Re: [PATCH net-next v7 1/5] mm: add a signature in struct page

[Matteo Croce]

On Sat, Jun 5, 2021 at 4:32 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sat, Jun 05, 2021 at 12:59:50AM +0200, Matteo Croce wrote:
> > On Fri, Jun 4, 2021 at 9:08 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> > >
> > > On Fri, Jun 04, 2021 at 08:33:45PM +0200, Matteo Croce wrote:
> > > > @@ -130,7 +137,10 @@ struct page {
> > > >                       };
> > > >               };
> > > >               struct {        /* Tail pages of compound page */
> > > > -                     unsigned long compound_head;    /* Bit zero is set */
> > > > +                     /* Bit zero is set
> > > > +                      * Bit one if pfmemalloc page
> > > > +                      */
> > > > +                     unsigned long compound_head;
> > >
> > > I would drop this hunk.  Bit 1 is not used for this purpose in tail
> > > pages; it's used for that purpose in head and base pages.
> > >
> > > I suppose we could do something like ...
> > >
> > >  static inline void set_page_pfmemalloc(struct page *page)
> > >  {
> > > -       page->index = -1UL;
> > > +       page->lru.next = (void *)2;
> > >  }
> > >
> > > if it's causing confusion.
> > >
> >

And change all the *_pfmemalloc functions to use page->lru.next like this?

@@ -1668,10 +1668,12 @@ struct address_space *page_mapping(struct page *page);
static inline bool page_is_pfmemalloc(const struct page *page)
{
       /*
-        * Page index cannot be this large so this must be
-        * a pfmemalloc page.
+        * This is not a tail page; compound_head of a head page is unused
+        * at return from the page allocator, and will be overwritten
+        * by callers who do not care whether the page came from the
+        * reserves.
        */
-       return page->index == -1UL;
+       return (uintptr_t)page->lru.next & BIT(1);
}

/*
@@ -1680,12 +1682,12 @@ static inline bool page_is_pfmemalloc(const
struct page *page)
 */
static inline void set_page_pfmemalloc(struct page *page)
{
-       page->index = -1UL;
+       page->lru.next = (void *)BIT(1);
}

static inline void clear_page_pfmemalloc(struct page *page)
{
-       page->index = 0;
+       page->lru.next = NULL;

}

-- 
per aspera ad upstream