Re: [PATCH net-next v11 0/4] fix the DMA API misuse problem for page_pool

[Yunsheng Lin <yunshenglin0825@gmail.com>]

On 3/7/2025 10:15 PM, Toke Høiland-Jørgensen wrote:

...

> 
> You are making this incredibly complicated. You've basically implemented
> a whole new slab allocator for those page_pool_item objects, and you're
> tracking every page handed out by the page pool instead of just the ones
> that are DMA-mapped. None of this is needed.
 > > I took a stab at implementing the xarray-based tracking first suggested
> by Mina[0]:

I did discuss Mina' suggestion with Ilias below in case you didn't
notice:
https://lore.kernel.org/all/0ef315df-e8e9-41e8-9ba8-dcb69492c616@huawei.com/

Anyway, It is great that you take the effort to actually implement
the idea to have some more concrete comparison here.

> 
> https://git.kernel.org/toke/c/e87e0edf9520
> 
> And, well, it's 50 lines of extra code, none of which are in the fast
> path.

I wonder what is the overhead for the xarray idea regarding the
time_bench_page_pool03_slow() testcase before we begin to discuss
if xarray idea is indeed possible.

> 
> Jesper has kindly helped with testing that it works for normal packet
> processing, but I haven't yet verified that it resolves the original
> crash. Will post the patch to the list once I have verified this (help
> welcome!).

RFC seems like a good way to show and discuss the basic idea.

I only took a glance at git code above, it seems reusing the
_pp_mapping_pad for pp_dma_index seems like a wrong direction
as mentioned in discussion with Ilias above as the field might
be used when a page is mmap'ed to user space, and reusing that
field in 'struct page' seems to disable the tcp_zerocopy feature,
see the below commit from Eric:
https://github.com/torvalds/linux/commit/577e4432f3ac810049cb7e6b71f4d96ec7c6e894

Also, I am not sure if a page_pool owned page can be spliced into the fs
subsystem yet, but if it does, I am not sure how is reusing the
page->mapping possible if that page is called in __filemap_add_folio()?

https://elixir.bootlin.com/linux/v6.14-rc5/source/mm/filemap.c#L882

> 
> -Toke
> 
> [0] https://lore.kernel.org/all/CAHS8izPg7B5DwKfSuzz-iOop_YRbk3Sd6Y4rX7KBG9DcVJcyWg@mail.gmail.com/
> 
>