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Message-ID: <20250722012324.GA63367@system.software.com>
Date: Tue, 22 Jul 2025 10:23:24 +0900
From: Byungchul Park <byungchul@...com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: Mina Almasry <almasrymina@...gle.com>,
David Hildenbrand <david@...hat.com>,
"willy@...radead.org" <willy@...radead.org>, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
kernel_team@...ynix.com, kuba@...nel.org,
ilias.apalodimas@...aro.org, harry.yoo@...cle.com, hawk@...nel.org,
akpm@...ux-foundation.org, davem@...emloft.net,
john.fastabend@...il.com, andrew+netdev@...n.ch, toke@...hat.com,
tariqt@...dia.com, edumazet@...gle.com, pabeni@...hat.com,
saeedm@...dia.com, leon@...nel.org, ast@...nel.org,
daniel@...earbox.net, lorenzo.stoakes@...cle.com,
Liam.Howlett@...cle.com, vbabka@...e.cz, rppt@...nel.org,
surenb@...gle.com, mhocko@...e.com, horms@...nel.org,
linux-rdma@...r.kernel.org, bpf@...r.kernel.org,
vishal.moola@...il.com, hannes@...xchg.org, ziy@...dia.com,
jackmanb@...gle.com
Subject: Re: [PATCH net-next v9 3/8] page_pool: access ->pp_magic through
struct netmem_desc in page_pool_page_is_pp()
On Thu, Jul 17, 2025 at 12:08:58PM +0900, Byungchul Park wrote:
> On Sat, Jul 12, 2025 at 02:58:14PM +0100, Pavel Begunkov wrote:
> > On 7/11/25 02:14, Byungchul Park wrote:
> > ...>>> +#ifdef CONFIG_PAGE_POOL
> > > > > +/* XXX: This would better be moved to mm, once mm gets its way to
> > > > > + * identify the type of page for page pool.
> > > > > + */
> > > > > +static inline bool page_pool_page_is_pp(struct page *page)
> > > > > +{
> > > > > + struct netmem_desc *desc = page_to_nmdesc(page);
> > > > > +
> > > > > + return (desc->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > > > > +}
> > > >
> > > > pages can be pp pages (where they have pp fields inside of them) or
> > > > non-pp pages (where they don't have pp fields inside them, because
> > > > they were never allocated from the page_pool).
> > > >
> > > > Casting a page to a netmem_desc, and then checking if the page was a
> > > > pp page doesn't makes sense to me on a fundamental level. The
> > > > netmem_desc is only valid if the page was a pp page in the first
> > > > place. Maybe page_to_nmdesc should reject the cast if the page is not
> > > > a pp page or something.
> > >
> > > Right, as you already know, the current mainline code already has the
> > > same problem but we've been using the werid way so far, in other words,
> > > mm code is checking if it's a pp page or not by using ->pp_magic, but
> > > it's ->lur, ->buddy_list, or ->pcp_list if it's not a pp page.
> > >
> > > Both the mainline code and this patch can make sense *only if* it's
> > > actually a pp page. It's unevitable until mm provides a way to identify
> > > the type of page for page pool. Thoughts?
> > Question to mm folks, can we add a new PGTY for page pool and use
> > that to filter page pool originated pages? Like in the incomplete
> > and untested diff below?
> >
> >
> > commit 8fc2347fb3ff4a3fc7929c70a5a21e1128935d4a
> > Author: Pavel Begunkov <asml.silence@...il.com>
> > Date: Sat Jul 12 14:29:52 2025 +0100
> >
> > net/mm: use PGTY for tracking page pool pages
> >
> > Currently, we use page->pp_magic to determine whether a page belongs to
> > a page pool. It's not ideal as the field is aliased with other page
> > types, and thus needs to to rely on elaborated rules to work. Add a new
> > page type for page pool.
>
> Hi Pavel,
>
> I need this work to be done to remove ->pp_magic in struct page. Will
> you let me work on this work? Or can you please refine and post this
No response I got. Thus, I started. I hope you understand.
Byungchul
> work? If you let me, I will go for this as a separate patch from this
> series.
>
> Byungchul
>
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0ef2ba0c667a..975a013f1f17 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4175,7 +4175,7 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
> > #ifdef CONFIG_PAGE_POOL
> > static inline bool page_pool_page_is_pp(struct page *page)
> > {
> > - return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > + return PageNetpp(page);
> > }
> > #else
> > static inline bool page_pool_page_is_pp(struct page *page)
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 4fe5ee67535b..9bd1dfded2fc 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -957,6 +957,7 @@ enum pagetype {
> > PGTY_zsmalloc = 0xf6,
> > PGTY_unaccepted = 0xf7,
> > PGTY_large_kmalloc = 0xf8,
> > + PGTY_netpp = 0xf9,
> >
> > PGTY_mapcount_underflow = 0xff
> > };
> > @@ -1101,6 +1102,11 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
> > PAGE_TYPE_OPS(Unaccepted, unaccepted, unaccepted)
> > FOLIO_TYPE_OPS(large_kmalloc, large_kmalloc)
> >
> > +/*
> > + * Marks page_pool allocated pages
> > + */
> > +PAGE_TYPE_OPS(Netpp, netpp, netpp)
> > +
> > /**
> > * PageHuge - Determine if the page belongs to hugetlbfs
> > * @page: The page to test.
> > diff --git a/include/net/netmem.h b/include/net/netmem.h
> > index de1d95f04076..20f5dbb08149 100644
> > --- a/include/net/netmem.h
> > +++ b/include/net/netmem.h
> > @@ -113,6 +113,8 @@ static inline bool netmem_is_net_iov(const netmem_ref netmem)
> > */
> > static inline struct page *__netmem_to_page(netmem_ref netmem)
> > {
> > + DEBUG_NET_WARN_ON_ONCE(netmem_is_net_iov(netmem));
> > +
> > return (__force struct page *)netmem;
> > }
> >
> > diff --git a/net/core/netmem_priv.h b/net/core/netmem_priv.h
> > index cd95394399b4..e38c64da1a78 100644
> > --- a/net/core/netmem_priv.h
> > +++ b/net/core/netmem_priv.h
> > @@ -13,16 +13,11 @@ static inline void netmem_or_pp_magic(netmem_ref netmem, unsigned long pp_magic)
> > __netmem_clear_lsb(netmem)->pp_magic |= pp_magic;
> > }
> >
> > -static inline void netmem_clear_pp_magic(netmem_ref netmem)
> > -{
> > - WARN_ON_ONCE(__netmem_clear_lsb(netmem)->pp_magic & PP_DMA_INDEX_MASK);
> > -
> > - __netmem_clear_lsb(netmem)->pp_magic = 0;
> > -}
> > -
> > static inline bool netmem_is_pp(netmem_ref netmem)
> > {
> > - return (netmem_get_pp_magic(netmem) & PP_MAGIC_MASK) == PP_SIGNATURE;
> > + if (netmem_is_net_iov(netmem))
> > + return true;
> > + return page_pool_page_is_pp(netmem_to_page(netmem));
> > }
> >
> > static inline void netmem_set_pp(netmem_ref netmem, struct page_pool *pool)
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index 05e2e22a8f7c..52120e2912a6 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -371,6 +371,13 @@ struct page_pool *page_pool_create(const struct page_pool_params *params)
> > }
> > EXPORT_SYMBOL(page_pool_create);
> >
> > +static void page_pool_set_page_pp_info(struct page_pool *pool,
> > + struct page *page)
> > +{
> > + __SetPageNetpp(page);
> > + page_pool_set_pp_info(page_to_netmem(page));
> > +}
> > +
> > static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem);
> >
> > static noinline netmem_ref page_pool_refill_alloc_cache(struct page_pool *pool)
> > @@ -534,7 +541,7 @@ static struct page *__page_pool_alloc_page_order(struct page_pool *pool,
> > }
> >
> > alloc_stat_inc(pool, slow_high_order);
> > - page_pool_set_pp_info(pool, page_to_netmem(page));
> > + page_pool_set_page_pp_info(pool, page);
> >
> > /* Track how many pages are held 'in-flight' */
> > pool->pages_state_hold_cnt++;
> > @@ -579,7 +586,7 @@ static noinline netmem_ref __page_pool_alloc_netmems_slow(struct page_pool *pool
> > continue;
> > }
> >
> > - page_pool_set_pp_info(pool, netmem);
> > + page_pool_set_page_pp_info(pool, __netmem_to_page(netmem));
> > pool->alloc.cache[pool->alloc.count++] = netmem;
> > /* Track how many pages are held 'in-flight' */
> > pool->pages_state_hold_cnt++;
> > @@ -654,7 +661,6 @@ s32 page_pool_inflight(const struct page_pool *pool, bool strict)
> > void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
> > {
> > netmem_set_pp(netmem, pool);
> > - netmem_or_pp_magic(netmem, PP_SIGNATURE);
> >
> > /* Ensuring all pages have been split into one fragment initially:
> > * page_pool_set_pp_info() is only called once for every page when it
> > @@ -669,7 +675,6 @@ void page_pool_set_pp_info(struct page_pool *pool, netmem_ref netmem)
> >
> > void page_pool_clear_pp_info(netmem_ref netmem)
> > {
> > - netmem_clear_pp_magic(netmem);
> > netmem_set_pp(netmem, NULL);
> > }
> >
> > @@ -730,8 +735,11 @@ static void page_pool_return_netmem(struct page_pool *pool, netmem_ref netmem)
> > trace_page_pool_state_release(pool, netmem, count);
> >
> > if (put) {
> > + struct page *page = netmem_to_page(netmem);
> > +
> > page_pool_clear_pp_info(netmem);
> > - put_page(netmem_to_page(netmem));
> > + __ClearPageNetpp(page);
> > + put_page(page);
> > }
> > /* An optimization would be to call __free_pages(page, pool->p.order)
> > * knowing page is not part of page-cache (thus avoiding a
> >
> > --
> > Pavel Begunkov
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