapped address for x86-64.

> 
> mmap_min_addr handling is a bit mess... As you say, we would return
> EPERM rather than ENOMEM which can be confusing but depleting the
> address space like that is quite unlikely on 64b unless I am missing.
> It is good to be in sync here with the generic implementation though,
> IMO.
> 

If we take a look on mm/mmap.c:
#ifndef HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
unsigned long
arch_get_unmapped_area_topdown(
…
	if (len > TASK_SIZE - mmap_min_addr)
		return -ENOMEM;
…
	info.low_limit = max(PAGE_SIZE, mmap_min_addr);

And this one looks like a generic implementation.
But for many other architectures like arch/parisc/kernel/sys_parisc.c 
or arch/x86/kernel/sys_x86_64.c

	info.low_limit = PAGE_SIZE;

What is looks like an issue for me.

Here is C code could be used as test-case:

#include <errno.h>
#include <stdio.h>
#include <sys/mman.h>

int main() {
  char buffer[1024];

  unsigned long len = 1ULL << 46;
  while(len) {
    void *ptr = mmap(4096, len, 0, MAP_PRIVATE | MAP_ANONYMOUS , -1, 0);
    if (ptr == MAP_FAILED) {
      if (errno == EPERM)
        break;
      if (errno == ENOMEM) {
        len >>= 1;
        continue;
      }
      return -1;
    }
  }
  if (errno == EPERM) {
    printf("Test failed, you have wrong ret code EPERM\n");
    sprintf(buffer, "cat /proc/%d/maps", getpid());
    system(buffer);
    return -1;
  }
  return 0;
}


>> 
>> +	if (addr < mmap_min_addr)
>> +		return false;
>> +
>> 	return (addr > DEFAULT_MAP_WINDOW) == (addr + len > DEFAULT_MAP_WINDOW);
> 
> But is this one necessary? We do sanitze hint address before going to
> get_unmapped_address AFAIR.
> 
I’m agree, looks like I was trying to fix something that already fine.

Thanks,
Ilya