linux-kernel - Re: + uprobe-use-original-page-when-all-uprobes-are-removed.patch added to -mm tree

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Date:   Mon, 29 Jul 2019 17:05:40 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     akpm@...ux-foundation.org
Cc:     kirill.shutemov@...ux.intel.com, matthew.wilcox@...cle.com,
        peterz@...radead.org, rostedt@...dmis.org, songliubraving@...com,
        srikar@...ux.vnet.ibm.com, william.kucharski@...cle.com,
        linux-kernel@...r.kernel.org
Subject: Re: + uprobe-use-original-page-when-all-uprobes-are-removed.patch
 added to -mm tree

I didn't see this version, so let me reply here.

On 07/26, Andrew Morton wrote:
>
> +	/* try orig_page only for unregister and anonymous old_page */
> +	if (!is_register && PageAnon(old_page)) {

Well, this is confusing... nothing really wrong, but we certainly do not
want to install the new anonymous page if !is_register && !PageAnon(old).
And in this case we do not even want to call __replace page().

OK, I won't insist, this should almost never happen, but again, please
see https://lore.kernel.org/lkml/20190726084423.GA16112@redhat.com/

> +		struct page *orig_page;
> +		pgoff_t index;
> +
> +		index = vaddr_to_offset(vma, vaddr & PAGE_MASK) >> PAGE_SHIFT;
> +		orig_page = find_get_page(vma->vm_file->f_inode->i_mapping,
> +					  index);
> +
> +		if (orig_page) {
> +			if (PageUptodate(orig_page) &&
> +			    pages_identical(new_page, orig_page)) {
> +				/* let go new_page */
> +				put_page(new_page);
> +				new_page = NULL;
> +
> +				/* dec_mm_counter for old_page */
> +				dec_mm_counter(mm, MM_ANONPAGES);

this assumes that __replace_page() can't fail, but it can. I think you
should move this into into __replace_page().

Oleg.