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

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Date:   Wed, 24 Jul 2019 10:57:36 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...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 07/24, Song Liu wrote:
>
>
> > On Jul 15, 2019, at 8:25 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> >> +	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,

Hmm. I don't think so.

> so
> this should be really rare case. But I guess we can check anyway.

Can? I think we should or this code is simply wrong...

> > 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.

But we need to do this anyway? At least with your patch replace_page() still
does this after page_add_file_rmap().

> I think current solution is better than
> that,

perhaps, I won't argue,

> as it saves a page fault.

I don't think it matters in this case.

Oleg.