On Wed, Nov 26, 2014 at 02:00:06PM -0800, Andrew Morton wrote:
> On Tue, 25 Nov 2014 14:48:41 -0500 Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> > The
> > same btw applies for the page_mkwrite case: how is mapping safe to
> > pass to balance_dirty_pages() after unlocking page table and page?
>
> I'm not sure which code you're referring to here, but it's likely that
> the switch-balancing-to-bdi approach will address that as well?
This code in do_wp_page():
pte_unmap_unlock(page_table, ptl);
[...]
put_page(dirty_page);
if (page_mkwrite) {
struct address_space *mapping = dirty_page->mapping;
set_page_dirty(dirty_page);
unlock_page(dirty_page);
page_cache_release(dirty_page);
if (mapping) {
/*
* Some device drivers do not set page.mapping
* but still dirty their pages
*/
balance_dirty_pages_ratelimited(mapping);
}
}
And there is also this code in do_shared_fault():
pte_unmap_unlock(pte, ptl);
if (set_page_dirty(fault_page))
dirtied = 1;
mapping = fault_page->mapping;
unlock_page(fault_page);
if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
/*
* Some device drivers do not set page.mapping but still
* dirty their pages
*/
balance_dirty_pages_ratelimited(mapping);
}
I don't see anything that ensures mapping stays alive by the time it's
passed to balance_dirty_pages() in either case.
Argh, but of course there is. The mmap_sem. That pins the vma, which
pins the file, which pins the inode. In all cases. So I think we can
just stick with passing mapping to balance_dirty_pages() for now.
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