Message-ID: <CAF=yD-L0ajGVrexnOVvvhC-A7vw6XP9tq5T3HCDTjQMS0mXdTQ@mail.gmail.com>
Date: Sat, 19 Aug 2023 10:08:51 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org, 
	brouer@...hat.com, "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, Ilias Apalodimas <ilias.apalodimas@...aro.org>, 
	Arnd Bergmann <arnd@...db.de>, David Ahern <dsahern@...nel.org>, 
	Sumit Semwal <sumit.semwal@...aro.org>, Christian König <christian.koenig@....com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Hari Ramakrishnan <rharix@...gle.com>, 
	Dan Williams <dan.j.williams@...el.com>, Andy Lutomirski <luto@...nel.org>, stephen@...workplumber.org, 
	sdf@...gle.com
Subject: Re: [RFC PATCH v2 06/11] page-pool: add device memory support

On Sat, Aug 19, 2023 at 5:51 AM Jesper Dangaard Brouer
<jbrouer@...hat.com> wrote:
>
>
>
> On 10/08/2023 03.57, Mina Almasry wrote:
> > Overload the LSB of struct page* to indicate that it's a page_pool_iov.
> >
> > Refactor mm calls on struct page * into helpers, and add page_pool_iov
> > handling on those helpers. Modify callers of these mm APIs with calls to
> > these helpers instead.
> >
>
> I don't like of this approach.
> This is adding code to the PP (page_pool) fast-path in multiple places.
>
> I've not had time to run my usual benchmarks, which are here:
>
> https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/bench_page_pool_simple.c
>
> But I'm sure it will affect performance.
>
> Regardless of performance, this approach is using ptr-LSB-bits, to hide
> that page-pointer are not really struct-pages, feels like force feeding
> a solution just to use the page_pool APIs.
>
>
> > In areas where struct page* is dereferenced, add a check for special
> > handling of page_pool_iov.
> >
> > The memory providers producing page_pool_iov can set the LSB on the
> > struct page* returned to the page pool.
> >
> > Note that instead of overloading the LSB of page pointers, we can
> > instead define a new union between struct page & struct page_pool_iov and
> > compact it in a new type. However, we'd need to implement the code churn
> > to modify the page_pool & drivers to use this new type. For this POC
> > that is not implemented (feedback welcome).
> >
>
> I've said before, that I prefer multiplexing on page->pp_magic.
> For your page_pool_iov the layout would have to match the offset of
> pp_magic, to do this. (And if insisting on using PP infra the refcnt
> would also need to align).

Perhaps I misunderstand, but this suggests continuing to using
struct page to demultiplex memory type?

I think the feedback has been strong to not multiplex yet another
memory type into that struct, that is not a real page. Which is why
we went into this direction. This latest series limits the impact largely
to networking structures and code.

One way or another, there will be a branch and multiplexing. Whether
that is in struct page, the page pool or a new netdev mem type as you
propose.

Any regression in page pool can be avoided in the common case that
does not use device mem by placing that behind a static_branch. Would
that address your performance concerns?

>
> On the allocation side, all drivers already use a driver helper
> page_pool_dev_alloc_pages() or we could add another (better named)
> helper to multiplex between other types of allocators, e.g. a devmem
> allocator.
>
> On free/return/recycle the functions napi_pp_put_page or skb_pp_recycle
> could multiplex on pp_magic and call another API.  The API could be an
> extension to PP helpers, but it could also be a devmap allocator helper.
>
> IMHO forcing/piggy-bagging everything into page_pool is not the right
> solution.  I really think netstack need to support different allocator
> types.

To me this is lifting page_pool into such a netstack alloctator pool.

Not sure adding another explicit layer of indirection would be cleaner
or faster (potentially more indirect calls).

As for the LSB trick: that avoided adding a lot of boilerplate churn
with new type and helper functions.



> The page pool have been leading the way, yes, but perhaps it is
> time to add an API layer that e.g. could be named netmem, that gives us
> the multiplexing between allocators.  In that process some of page_pool
> APIs would be lifted out as common blocks and others remain.
>
> --Jesper
>
> > I have a sample implementation of adding a new page_pool_token type
> > in the page_pool to give a general idea here:
> > https://github.com/torvalds/linux/commit/3a7628700eb7fd02a117db036003bca50779608d
> >
> > Full branch here:
> > https://github.com/torvalds/linux/compare/master...mina:linux:tcpdevmem-pp-tokens
> >
> > (In the branches above, page_pool_iov is called devmem_slice).
> >
> > Could also add static_branch to speed up the checks in page_pool_iov
> > memory providers are being used.
> >
> > Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> > ---
> >   include/net/page_pool.h | 74 ++++++++++++++++++++++++++++++++++-
> >   net/core/page_pool.c    | 85 ++++++++++++++++++++++++++++-------------
> >   2 files changed, 131 insertions(+), 28 deletions(-)
> >
> > diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> > index 537eb36115ed..f08ca230d68e 100644
> > --- a/include/net/page_pool.h
> > +++ b/include/net/page_pool.h
> > @@ -282,6 +282,64 @@ static inline struct page_pool_iov *page_to_page_pool_iov(struct page *page)
> >       return NULL;
> >   }
> >
> > +static inline int page_pool_page_ref_count(struct page *page)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return page_pool_iov_refcount(page_to_page_pool_iov(page));
> > +
> > +     return page_ref_count(page);
> > +}
> > +
> > +static inline void page_pool_page_get_many(struct page *page,
> > +                                        unsigned int count)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return page_pool_iov_get_many(page_to_page_pool_iov(page),
> > +                                           count);
> > +
> > +     return page_ref_add(page, count);
> > +}
> > +
> > +static inline void page_pool_page_put_many(struct page *page,
> > +                                        unsigned int count)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return page_pool_iov_put_many(page_to_page_pool_iov(page),
> > +                                           count);
> > +
> > +     if (count > 1)
> > +             page_ref_sub(page, count - 1);
> > +
> > +     put_page(page);
> > +}
> > +
> > +static inline bool page_pool_page_is_pfmemalloc(struct page *page)
> > +{
> > +     if (page_is_page_pool_iov(page))
> > +             return false;
> > +
> > +     return page_is_pfmemalloc(page);
> > +}
> > +
> > +static inline bool page_pool_page_is_pref_nid(struct page *page, int pref_nid)
> > +{
> > +     /* Assume page_pool_iov are on the preferred node without actually
> > +      * checking...
> > +      *
> > +      * This check is only used to check for recycling memory in the page
> > +      * pool's fast paths. Currently the only implementation of page_pool_iov
> > +      * is dmabuf device memory. It's a deliberate decision by the user to
> > +      * bind a certain dmabuf to a certain netdev, and the netdev rx queue
> > +      * would not be able to reallocate memory from another dmabuf that
> > +      * exists on the preferred node, so, this check doesn't make much sense
> > +      * in this case. Assume all page_pool_iovs can be recycled for now.
> > +      */
> > +     if (page_is_page_pool_iov(page))
> > +             return true;
> > +
> > +     return page_to_nid(page) == pref_nid;
> > +}
> > +
> >   struct page_pool {
> >       struct page_pool_params p;
> >
> > @@ -434,6 +492,9 @@ static inline long page_pool_defrag_page(struct page *page, long nr)
> >   {
> >       long ret;
> >
> > +     if (page_is_page_pool_iov(page))
> > +             return -EINVAL;
> > +
> >       /* If nr == pp_frag_count then we have cleared all remaining
> >        * references to the page. No need to actually overwrite it, instead
> >        * we can leave this to be overwritten by the calling function.
> > @@ -494,7 +555,12 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
> >
> >   static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> >   {
> > -     dma_addr_t ret = page->dma_addr;
> > +     dma_addr_t ret;
> > +
> > +     if (page_is_page_pool_iov(page))
> > +             return page_pool_iov_dma_addr(page_to_page_pool_iov(page));
> > +
> > +     ret = page->dma_addr;
> >
> >       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> >               ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
> > @@ -504,6 +570,12 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
> >
> >   static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
> >   {
> > +     /* page_pool_iovs are mapped and their dma-addr can't be modified. */
> > +     if (page_is_page_pool_iov(page)) {
> > +             DEBUG_NET_WARN_ON_ONCE(true);
> > +             return;
> > +     }
> > +
> >       page->dma_addr = addr;
> >       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
> >               page->dma_addr_upper = upper_32_bits(addr);
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index 0a7c08d748b8..20c1f74fd844 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -318,7 +318,7 @@ static struct page *page_pool_refill_alloc_cache(struct page_pool *pool)
> >               if (unlikely(!page))
> >                       break;
> >
> > -             if (likely(page_to_nid(page) == pref_nid)) {
> > +             if (likely(page_pool_page_is_pref_nid(page, pref_nid))) {
> >                       pool->alloc.cache[pool->alloc.count++] = page;
> >               } else {
> >                       /* NUMA mismatch;
> > @@ -363,7 +363,15 @@ static void page_pool_dma_sync_for_device(struct page_pool *pool,
> >                                         struct page *page,
> >                                         unsigned int dma_sync_size)
> >   {
> > -     dma_addr_t dma_addr = page_pool_get_dma_addr(page);
> > +     dma_addr_t dma_addr;
> > +
> > +     /* page_pool_iov memory provider do not support PP_FLAG_DMA_SYNC_DEV */
> > +     if (page_is_page_pool_iov(page)) {
> > +             DEBUG_NET_WARN_ON_ONCE(true);
> > +             return;
> > +     }
> > +
> > +     dma_addr = page_pool_get_dma_addr(page);
> >
> >       dma_sync_size = min(dma_sync_size, pool->p.max_len);
> >       dma_sync_single_range_for_device(pool->p.dev, dma_addr,
> > @@ -375,6 +383,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
> >   {
> >       dma_addr_t dma;
> >
> > +     if (page_is_page_pool_iov(page)) {
> > +             /* page_pool_iovs are already mapped */
> > +             DEBUG_NET_WARN_ON_ONCE(true);
> > +             return true;
> > +     }
> > +
> >       /* Setup DMA mapping: use 'struct page' area for storing DMA-addr
> >        * since dma_addr_t can be either 32 or 64 bits and does not always fit
> >        * into page private data (i.e 32bit cpu with 64bit DMA caps)
> > @@ -398,14 +412,24 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
> >   static void page_pool_set_pp_info(struct page_pool *pool,
> >                                 struct page *page)
> >   {
> > -     page->pp = pool;
> > -     page->pp_magic |= PP_SIGNATURE;
> > +     if (!page_is_page_pool_iov(page)) {
> > +             page->pp = pool;
> > +             page->pp_magic |= PP_SIGNATURE;
> > +     } else {
> > +             page_to_page_pool_iov(page)->pp = pool;
> > +     }
> > +
> >       if (pool->p.init_callback)
> >               pool->p.init_callback(page, pool->p.init_arg);
> >   }
> >
> >   static void page_pool_clear_pp_info(struct page *page)
> >   {
> > +     if (page_is_page_pool_iov(page)) {
> > +             page_to_page_pool_iov(page)->pp = NULL;
> > +             return;
> > +     }
> > +
> >       page->pp_magic = 0;
> >       page->pp = NULL;
> >   }
> > @@ -615,7 +639,7 @@ static bool page_pool_recycle_in_cache(struct page *page,
> >               return false;
> >       }
> >
> > -     /* Caller MUST have verified/know (page_ref_count(page) == 1) */
> > +     /* Caller MUST have verified/know (page_pool_page_ref_count(page) == 1) */
> >       pool->alloc.cache[pool->alloc.count++] = page;
> >       recycle_stat_inc(pool, cached);
> >       return true;
> > @@ -638,9 +662,10 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
> >        * refcnt == 1 means page_pool owns page, and can recycle it.
> >        *
> >        * page is NOT reusable when allocated when system is under
> > -      * some pressure. (page_is_pfmemalloc)
> > +      * some pressure. (page_pool_page_is_pfmemalloc)
> >        */
> > -     if (likely(page_ref_count(page) == 1 && !page_is_pfmemalloc(page))) {
> > +     if (likely(page_pool_page_ref_count(page) == 1 &&
> > +                !page_pool_page_is_pfmemalloc(page))) {
> >               /* Read barrier done in page_ref_count / READ_ONCE */
> >
> >               if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> > @@ -741,7 +766,8 @@ static struct page *page_pool_drain_frag(struct page_pool *pool,
> >       if (likely(page_pool_defrag_page(page, drain_count)))
> >               return NULL;
> >
> > -     if (page_ref_count(page) == 1 && !page_is_pfmemalloc(page)) {
> > +     if (page_pool_page_ref_count(page) == 1 &&
> > +         !page_pool_page_is_pfmemalloc(page)) {
> >               if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> >                       page_pool_dma_sync_for_device(pool, page, -1);
> >
> > @@ -818,9 +844,9 @@ static void page_pool_empty_ring(struct page_pool *pool)
> >       /* Empty recycle ring */
> >       while ((page = ptr_ring_consume_bh(&pool->ring))) {
> >               /* Verify the refcnt invariant of cached pages */
> > -             if (!(page_ref_count(page) == 1))
> > +             if (!(page_pool_page_ref_count(page) == 1))
> >                       pr_crit("%s() page_pool refcnt %d violation\n",
> > -                             __func__, page_ref_count(page));
> > +                             __func__, page_pool_page_ref_count(page));
> >
> >               page_pool_return_page(pool, page);
> >       }
> > @@ -977,19 +1003,24 @@ bool page_pool_return_skb_page(struct page *page, bool napi_safe)
> >       struct page_pool *pp;
> >       bool allow_direct;
> >
> > -     page = compound_head(page);
> > +     if (!page_is_page_pool_iov(page)) {
> > +             page = compound_head(page);
> >
> > -     /* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation
> > -      * in order to preserve any existing bits, such as bit 0 for the
> > -      * head page of compound page and bit 1 for pfmemalloc page, so
> > -      * mask those bits for freeing side when doing below checking,
> > -      * and page_is_pfmemalloc() is checked in __page_pool_put_page()
> > -      * to avoid recycling the pfmemalloc page.
> > -      */
> > -     if (unlikely((page->pp_magic & ~0x3UL) != PP_SIGNATURE))
> > -             return false;
> > +             /* page->pp_magic is OR'ed with PP_SIGNATURE after the
> > +              * allocation in order to preserve any existing bits, such as
> > +              * bit 0 for the head page of compound page and bit 1 for
> > +              * pfmemalloc page, so mask those bits for freeing side when
> > +              * doing below checking, and page_pool_page_is_pfmemalloc() is
> > +              * checked in __page_pool_put_page() to avoid recycling the
> > +              * pfmemalloc page.
> > +              */
> > +             if (unlikely((page->pp_magic & ~0x3UL) != PP_SIGNATURE))
> > +                     return false;
> >
> > -     pp = page->pp;
> > +             pp = page->pp;
> > +     } else {
> > +             pp = page_to_page_pool_iov(page)->pp;
> > +     }
> >
> >       /* Allow direct recycle if we have reasons to believe that we are
> >        * in the same context as the consumer would run, so there's
> > @@ -1273,9 +1304,9 @@ static bool mp_huge_busy(struct mp_huge *hu, unsigned int idx)
> >
> >       for (j = 0; j < (1 << MP_HUGE_ORDER); j++) {
> >               page = hu->page[idx] + j;
> > -             if (page_ref_count(page) != 1) {
> > +             if (page_pool_page_ref_count(page) != 1) {
> >                       pr_warn("Page with ref count %d at %u, %u. Can't safely destory, leaking memory!\n",
> > -                             page_ref_count(page), idx, j);
> > +                             page_pool_page_ref_count(page), idx, j);
> >                       return true;
> >               }
> >       }
> > @@ -1330,7 +1361,7 @@ static struct page *mp_huge_alloc_pages(struct page_pool *pool, gfp_t gfp)
> >                       continue;
> >
> >               if ((page->pp_magic & ~0x3UL) == PP_SIGNATURE ||
> > -                 page_ref_count(page) != 1) {
> > +                 page_pool_page_ref_count(page) != 1) {
> >                       atomic_inc(&mp_huge_ins_b);
> >                       continue;
> >               }
> > @@ -1458,9 +1489,9 @@ static void mp_huge_1g_destroy(struct page_pool *pool)
> >       free = true;
> >       for (i = 0; i < MP_HUGE_1G_CNT; i++) {
> >               page = hu->page + i;
> > -             if (page_ref_count(page) != 1) {
> > +             if (page_pool_page_ref_count(page) != 1) {
> >                       pr_warn("Page with ref count %d at %u. Can't safely destory, leaking memory!\n",
> > -                             page_ref_count(page), i);
> > +                             page_pool_page_ref_count(page), i);
> >                       free = false;
> >                       break;
> >               }
> > @@ -1489,7 +1520,7 @@ static struct page *mp_huge_1g_alloc_pages(struct page_pool *pool, gfp_t gfp)
> >               page = hu->page + page_i;
> >
> >               if ((page->pp_magic & ~0x3UL) == PP_SIGNATURE ||
> > -                 page_ref_count(page) != 1) {
> > +                 page_pool_page_ref_count(page) != 1) {
> >                       atomic_inc(&mp_huge_ins_b);
> >                       continue;
> >               }
> > --
> > 2.41.0.640.ga95def55d0-goog
> >
>

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