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

[Oleg Nesterov <oleg@redhat.com>]

On 07/24, Song Liu wrote:
>
>
> > On Jul 15, 2019, at 8:25 AM, Oleg Nesterov <oleg@redhat.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.