On Wed, May 28, 2025 at 10:07:52AM +0100, Pavel Begunkov wrote:
> On 5/28/25 09:14, Byungchul Park wrote:
> > On Wed, May 28, 2025 at 08:51:47AM +0100, Pavel Begunkov wrote:
> > > On 5/26/25 03:23, Byungchul Park wrote:
> > > > On Fri, May 23, 2025 at 10:21:17AM -0700, Mina Almasry wrote:
> > > > > On Thu, May 22, 2025 at 8:26 PM Byungchul Park <byungchul@xxxxxx> wrote:
> > > > > >
> > > > > > To simplify struct page, the effort to seperate its own descriptor from
> > > > > > struct page is required and the work for page pool is on going.
> > > > > >
> > > > > > To achieve that, all the code should avoid accessing page pool members
> > > > > > of struct page directly, but use safe APIs for the purpose.
> > > > > >
> > > > > > Use netmem_is_pp() instead of directly accessing page->pp_magic in
> > > > > > page_pool_page_is_pp().
> > > > > >
> > > > > > Signed-off-by: Byungchul Park <byungchul@xxxxxx>
> > > > > > ---
> > > > > > include/linux/mm.h | 5 +----
> > > > > > net/core/page_pool.c | 5 +++++
> > > > > > 2 files changed, 6 insertions(+), 4 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > > > index 8dc012e84033..3f7c80fb73ce 100644
> > > > > > --- a/include/linux/mm.h
> > > > > > +++ b/include/linux/mm.h
> > > > > > @@ -4312,10 +4312,7 @@ int arch_lock_shadow_stack_status(struct task_struct *t, unsigned long status);
> > > > > > #define PP_MAGIC_MASK ~(PP_DMA_INDEX_MASK | 0x3UL)
> > > > > >
> > > > > > #ifdef CONFIG_PAGE_POOL
> > > > > > -static inline bool page_pool_page_is_pp(struct page *page)
> > > > > > -{
> > > > > > - return (page->pp_magic & PP_MAGIC_MASK) == PP_SIGNATURE;
> > > > > > -}
> > > > >
> > > > > I vote for keeping this function as-is (do not convert it to netmem),
> > > > > and instead modify it to access page->netmem_desc->pp_magic.
> > > >
> > > > Once the page pool fields are removed from struct page, struct page will
> > > > have neither struct netmem_desc nor the fields..
> > > >
> > > > So it's unevitable to cast it to netmem_desc in order to refer to
> > > > pp_magic. Again, pp_magic is no longer associated to struct page.
> > > >
> > > > Thoughts?
> > >
> > > Once the indirection / page shrinking is realized, the page is
> > > supposed to have a type field, isn't it? And all pp_magic trickery
> > > will be replaced with something like
> > >
> > > page_pool_page_is_pp() { return page->type == PAGE_TYPE_PP; }
> >
> > Agree, but we need a temporary solution until then. I will use the
> > following way for now:
>
> The question is what is the problem that you need another temporary
> solution? If, for example, we go the placeholder way, page_pool_page_is_pp()
I prefer using the place-holder, but Matthew does not. I explained it:
https://lore.kernel.org/all/20250528013145.GB2986@xxxxxxxxxxxxxxxxxxx/
Now, I'm going with the same way as the other approaches e.g. ptdesc.
Byungchul
> can continue using page->netmem_desc->pp_magic as before, and mm folks
> will fix it up to page->type when it's time for that. And the compiler
> will help by failing compilation if forgotten. You should be able to do
> the same with the overlay option.
>
> And, AFAIU, they want to remove/move the lru field in the same way?
> In which case we'll get the same problem and need to re-alias it to
> something else.
>
> --
> Pavel Begunkov
