Re: [net-next v1 09/16] page_pool: device memory support

[Mina Almasry]

On Mon, Dec 11, 2023 at 3:51 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
>
> On 2023/12/11 12:04, Mina Almasry wrote:
> > On Sun, Dec 10, 2023 at 6:26 PM Mina Almasry <almasrymina@xxxxxxxxxx> wrote:
> >>
> >> On Sun, Dec 10, 2023 at 6:04 PM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>>
> >>> On 2023/12/9 0:05, Mina Almasry wrote:
> >>>> On Fri, Dec 8, 2023 at 1:30 AM Yunsheng Lin <linyunsheng@xxxxxxxxxx> wrote:
> >>>>>
> >>>>>
> >>>>> As mentioned before, it seems we need to have the above checking every
> >>>>> time we need to do some per-page handling in page_pool core, is there
> >>>>> a plan in your mind how to remove those kind of checking in the future?
> >>>>>
> >>>>
> >>>> I see 2 ways to remove the checking, both infeasible:
> >>>>
> >>>> 1. Allocate a wrapper struct that pulls out all the fields the page pool needs:
> >>>>
> >>>> struct netmem {
> >>>>         /* common fields */
> >>>>         refcount_t refcount;
> >>>>         bool is_pfmemalloc;
> >>>>         int nid;
> >>>>         ...
> >>>>         union {
> >>>>                 struct dmabuf_genpool_chunk_owner *owner;
> >>>>                 struct page * page;
> >>>>         };
> >>>> };
> >>>>
> >>>> The page pool can then not care if the underlying memory is iov or
> >>>> page. However this introduces significant memory bloat as this struct
> >>>> needs to be allocated for each page or ppiov, which I imagine is not
> >>>> acceptable for the upside of removing a few static_branch'd if
> >>>> statements with no performance cost.
> >>>>
> >>>> 2. Create a unified struct for page and dmabuf memory, which the mm
> >>>> folks have repeatedly nacked, and I imagine will repeatedly nack in
> >>>> the future.
> >>>>
> >>>> So I imagine the special handling of ppiov in some form is critical
> >>>> and the checking may not be removable.
> >>>
> >>> If the above is true, perhaps devmem is not really supposed to be intergated
> >>> into page_pool.
> >>>
> >>> Adding a checking for every per-page handling in page_pool core is just too
> >>> hacky to be really considerred a longterm solution.
> >>>
> >>
> >> The only other option is to implement another page_pool for ppiov and
> >> have the driver create page_pool or ppiov_pool depending on the state
> >> of the netdev_rx_queue (or some helper in the net stack to do that for
> >> the driver). This introduces some code duplication. The ppiov_pool &
> >> page_pool would look similar in implementation.
>
> I think there is a design pattern already to deal with this kind of problem,
> refactoring common code used by both page_pool and ppiov into a library to
> aovid code duplication if most of them have similar implementation.
>

Code can be refactored if it's identical, not if it is similar. I
suspect the page_pools will be only similar, and if you're not willing
to take devmem handling into the page pool then refactoring page_pool
code into helpers that do devmem handling may also not be an option.

> >>
> >> But this was all discussed in detail in RFC v2 and the last response I
> >> heard from Jesper was in favor if this approach, if I understand
> >> correctly:
> >>
> >> https://lore.kernel.org/netdev/7aedc5d5-0daf-63be-21bc-3b724cc1cab9@xxxxxxxxxx/
> >>
> >> Would love to have the maintainer weigh in here.
> >>
> >
> > I should note we may be able to remove some of the checking, but maybe not all.
> >
> > - Checks that disable page fragging for ppiov can be removed once
> > ppiov has frag support (in this series or follow up).
> >
> > - If we use page->pp_frag_count (or page->pp_ref_count) for
> > refcounting ppiov, we can remove the if checking in the refcounting.
> >

I'm not sure this is actually possible in the short term. The
page_pool uses both page->_refcount and page->pp_frag_count for
refcounting, and I will not be able to remove the special handling
around page->_refcount as i'm not allowed to call page_ref_*() APIs on
a non-struct page.

> > - We may be able to store the dma_addr of the ppiov in page->dma_addr,
> > but I'm unsure if that actually works, because the dma_buf dmaddr is
> > dma_addr_t (u32 or u64), but page->dma_addr is unsigned long (4 bytes
> > I think). But if it works for pages I may be able to make it work for
> > ppiov as well.
> >
> > - Checks that obtain the page->pp can work with ppiov if we align the
> > offset of page->pp and ppiov->pp.
> >
> > - Checks around page->pp_magic can be removed if we also have offset
> > aligned ppiov->pp_magic.
> >
> > Sadly I don't see us removing the checking for these other cases:
> >
> > - page_is_pfmemalloc(): I'm not allowed to pass a non-struct page into
> > that helper.
>
> We can do similar trick like above as bit 1 of page->pp_magic is used to
> indicate that if it is a pfmemalloc page.
>

Likely yes.

> >
> > - page_to_nid(): I'm not allowed to pass a non-struct page into that helper.
>
> Yes, this one need special case.
>
> >
> > - page_pool_free_va(): ppiov have no va.
>
> Doesn't the skb_frags_readable() checking will protect the page_pool_free_va()
> from being called on devmem?
>

This function seems to be only called from veth which doesn't support
devmem. I can remove the handling there.

> >
> > - page_pool_sync_for_dev/page_pool_dma_map: ppiov backed by dma-buf
> > fundamentally can't get mapped again.
>
> Can we just fail the page_pool creation with PP_FLAG_DMA_MAP and
> DMA_ATTR_SKIP_CPU_SYNC flags for devmem provider?
>

Jakub says PP_FLAG_DMA_MAP must be enabled for devmem, such that the
page_pool handles the dma mapping of the devmem and the driver doesn't
use it on its own.

We may fail creating the page pool on  PP_FLAG_DMA_SYNC_DEV maybe, and
remove the checking from page_pool_sync_for_dev(), I think.

> >
> > Are the removal (or future removal) of these checks enough to resolve this?
>
> Yes, that is somewhat similar to my proposal, the biggest objection seems to
> be that we need to have a safe type checking for it to work correctly.
>
> >
> >>> It is somewhat ironical that devmem is using static_branch to alliviate the
> >>> performance impact for normal memory at the possible cost of performance
> >>> degradation for devmem, does it not defeat some purpose of intergating devmem
> >>> to page_pool?
> >>>
> >>
> >> I don't see the issue. The static branch sets the non-ppiov path as
> >> default if no memory providers are in use, and flips it when they are,
> >> making the default branch prediction ideal in both cases.
>
> You are assuming the we are not using page pool for both normal memory and
> devmem at the same. But a generic solution should not have that assumption
> as my understanding.
>
> >>
> >>>>
> >>>>> Even though a static_branch check is added in page_is_page_pool_iov(), it
> >>>>> does not make much sense that a core has tow different 'struct' for its
> >>>>> most basic data.
> >>>>>
> >>>>> IMHO, the ppiov for dmabuf is forced fitting into page_pool without much
> >>>>> design consideration at this point.
> >>>>>
> >>>> ...
> >>>>>
> >>>>> For now, the above may work for the the rx part as it seems that you are
> >>>>> only enabling rx for dmabuf for now.
> >>>>>
> >>>>> What is the plan to enable tx for dmabuf? If it is also intergrated into
> >>>>> page_pool? There was a attempt to enable page_pool for tx, Eric seemed to
> >>>>> have some comment about this:
> >>>>> https://lkml.kernel.org/netdev/2cf4b672-d7dc-db3d-ce90-15b4e91c4005@xxxxxxxxxx/T/#mb6ab62dc22f38ec621d516259c56dd66353e24a2
> >>>>>
> >>>>> If tx is not intergrated into page_pool, do we need to create a new layer for
> >>>>> the tx dmabuf?
> >>>>>
> >>>>
> >>>> I imagine the TX path will reuse page_pool_iov, page_pool_iov_*()
> >>>> helpers, and page_pool_page_*() helpers, but will not need any core
> >>>> page_pool changes. This is because the TX path will have to piggyback
> >>>
> >>> We may need another bit/flags checking to demux between page_pool owned
> >>> devmem and non-page_pool owned devmem.
> >>>
> >>
> >> The way I'm imagining the support, I don't see the need for such
> >> flags. We'd be re-using generic helpers like
> >> page_pool_iov_get_dma_address() and what not that don't need that
> >> checking.
> >>
> >>> Also calling page_pool_*() on non-page_pool owned devmem is confusing
> >>> enough that we may need a thin layer handling non-page_pool owned devmem
> >>> in the end.
> >>>
> >>
> >> The page_pool_page* & page_pool_iov* functions can be renamed if
> >> confusing. I would think that's no issue (note that the page_pool_*
>
> When you rename those functions, you will have a thin layer automatically.
>
> >> functions need not be called for TX path).
> >>
> >>>> on MSG_ZEROCOPY (devmem is not copyable), so no memory allocation from
> >>>> the page_pool (or otherwise) is needed or possible. RFCv1 had a TX
> >>>> implementation based on dmabuf pages without page_pool involvement, I
> >>>> imagine I'll do something similar.
> >>> It would be good to have a tx implementation for the next version, so
> >>> that we can have a whole picture of devmem.



-- 
Thanks,
Mina