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

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Message-ID: <20190605100207.GD32406@redhat.com>
Date:   Wed, 5 Jun 2019 12:02:07 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        peterz@...radead.org, rostedt@...dmis.org, mhiramat@...nel.org,
        kirill.shutemov@...ux.intel.com, kernel-team@...com,
        william.kucharski@...cle.com
Subject: Re: [PATCH uprobe, thp v2 2/5] uprobe: use original page when all
 uprobes are removed

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.

> +	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?

Oleg.