Re: [PATCH net-next v13 04/14] mm: page_frag: add '_va' suffix to page_frag API

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Aug 16, 2024 at 4:55 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>
> On 2024/8/15 23:00, Alexander Duyck wrote:
> > On Wed, Aug 14, 2024 at 8:00 PM Yunsheng Lin <linyunsheng@huawei.com> wrote:
> >>
> >> On 2024/8/14 23:49, Alexander H Duyck wrote:
> >>> On Thu, 2024-08-08 at 20:37 +0800, Yunsheng Lin wrote:
> >>>> Currently the page_frag API is returning 'virtual address'
> >>>> or 'va' when allocing and expecting 'virtual address' or
> >>>> 'va' as input when freeing.
> >>>>
> >>>> As we are about to support new use cases that the caller
> >>>> need to deal with 'struct page' or need to deal with both
> >>>> 'va' and 'struct page'. In order to differentiate the API
> >>>> handling between 'va' and 'struct page', add '_va' suffix
> >>>> to the corresponding API mirroring the page_pool_alloc_va()
> >>>> API of the page_pool. So that callers expecting to deal with
> >>>> va, page or both va and page may call page_frag_alloc_va*,
> >>>> page_frag_alloc_pg*, or page_frag_alloc* API accordingly.
> >>>>
> >>>> CC: Alexander Duyck <alexander.duyck@gmail.com>
> >>>> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
> >>>> Reviewed-by: Subbaraya Sundeep <sbhatta@marvell.com>
> >>>> Acked-by: Chuck Lever <chuck.lever@oracle.com>
> >>>> Acked-by: Sagi Grimberg <sagi@grimberg.me>
> >>>> ---
> >>>>  drivers/net/ethernet/google/gve/gve_rx.c      |  4 ++--
> >>>>  drivers/net/ethernet/intel/ice/ice_txrx.c     |  2 +-
> >>>>  drivers/net/ethernet/intel/ice/ice_txrx.h     |  2 +-
> >>>>  drivers/net/ethernet/intel/ice/ice_txrx_lib.c |  2 +-
> >>>>  .../net/ethernet/intel/ixgbevf/ixgbevf_main.c |  4 ++--
> >>>>  .../marvell/octeontx2/nic/otx2_common.c       |  2 +-
> >>>>  drivers/net/ethernet/mediatek/mtk_wed_wo.c    |  4 ++--
> >>>>  drivers/nvme/host/tcp.c                       |  8 +++----
> >>>>  drivers/nvme/target/tcp.c                     | 22 +++++++++----------
> >>>>  drivers/vhost/net.c                           |  6 ++---
> >>>>  include/linux/page_frag_cache.h               | 21 +++++++++---------
> >>>>  include/linux/skbuff.h                        |  2 +-
> >>>>  kernel/bpf/cpumap.c                           |  2 +-
> >>>>  mm/page_frag_cache.c                          | 12 +++++-----
> >>>>  net/core/skbuff.c                             | 16 +++++++-------
> >>>>  net/core/xdp.c                                |  2 +-
> >>>>  net/rxrpc/txbuf.c                             | 15 +++++++------
> >>>>  net/sunrpc/svcsock.c                          |  6 ++---
> >>>>  .../selftests/mm/page_frag/page_frag_test.c   | 13 ++++++-----
> >>>>  19 files changed, 75 insertions(+), 70 deletions(-)
> >>>>
> >>>
> >>> I still say no to this patch. It is an unnecessary name change and adds
> >>> no value. If you insist on this patch I will reject the set every time.
> >>>
> >>> The fact is it is polluting the git history and just makes things
> >>> harder to maintain without adding any value as you aren't changing what
> >>> the function does and there is no need for this. In addition it just
> >>
> >> I guess I have to disagree with the above 'no need for this' part for
> >> now, as mentioned in [1]:
> >>
> >> "There are three types of API as proposed in this patchset instead of
> >> two types of API:
> >> 1. page_frag_alloc_va() returns [va].
> >> 2. page_frag_alloc_pg() returns [page, offset].
> >> 3. page_frag_alloc() returns [va] & [page, offset].
> >>
> >> You seemed to miss that we need a third naming for the type 3 API.
> >> Do you see type 3 API as a valid API? if yes, what naming are you
> >> suggesting for it? if no, why it is not a valid API?"
> >
> > I didn't. I just don't see the point in pushing out the existing API
> > to support that. In reality 2 and 3 are redundant. You probably only
> > need 3. Like I mentioned earlier you can essentially just pass a
>
> If the caller just expect [page, offset], do you expect the caller also
> type 3 API, which return both [va] and [page, offset]?
>
> I am not sure if I understand why you think 2 and 3 are redundant here?
> If you think 2 and 3 are redundant here, aren't 1 and 3 also redundant
> as the similar agrument?

The big difference is the need to return page and offset. Basically to
support returning page and offset you need to pass at least one value
as a pointer so you can store the return there.

The reason why 3 is just a redundant form of 2 is that you will
normally just be converting from a va to a page and offset so the va
should already be easily accessible.

> > page_frag via pointer to the function. With that you could also look
> > at just returning a virtual address as well if you insist on having
> > something that returns all of the above. No point in having 2 and 3 be
> > seperate functions.
>
> Let's be more specific about what are your suggestion here: which way
> is the prefer way to return the virtual address. It seems there are two
> options:
>
> 1. Return the virtual address by function returning as below:
> void *page_frag_alloc_bio(struct page_frag_cache *nc, struct bio_vec *bio);
>
> 2. Return the virtual address by double pointer as below:
> int page_frag_alloc_bio(struct page_frag_cache *nc, struct bio_vec *bio,
>                         void **va);

I was thinking more of option 1. Basically this is a superset of
page_frag_alloc_va that is also returning the page and offset via a
page frag. However instead of bio_vec I would be good with "struct
page_frag *" being the value passed to the function to play the role
of container. Basically the big difference between 1 and 2/3 if I am
not mistaken is the fact that for 1 you pass the size, whereas with
2/3 you are peeling off the page frag from the larger page frag cache
after the fact via a commit type action.