Re: [PATCH 05/16] page-flags: define behavior of FS/IO-related flags on compound pages

[Kirill A. Shutemov]

On Tue, Mar 24, 2015 at 03:54:00PM -0700, Hugh Dickins wrote:
> On Mon, 23 Mar 2015, Kirill A. Shutemov wrote:
> > Should we avoid dirtying them in the first place?
> 
> I don't think so: to do so would add more branches in hot paths,
> just to avoid a rare case which works fine without them; and
> prevent a driver from using it, in the unlikely case that's so.

It's branches vs. useless atomic oprations.

> > GUP pin would screw up page_mapcount() on these pages. It would affect
> > memory stats for the process and probably something else.
> 
> Yes, the GUP pin would increment page_mapcount() without an additional
> mapping - but can only happen once the page has already been mapped,
> so FILE_MAPPED stats unaffected?  I'm not sure; but surely it wouldn't
> work as well when unmapped before unpinned, since the unmapping will
> see "still mapped" and the unpinning won't do anything with FILE_MAPPED.
> 
> Unmapping before unpinning is an uncommon path; but it can't be ignored,
> it is the path which demanded __GFP_COMP in the first place.
> 
> Looks like extending THP by-mapcount refcounting to other compound pages
> was not such a good idea.  But since nobody has noticed, we may not need
> a more urgent fix than your simplification of THP refcounting.

I think PSS and /proc/kpagecount are broken by this.

> > I think we can get __compound_tail_refcounted() ignore these pages by
> > checking if page->mapping is NULL.
> 
> I forget what's in page->mapping on the THP tails.

NULL. We never set ->mapping on any tail pages. That's why I want outlaw
using that value: it's just doesn't match with head page ->mapping for
some of compound pages. And for others it matches just because nobody
touches it for any subpage.

> Or do you mean page->mapping of head?  It would be better not to rely on
> that, I'm not certain that no driver could set page->mapping of compound
> head.  There's probably some field or flag on the tails that you could
> use; but I don't know that it's needed in a hurry.

We only need tail refcounting for THP, so I think this should fix the issue:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 4a3a38522ab4..9ab432660adb 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -456,7 +456,7 @@ static inline int page_count(struct page *page)
 
 static inline bool __compound_tail_refcounted(struct page *page)
 {
-       return !PageSlab(page) && !PageHeadHuge(page);
+       return !PageSlab(page) && !PageHeadHuge(page) && PageAnon(page);
 }
 
 /*

-- 
 Kirill A. Shutemov
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