Re: [PATCH net-next v6 2/2] page_pool: Track DMA-mapped pages and unmap them when destroying the pool

Pavel Begunkov
2025-04-02T11:39:11Z

On 4/2/25 12:10, Toke Høiland-Jørgensen wrote:
> Pavel Begunkov <asml.silence@gmail.com> writes:
>
>> On 4/1/25 10:27, Toke Høiland-Jørgensen wrote:
>> ...
>>> Reported-by: Yonglong Liu <liuyonglong@huawei.com>
>>> Closes: https://lore.kernel.org/r/8743264a-9700-4227-a556-5f931c720211@huawei.com
>>> Fixes: ff7d6b27f894 ("page_pool: refurbish version of page_pool code")
>>> Suggested-by: Mina Almasry <almasrymina@google.com>
>>> Reviewed-by: Mina Almasry <almasrymina@google.com>
>>> Reviewed-by: Jesper Dangaard Brouer <hawk@kernel.org>
>>> Tested-by: Jesper Dangaard Brouer <hawk@kernel.org>
>>> Tested-by: Qiuling Ren <qren@redhat.com>
>>> Tested-by: Yuying Ma <yuma@redhat.com>
>>> Tested-by: Yonglong Liu <liuyonglong@huawei.com>
>>> Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>
>> I haven't looked into the bit carving, but the rest looks
>> good to me. A few nits below,
>>
>> ...
>>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>>> index 7745ad924ae2d801580a6760eba9393e1cf67b01..52b5ddab7ecb405066fd55b8d61abfd4186b9dcf 100644
>>> --- a/net/core/page_pool.c
>>> +++ b/net/core/page_pool.c
>>> @@ -227,6 +227,8 @@ static int page_pool_init(struct page_pool *pool,
>>> return -EINVAL;
>>>
>>> pool->dma_map = true;
>>> +
>>> + xa_init_flags(&pool->dma_mapped, XA_FLAGS_ALLOC1);
>>
>> nit: might be better to init/destroy unconditionally, it doesn't
>> allocate any memory.
>
> Hmm, yeah, suppose both could work; I do think this makes it clearer
> that it's tied to DMA mapping, but I won't insist. Not sure it's worth
> respinning just for this, though (see below).

That's a somewhat safer way, but yes, I agree, it's not worth of
a respin.

>
>>> }
>>>
>>> if (pool->slow.flags & PP_FLAG_DMA_SYNC_DEV) {
>>> @@ -276,9 +278,6 @@ static int page_pool_init(struct page_pool *pool,
>>> /* Driver calling page_pool_create() also call page_pool_destroy() */
>>> refcount_set(&pool->user_cnt, 1);
>>>
>>> - if (pool->dma_map)
>>> - get_device(pool->p.dev);
>>> -
>>> if (pool->slow.flags & PP_FLAG_ALLOW_UNREADABLE_NETMEM) {
>>> netdev_assert_locked(pool->slow.netdev);
>>> rxq = __netif_get_rx_queue(pool->slow.netdev,
>>> @@ -322,7 +321,7 @@ static void page_pool_uninit(struct page_pool *pool)
>>> ptr_ring_cleanup(&pool->ring, NULL);
>>>
>>> if (pool->dma_map)
>>> - put_device(pool->p.dev);
>>> + xa_destroy(&pool->dma_mapped);
>>>
>>> #ifdef CONFIG_PAGE_POOL_STATS
>>> if (!pool->system)
>>> @@ -463,13 +462,21 @@ page_pool_dma_sync_for_device(const struct page_pool *pool,
>>> netmem_ref netmem,
>>> u32 dma_sync_size)
>>> {
>>> - if (pool->dma_sync && dma_dev_need_sync(pool->p.dev))
>>> - __page_pool_dma_sync_for_device(pool, netmem, dma_sync_size);
>>> + if (READ_ONCE(pool->dma_sync) && dma_dev_need_sync(pool->p.dev)) {
>>> + rcu_read_lock();
>>> + /* re-check under rcu_read_lock() to sync with page_pool_scrub() */
>>> + if (READ_ONCE(pool->dma_sync))
>>> + __page_pool_dma_sync_for_device(pool, netmem,
>>> + dma_sync_size);
>>> + rcu_read_unlock();
>>> + }
>>> }
>>>
>>> -static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem)
>>> +static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem, gfp_t gfp)
>>> {
>>> dma_addr_t dma;
>>> + int err;
>>> + u32 id;
>>>
>>> /* 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
>>> @@ -483,15 +490,28 @@ static bool page_pool_dma_map(struct page_pool *pool, netmem_ref netmem)
>>> if (dma_mapping_error(pool->p.dev, dma))
>>> return false;
>>>
>>> - if (page_pool_set_dma_addr_netmem(netmem, dma))
>>> + if (in_softirq())
>>> + err = xa_alloc(&pool->dma_mapped, &id, netmem_to_page(netmem),
>>> + PP_DMA_INDEX_LIMIT, gfp);
>>> + else
>>> + err = xa_alloc_bh(&pool->dma_mapped, &id, netmem_to_page(netmem),
>>> + PP_DMA_INDEX_LIMIT, gfp);
>>
>> Is it an optimisation? bh disable should be reentrable and could
>> just be xa_alloc_bh().
>
> Yeah, it's an optimisation. We do the same thing in
> page_pool_recycle_in_ring(), so I just kept the same pattern.

Got it

>
>> KERN_{NOTICE,INFO} Maybe?
>
> Erm? Was this supposed to be part of the comment below?

Oops, yes, that's for the warning below

>>> + if (err) {
>>> + WARN_ONCE(1, "couldn't track DMA mapping, please report to netdev@");
>>
>> That can happen with enough memory pressure, I don't think
>> it should be a warning. Maybe some pr_info?
>
> So my reasoning here was that this code is only called in the alloc
> path, so if we're under memory pressure, the page allocation itself
> should fail before the xarray alloc does. And if it doesn't (i.e., if
> the use of xarray itself causes allocation failures), we really want to
> know about it so we can change things. Hence the loud warning.

There is a gap between allocations, one doesn't guarantee
another. I'd say the mental test here is whether we can reasonably
cause it from user space (including by abusive users), because crash
on warning setups exist, and it'll let you know about itself too
loudly, when it could've been tolerated just fine. Not going to
insist though.

> @maintainers, given the comments above I'm not going to respin for this
> unless you tell me to, but let me know! :)
>
> -Toke
>

--
Pavel Begunkov