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    SubjectRe: [PATCH net-next v11 1/6] page_pool: fragment API support for 32-bit arch with 64-bit DMA
    Hi Yunsheng

    re-sending, HTML was somehow enabled and the ML dropped the public part.

    Apologies for the late reply (and the noise)


    On Fri, 13 Oct 2023 at 09:47, Yunsheng Lin <linyunsheng@huawei.com> wrote:
    >
    > Currently page_pool_alloc_frag() is not supported in 32-bit
    > arch with 64-bit DMA because of the overlap issue between
    > pp_frag_count and dma_addr_upper in 'struct page' for those
    > arches, which seems to be quite common, see [1], which means
    > driver may need to handle it when using fragment API.
    >
    > It is assumed that the combination of the above arch with an
    > address space >16TB does not exist, as all those arches have
    > 64b equivalent, it seems logical to use the 64b version for a
    > system with a large address space. It is also assumed that dma
    > address is page aligned when we are dma mapping a page aligned
    > buffer, see [2].
    >
    > That means we're storing 12 bits of 0 at the lower end for a
    > dma address, we can reuse those bits for the above arches to
    > support 32b+12b, which is 16TB of memory.
    >
    > If we make a wrong assumption, a warning is emitted so that
    > user can report to us.
    >
    > 1. https://lore.kernel.org/all/20211117075652.58299-1-linyunsheng@huawei.com/
    > 2. https://lore.kernel.org/all/20230818145145.4b357c89@kernel.org/
    >
    > Tested-by: Alexander Lobakin <aleksander.lobakin@intel.com>
    > Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    > Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
    > CC: Lorenzo Bianconi <lorenzo@kernel.org>
    > CC: Alexander Duyck <alexander.duyck@gmail.com>
    > CC: Liang Chen <liangchen.linux@gmail.com>
    > CC: Alexander Lobakin <aleksander.lobakin@intel.com>
    > CC: Guillaume Tucker <guillaume.tucker@collabora.com>
    > CC: Matthew Wilcox <willy@infradead.org>
    > CC: Linux-MM <linux-mm@kvack.org>
    > ---
    > include/linux/mm_types.h | 13 +------------
    > include/net/page_pool/helpers.h | 20 ++++++++++++++------
    > net/core/page_pool.c | 14 +++++++++-----
    > 3 files changed, 24 insertions(+), 23 deletions(-)
    >
    > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
    > index 36c5b43999e6..74b49c4c7a52 100644
    > --- a/include/linux/mm_types.h
    > +++ b/include/linux/mm_types.h
    > @@ -125,18 +125,7 @@ struct page {
    > struct page_pool *pp;
    > unsigned long _pp_mapping_pad;
    > unsigned long dma_addr;
    > - union {
    > - /**
    > - * dma_addr_upper: might require a 64-bit
    > - * value on 32-bit architectures.
    > - */
    > - unsigned long dma_addr_upper;
    > - /**
    > - * For frag page support, not supported in
    > - * 32-bit architectures with 64-bit DMA.
    > - */
    > - atomic_long_t pp_frag_count;
    > - };
    > + atomic_long_t pp_frag_count;
    > };
    > struct { /* Tail pages of compound page */
    > unsigned long compound_head; /* Bit zero is set */
    > diff --git a/include/net/page_pool/helpers.h b/include/net/page_pool/helpers.h
    > index 8e7751464ff5..8f64adf86f5b 100644
    > --- a/include/net/page_pool/helpers.h
    > +++ b/include/net/page_pool/helpers.h
    > @@ -197,7 +197,7 @@ static inline void page_pool_recycle_direct(struct page_pool *pool,
    > page_pool_put_full_page(pool, page, true);
    > }
    >
    > -#define PAGE_POOL_DMA_USE_PP_FRAG_COUNT \
    > +#define PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA \
    > (sizeof(dma_addr_t) > sizeof(unsigned long))
    >
    > /**
    > @@ -211,17 +211,25 @@ static inline dma_addr_t page_pool_get_dma_addr(struct page *page)
    > {
    > dma_addr_t ret = page->dma_addr;
    >
    > - if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
    > - ret |= (dma_addr_t)page->dma_addr_upper << 16 << 16;
    > + if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA)
    > + ret <<= PAGE_SHIFT;
    >
    > return ret;
    > }
    >
    > -static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
    > +static inline bool page_pool_set_dma_addr(struct page *page, dma_addr_t addr)
    > {
    > + if (PAGE_POOL_32BIT_ARCH_WITH_64BIT_DMA) {
    > + page->dma_addr = addr >> PAGE_SHIFT;
    > +
    > + /* We assume page alignment to shave off bottom bits,
    > + * if this "compression" doesn't work we need to drop.
    > + */
    > + return addr != (dma_addr_t)page->dma_addr << PAGE_SHIFT;
    > + }
    > +
    > page->dma_addr = addr;
    > - if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
    > - page->dma_addr_upper = upper_32_bits(addr);
    > + return false;
    > }
    >
    > static inline bool page_pool_put(struct page_pool *pool)
    > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
    > index 77cb75e63aca..8a9868ea5067 100644
    > --- a/net/core/page_pool.c
    > +++ b/net/core/page_pool.c
    > @@ -211,10 +211,6 @@ static int page_pool_init(struct page_pool *pool,
    > */
    > }
    >
    > - if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&
    > - pool->p.flags & PP_FLAG_PAGE_FRAG)
    > - return -EINVAL;
    > -
    > #ifdef CONFIG_PAGE_POOL_STATS
    > pool->recycle_stats = alloc_percpu(struct page_pool_recycle_stats);
    > if (!pool->recycle_stats)
    > @@ -359,12 +355,20 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
    > if (dma_mapping_error(pool->p.dev, dma))
    > return false;
    >
    > - page_pool_set_dma_addr(page, dma);
    > + if (page_pool_set_dma_addr(page, dma))
    > + goto unmap_failed;
    >
    > if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
    > page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
    >
    > return true;
    > +
    > +unmap_failed:
    > + WARN_ON_ONCE("unexpected DMA address, please report to netdev@");
    > + dma_unmap_page_attrs(pool->p.dev, dma,
    > + PAGE_SIZE << pool->p.order, pool->p.dma_dir,
    > + DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_WEAK_ORDERING);
    > + return false;
    > }
    >
    > static void page_pool_set_pp_info(struct page_pool *pool,
    > --
    > 2.33.0
    >

    That looks fine wrt what we discussed with Jakub,

    Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

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