Re: [PATCHv3 17/41] filemap: handle huge pages in filemap_fdatawait_range()

Kirill A. Shutemov
2016-10-13T12:08:44Z

On Thu, Oct 13, 2016 at 11:44:41AM +0200, Jan Kara wrote:
> On Thu 15-09-16 14:54:59, Kirill A. Shutemov wrote:
> > We writeback whole huge page a time.
>
> This is one of the things I don't understand. Firstly I didn't see where
> changes of writeback like this would happen (maybe they come later).
> Secondly I'm not sure why e.g. writeback should behave atomically wrt huge
> pages. Is this because radix-tree multiorder entry tracks dirtiness for us
> at that granularity?

We track dirty/writeback on per-compound pages: meaning we have one
dirty/writeback flag for whole compound page, not on every individual
4k subpage. The same story for radix-tree tags.

> BTW, can you also explain why do we need multiorder entries? What do
> they solve for us?

It helps us having coherent view on tags in radix-tree: no matter which
index we refer from the range huge page covers we will get the same
answer on which tags set.

> I'm sorry for these basic questions but I'd just like to understand how is
> this supposed to work...
>
> Honza
>
>
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> > mm/filemap.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 05b42d3e5ed8..53da93156e60 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -372,9 +372,14 @@ static int __filemap_fdatawait_range(struct address_space *mapping,
> > if (page->index > end)
> > continue;
> >
> > + page = compound_head(page);
> > wait_on_page_writeback(page);
> > if (TestClearPageError(page))
> > ret = -EIO;
> > + if (PageTransHuge(page)) {
> > + index = page->index + HPAGE_PMD_NR;
> > + i += index - pvec.pages[i]->index - 1;
> > + }
> > }
> > pagevec_release(&pvec);
> > cond_resched();
> > --
> > 2.9.3
> >
> >
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR
>
> --
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@kvack.org. For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Don't email: email@kvack.org

--
Kirill A. Shutemov