linux-kernel - Re: [PATCH uprobe, thp v2 2/5] uprobe: use original page when all uprobes are removed

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Message-ID: <0B1DEAD9-DB1E-46C5-9F5C-1049D0DC043F@fb.com>
Date:   Wed, 5 Jun 2019 16:29:36 +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>,
        "Peter Zijlstra" <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Masami Hiramatsu" <mhiramat@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH uprobe, thp v2 2/5] uprobe: use original page when all
 uprobes are removed

Hi Oleg,

Thanks for your kind review!

> On Jun 5, 2019, at 3:02 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> On 06/04, Song Liu wrote:
>> 
>> Currently, uprobe swaps the target page with a anonymous page in both
>> install_breakpoint() and remove_breakpoint(). When all uprobes on a page
>> are removed, the given mm is still using an anonymous page (not the
>> original page).
> 
> Agreed, it would be nice to avoid this,
> 
>> @@ -461,9 +471,10 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>> 			unsigned long vaddr, uprobe_opcode_t opcode)
>> {
>> 	struct uprobe *uprobe;
>> -	struct page *old_page, *new_page;
>> +	struct page *old_page, *new_page, *orig_page = NULL;
>> 	struct vm_area_struct *vma;
>> 	int ret, is_register, ref_ctr_updated = 0;
>> +	pgoff_t index;
>> 
>> 	is_register = is_swbp_insn(&opcode);
>> 	uprobe = container_of(auprobe, struct uprobe, arch);
>> @@ -501,6 +512,19 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
>> 	copy_highpage(new_page, old_page);
>> 	copy_to_page(new_page, vaddr, &opcode, UPROBE_SWBP_INSN_SIZE);
>> 
>> +	index = vaddr_to_offset(vma, vaddr & PAGE_MASK) >> PAGE_SHIFT;
>> +	orig_page = find_get_page(vma->vm_file->f_inode->i_mapping, index);
> 
> I think you should take is_register into account, if it is true we are going
> to install the breakpoint so we can avoid find_get_page/pages_identical.

Good idea! I will add this to v3. 

> 
>> +	if (orig_page) {
>> +		if (pages_identical(new_page, orig_page)) {
>> +			/* if new_page matches orig_page, use orig_page */
>> +			put_page(new_page);
>> +			new_page = orig_page;
> 
> Hmm. can't we simply unmap the page in this case?

I haven't found an easier way here. I tried with zap_vma_ptes() and 
unmap_page_range(). But neither of them works well here. 

Also, we need to deal with *_mm_counter, rmap, etc. So I guess reusing
__replace_page() (as current patch) is probably the easiest solution. 

Did I miss anything? 

Thanks again,
Song