linux-kernel - Re: [PATCH v2 1/1] xsk: clear page contiguity bit when unmapping pool

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Message-ID: <CAJ8uoz2Gcs+oYorgHofz=0ZOznNoKS8hdyOK=rWwhRtYXO=RHw@mail.gmail.com>
Date:   Tue, 28 Jun 2022 09:47:11 +0200
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Ivan Malov <ivan.malov@...etlabs.ru>
Cc:     Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Network Development <netdev@...r.kernel.org>,
        Andrew Rybchenko <andrew.rybchenko@...etlabs.ru>,
        bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>
Subject: Re: [PATCH v2 1/1] xsk: clear page contiguity bit when unmapping pool

On Tue, Jun 28, 2022 at 2:18 AM Ivan Malov <ivan.malov@...etlabs.ru> wrote:
>
> When a XSK pool gets mapped, xp_check_dma_contiguity() adds bit 0x1
> to pages' DMA addresses that go in ascending order and at 4K stride.
> The problem is that the bit does not get cleared before doing unmap.
> As a result, a lot of warnings from iommu_dma_unmap_page() are seen
> suggesting mapping lookup failures at drivers/iommu/dma-iommu.c:848.

Thanks Ivan for spotting this. Please add if this patch is for bpf or
net in your subject line. E.g., [PATCH net].

Also, I cannot find a warning at drivers/iommu/dma-iommu.c:848. For
net and bpf I have a WARN() at line 679 in __iommu_dma_unmap(). Maybe
it would be better to just refer to __iommu_dma_unmap() and the
warning in that function. Line numbers tend to change.

> Fixes: 2b43470add8c ("xsk: Introduce AF_XDP buffer allocation API")
> Signed-off-by: Ivan Malov <ivan.malov@...etlabs.ru>
> ---
>  v1 -> v2: minor adjustments to dispose of the "Fixes:" tag warning
>
>  net/xdp/xsk_buff_pool.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> index 87bdd71c7bb6..f70112176b7c 100644
> --- a/net/xdp/xsk_buff_pool.c
> +++ b/net/xdp/xsk_buff_pool.c
> @@ -332,6 +332,7 @@ static void __xp_dma_unmap(struct xsk_dma_map *dma_map, unsigned long attrs)
>         for (i = 0; i < dma_map->dma_pages_cnt; i++) {
>                 dma = &dma_map->dma_pages[i];
>                 if (*dma) {
> +                       *dma &= ~XSK_NEXT_PG_CONTIG_MASK;
>                         dma_unmap_page_attrs(dma_map->dev, *dma, PAGE_SIZE,
>                                              DMA_BIDIRECTIONAL, attrs);
>                         *dma = 0;
> --
> 2.30.2
>