From: Byungchul Park <byungchul@sk•com>
To: "David Hildenbrand (Arm)" <david@kernel•org>
Cc: Dragos Tatulea <dtatulea@nvidia•com>,
linux-mm@kvack•org, akpm@linux-foundation•org,
netdev@vger•kernel.org, linux-kernel@vger•kernel.org,
kernel_team@skhynix•com, harry.yoo@oracle•com, ast@kernel•org,
daniel@iogearbox•net, davem@davemloft•net, kuba@kernel•org,
hawk@kernel•org, john.fastabend@gmail•com, sdf@fomichev•me,
saeedm@nvidia•com, leon@kernel•org, tariqt@nvidia•com,
mbloch@nvidia•com, andrew+netdev@lunn•ch, edumazet@google•com,
pabeni@redhat•com, lorenzo.stoakes@oracle•com,
Liam.Howlett@oracle•com, vbabka@suse•cz, rppt@kernel•org,
surenb@google•com, mhocko@suse•com, horms@kernel•org,
jackmanb@google•com, hannes@cmpxchg•org, ziy@nvidia•com,
ilias.apalodimas@linaro•org, willy@infradead•org,
brauner@kernel•org, kas@kernel•org, yuzhao@google•com,
usamaarif642@gmail•com, baolin.wang@linux•alibaba.com,
almasrymina@google•com, toke@redhat•com, asml.silence@gmail•com,
bpf@vger•kernel.org, linux-rdma@vger•kernel.org,
sfr@canb•auug.org.au, dw@davidwei•uk, ap420073@gmail•com
Subject: Re: [PATCH v4] mm: introduce a new page type for page pool in page type
Date: Wed, 13 May 2026 21:39:20 +0900 [thread overview]
Message-ID: <20260513123920.GA51788@system.software.com> (raw)
In-Reply-To: <8348d867-f8a1-432c-be2d-699ad96b2e93@kernel.org>
On Wed, May 13, 2026 at 02:29:46PM +0200, David Hildenbrand (Arm) wrote:
On 5/13/26 14:18, Byungchul Park wrote:
> > On Wed, May 13, 2026 at 11:00:51AM +0200, Dragos Tatulea wrote:
> >> On 24.02.26 06:13, Byungchul Park wrote:
> >>> Currently, the condition 'page->pp_magic == PP_SIGNATURE' is used to
> >>> determine if a page belongs to a page pool. However, with the planned
> >>> removal of @pp_magic, we should instead leverage the page_type in struct
> >>> page, such as PGTY_netpp, for this purpose.
> >>>
> >>> Introduce and use the page type APIs e.g. PageNetpp(), __SetPageNetpp(),
> >>> and __ClearPageNetpp() instead, and remove the existing APIs accessing
> >>> @pp_magic e.g. page_pool_page_is_pp(), netmem_or_pp_magic(), and
> >>> netmem_clear_pp_magic().
> >>>
> >>> Plus, add @page_type to struct net_iov at the same offset as struct page
> >>> so as to use the page_type APIs for struct net_iov as well. While at it,
> >>> reorder @type and @owner in struct net_iov to avoid a hole and
> >>> increasing the struct size.
> >>>
> >>> This work was inspired by the following link:
> >>>
> >>> https://lore.kernel.org/all/582f41c0-2742-4400-9c81-0d46bf4e8314@gmail.com/
> >>>
> >>> While at it, move the sanity check for page pool to on the free path.
> >>>
> >>> Suggested-by: David Hildenbrand <david@redhat•com>
> >>> Co-developed-by: Pavel Begunkov <asml.silence@gmail•com>
> >>> Signed-off-by: Pavel Begunkov <asml.silence@gmail•com>
> >>> Signed-off-by: Byungchul Park <byungchul@sk•com>
> >>> Acked-by: David Hildenbrand <david@redhat•com>
> >>> Acked-by: Zi Yan <ziy@nvidia•com>
> >>> Acked-by: Vlastimil Babka <vbabka@suse•cz>
> >>> Reviewed-by: Toke Høiland-Jørgensen <toke@redhat•com>
> >>> ---
> >>
> >> Seems like this patch broke tcp_mmap because
> >> validate_page_before_insert() returns -EINVAL due
> >> to a page having a type. Here's the full flow:
> >>
> >> getsockopt(TCP_ZEROCOPY_RECEIVE) returns -EINVAL because of the
> >> below flow in the kernel:
> >>
> >> tcp_zerocopy_receive()
> >> -> tcp_zerocopy_vm_insert_batch()
> >> -> vm_insert_pages()
> >> -> insert_pages()
> >> -> insert_page_in_batch_locked()
> >> -> validate_page_before_insert() returns -EINVAL
> >> because page_has_type(page) is now true.
> >>
> >> The patch below fixes the issue. But is this a valid fix?
> >
> > Hi,
> >
> > The problem comes from the fact that page_type and _mapcount are
> > union'ed but there is a case where these two information should be kept
> > at the same time.
> >
> > Why don't we allow these two information can be kept in the 4 bytes at
> > the same time until Zi Yan's work on _mapcount and page_type will be
> > done, instead of taking a step back?
> >
> > It can be more optimized but I suggest the approach I just mentioned:
> > ---
> > diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> > index 64dc44832808..e5ec204866dc 100644
> > --- a/fs/proc/internal.h
> > +++ b/fs/proc/internal.h
> > @@ -185,8 +185,7 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
> > {
> > int mapcount = atomic_read(&page->_mapcount) + 1;
> >
> > - if (page_mapcount_is_type(mapcount))
> > - mapcount = 0;
> > + mapcount = page_mapcount_clear_type(mapcount);
> > if (folio_test_large(folio))
> > mapcount += folio_entire_mapcount(folio);
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 8260e28205e9..f45064796313 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1865,8 +1865,7 @@ static inline int folio_mapcount(const struct folio *folio)
> >
> > if (likely(!folio_test_large(folio))) {
> > mapcount = atomic_read(&folio->_mapcount) + 1;
> > - if (page_mapcount_is_type(mapcount))
> > - mapcount = 0;
> > + mapcount = page_mapcount_clear_type(mapcount);
> > return mapcount;
> > }
> > return folio_large_mapcount(folio);
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 0e03d816e8b9..f3b0d1fa262d 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -934,9 +934,9 @@ static inline bool page_type_has_type(int page_type)
> > }
> >
> > /* This takes a mapcount which is one more than page->_mapcount */
> > -static inline bool page_mapcount_is_type(unsigned int mapcount)
> > +static inline unsigned int page_mapcount_clear_type(unsigned int mapcount)
> > {
> > - return page_type_has_type(mapcount - 1);
> > + return (unsigned int)(((int)(mapcount << 8)) >> 8);
> > }
> >
> > static inline bool page_has_type(const struct page *page)
> > @@ -953,16 +953,20 @@ static __always_inline void __folio_set_##fname(struct folio *folio) \
> > { \
> > if (folio_test_##fname(folio)) \
> > return; \
> > - VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX, \
> > + VM_BUG_ON_FOLIO(page_type_has_type(data_race(folio->page.page_type)), \
> > folio); \
> > - folio->page.page_type = (unsigned int)PGTY_##lname << 24; \
> > + folio->page.page_type &= ~(PGTY_mapcount_underflow << 24); \
> > + folio->page.page_type |= (unsigned int)PGTY_##lname << 24; \
> > } \
> > static __always_inline void __folio_clear_##fname(struct folio *folio) \
> > { \
> > - if (folio->page.page_type == UINT_MAX) \
> > + int mapcount; \
> > + \
> > + if (!page_type_has_type(folio->page.page_type)) \
> > return; \
> > VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio); \
> > - folio->page.page_type = UINT_MAX; \
> > + mapcount = atomic_read(&folio->page._mapcount); \
> > + folio->page.page_type = page_mapcount_clear_type(mapcount); \
> > }
> >
> > #define PAGE_TYPE_OPS(uname, lname, fname) \
> > @@ -975,15 +979,20 @@ static __always_inline void __SetPage##uname(struct page *page) \
> > { \
> > if (Page##uname(page)) \
> > return; \
> > - VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page); \
> > - page->page_type = (unsigned int)PGTY_##lname << 24; \
> > + VM_BUG_ON_PAGE(page_type_has_type(data_race(page->page_type)), \
> > + page); \
> > + page->page_type &= ~(PGTY_mapcount_underflow << 24); \
> > + page->page_type |= (unsigned int)PGTY_##lname << 24; \
> > } \
> > static __always_inline void __ClearPage##uname(struct page *page) \
> > { \
> > - if (page->page_type == UINT_MAX) \
> > + int mapcount; \
> > + \
> > + if (!page_type_has_type(page->page_type)) \
> > return; \
> > VM_BUG_ON_PAGE(!Page##uname(page), page); \
> > - page->page_type = UINT_MAX; \
> > + mapcount = atomic_read(&page->_mapcount); \
> > + page->page_type = page_mapcount_clear_type(mapcount); \
> > }
> >
> > /*
> > diff --git a/mm/debug.c b/mm/debug.c
> > index 77fa8fe1d641..9a932ded09d4 100644
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> > @@ -74,8 +74,7 @@ static void __dump_folio(const struct folio *folio, const struct page *page,
> > int mapcount = atomic_read(&page->_mapcount) + 1;
> > char *type = "";
> >
> > - if (page_mapcount_is_type(mapcount))
> > - mapcount = 0;
> > + mapcount = page_mapcount_clear_type(mapcount);
> >
> > pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
> > folio_ref_count(folio), mapcount, mapping,
> > ---
> >
> > Thoughts?
>
> God no.
This is not final patch, but for sharing the rough idea *with code* -
maybe there are more points in code that should be adjusted by the
change. I just typed the draft patch quick just for sharing idea.
If we should allow pp type pages to be used in mapping as well, then
we should allow a page to keep both its type and mapcount at the same
time. Am I missing something?
Byungchul
> --
> Cheers,
>
> David
next prev parent reply other threads:[~2026-05-13 12:39 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-02-24 5:13 [PATCH v4] mm: introduce a new page type for page pool in page type Byungchul Park
2026-02-25 7:19 ` Mike Rapoport
2026-02-26 18:49 ` Johannes Weiner
2026-03-16 22:29 ` Byungchul Park
2026-03-16 22:31 ` [PATCH v5] " Byungchul Park
2026-03-17 9:20 ` Jesper Dangaard Brouer
2026-03-17 10:03 ` Ilias Apalodimas
2026-03-17 11:06 ` Dragos Tatulea
2026-03-19 23:31 ` Jakub Kicinski
2026-03-18 2:02 ` Byungchul Park
2026-03-20 11:44 ` Jesper Dangaard Brouer
2026-03-23 12:16 ` Ilias Apalodimas
2026-03-19 23:31 ` Jakub Kicinski
2026-05-13 9:00 ` [PATCH v4] " Dragos Tatulea
2026-05-13 9:12 ` Vlastimil Babka (SUSE)
2026-05-13 9:26 ` Pedro Falcato
2026-05-13 9:36 ` David Hildenbrand (Arm)
2026-05-13 12:06 ` Dragos Tatulea
2026-05-13 12:11 ` David Hildenbrand (Arm)
2026-05-14 8:54 ` Byungchul Park
2026-05-14 9:24 ` Dragos Tatulea
2026-05-15 0:01 ` Byungchul Park
2026-05-13 9:34 ` David Hildenbrand (Arm)
2026-05-13 12:18 ` Byungchul Park
2026-05-13 12:29 ` David Hildenbrand (Arm)
2026-05-13 12:39 ` Byungchul Park [this message]
2026-05-13 13:02 ` David Hildenbrand (Arm)
2026-05-13 13:26 ` Byungchul Park
2026-05-18 9:33 ` David Hildenbrand (Arm)
2026-05-13 9:42 ` Lorenzo Stoakes
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