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

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Date:   Sun, 6 Jun 2021 03:50:54 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     netdev@...r.kernel.org, linux-mm@...ck.org,
        Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Marcin Wojtas <mw@...ihalf.com>,
        Russell King <linux@...linux.org.uk>,
        Mirko Lindner <mlindner@...vell.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Boris Pismenny <borisp@...dia.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yu Zhao <yuzhao@...gle.com>,
        Will Deacon <will@...nel.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Roman Gushchin <guro@...com>, Hugh Dickins <hughd@...gle.com>,
        Peter Xu <peterx@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Alexander Lobakin <alobakin@...me>,
        Cong Wang <cong.wang@...edance.com>, wenxu <wenxu@...oud.cn>,
        Kevin Hao <haokexin@...il.com>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Marco Elver <elver@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Yunsheng Lin <linyunsheng@...wei.com>,
        Guillaume Nault <gnault@...hat.com>,
        linux-kernel@...r.kernel.org, linux-rdma@...r.kernel.org,
        bpf@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>,
        David Ahern <dsahern@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrew Lunn <andrew@...n.ch>, Paolo Abeni <pabeni@...hat.com>,
        Sven Auhagen <sven.auhagen@...eatech.de>
Subject: Re: [PATCH net-next v7 1/5] mm: add a signature in struct page

On Sat, Jun 5, 2021 at 4:32 PM Matthew Wilcox <willy@...radead.org> 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@...radead.org> 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;

}

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