linux-kernel - Re: [PATCH v7 2/4] uprobe: use original page when all uprobes are removed

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Message-ID: <EA58E3BD-7EB1-4433-8F7F-1E3894F8D563@fb.com>
Date:   Wed, 24 Jul 2019 08:23:53 +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>,
        Matthew Wilcox <matthew.wilcox@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kernel Team <Kernel-team@...com>,
        William Kucharski <william.kucharski@...cle.com>
Subject: Re: [PATCH v7 2/4] uprobe: use original page when all uprobes are
 removed



> On Jul 15, 2019, at 8:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> On 06/25, Song Liu wrote:
>> 
>> This patch allows uprobe to use original page when possible (all uprobes
>> on the page are already removed).
> 
> I can't review. I do not understand vm enough.
> 
>> +	if (!is_register) {
>> +		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 (pages_identical(new_page, orig_page)) {
> 
> Shouldn't we at least check PageUptodate?

For page cache, we only do ClearPageUptodate() on read failures, so 
this should be really rare case. But I guess we can check anyway. 

> 
> and I am a bit surprised there is no simple way to unmap the old page
> in this case... 

The easiest way I have found requires flush_cache_page() plus a few
mmu_notifier calls around it. I think current solution is better than
that, as it saves a page fault. 

Thanks,
Song