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From: Yang Shi <yang.shi@linux.alibaba.com>
Subject: [Question] Why do clear VM_ACCOUNT before do_munmap() in mremap()
Message-ID: <80280fc6-3916-d6e0-7fb0-c5cbc7013221@linux.alibaba.com>
Date: Fri, 21 Sep 2018 17:00:52 -0700
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To: "linux-mm@kvack.org" <linux-mm@kvack.org>

Hi folks,


When reading the mremap() code, I found the below code fragmentation:


 A A A A A A A  /* Conceal VM_ACCOUNT so old reservation is not undone */
 A A A A A A A  if (vm_flags & VM_ACCOUNT) {
 A A A A A A A A A A A A A A A  vma->vm_flags &= ~VM_ACCOUNT;
 A A A A A A A A A A A A A A A  excess = vma->vm_end - vma->vm_start - old_len;
 A A A A A A A A A A A A A A A  if (old_addr > vma->vm_start &&
 A A A A A A A A A A A A A A A A A A A  old_addr + old_len < vma->vm_end)
 A A A A A A A A A A A A A A A A A A A A A A A  split = 1;
 A A A A A A A  }

 A A A A A A A  ...

 A A A A A A A  do_munmap(mm, old_addr, old_len, uf_unmap)

 A A A A A A A  ...

 A A A A A A A  /* Restore VM_ACCOUNT if one or two pieces of vma left */
 A A A A A A A  if (excess) {
 A A A A A A A A A A A A A A A  vma->vm_flags |= VM_ACCOUNT;
 A A A A A A A A A A A A A A A  if (split)
 A A A A A A A A A A A A A A A A A A A A A A A  vma->vm_next->vm_flags |= VM_ACCOUNT;
 A A A A A A A  }


I don't get why it conceals VM_ACCOUNT, then restores it. This change is 
pre git period, so there is not commit log about why this is needed. Any 
hint is appreciated.


Thanks,

Yang