linux-kernel - Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are removed

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Message-ID: <BCE000B2-3F72-4148-A75C-738274917282@fb.com>
Date:   Wed, 24 Jul 2019 18:52:19 +0000
From:   Song Liu <songliubraving@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "Kernel Team" <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are
 removed



> On Jul 24, 2019, at 4:37 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> On 07/24, Song Liu wrote:
>> 
>> 	lock_page(old_page);
>> @@ -177,15 +180,24 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
>> 	mmu_notifier_invalidate_range_start(&range);
>> 	err = -EAGAIN;
>> 	if (!page_vma_mapped_walk(&pvmw)) {
>> -		mem_cgroup_cancel_charge(new_page, memcg, false);
>> +		if (!orig)
>> +			mem_cgroup_cancel_charge(new_page, memcg, false);
>> 		goto unlock;
>> 	}
>> 	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
>> 
>> 	get_page(new_page);
>> -	page_add_new_anon_rmap(new_page, vma, addr, false);
>> -	mem_cgroup_commit_charge(new_page, memcg, false, false);
>> -	lru_cache_add_active_or_unevictable(new_page, vma);
>> +	if (orig) {
>> +		lock_page(new_page);  /* for page_add_file_rmap() */
>> +		page_add_file_rmap(new_page, false);
> 
> 
> Shouldn't we re-check new_page->mapping after lock_page() ? Or we can't
> race with truncate?

We can't race with truncate, because the file is open as binary and 
protected with DENYWRITE (ETXTBSY). 

> 
> 
> and I am worried this code can try to lock the same page twice...
> Say, the probed application does MADV_DONTNEED and then writes "int3"
> into vma->vm_file at the same address to fool verify_opcode().
> 

Do you mean the case where old_page == new_page? I think this won't 
happen, because in uprobe_write_opcode() we only do orig_page for 
!is_register case. 

Thanks,
Song