Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator use multipage folios

Alexander H Duyck
2023-05-29T17:42:55Z

On Fri, 2023-05-26 at 19:56 +0800, Yunsheng Lin wrote:
> On 2023/5/24 23:33, David Howells wrote:
> > Change the page_frag_cache allocator to use multipage folios rather than
> > groups of pages. This reduces page_frag_free to just a folio_put() or
> > put_page().
>
> Hi, David
>
> put_page() is not used in this patch, perhaps remove it to avoid
> the confusion?
> Also, Is there any significant difference between __free_pages()
> and folio_put()? IOW, what does the 'reduces' part means here?
>
> I followed some disscusion about folio before, but have not really
> understood about real difference between 'multipage folios' and
> 'groups of pages' yet. Is folio mostly used to avoid the confusion
> about whether a page is 'headpage of compound page', 'base page' or
> 'tailpage of compound page'? Or is there any abvious benefit about
> folio that I missed?
>
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 306a3d1a0fa6..d7c52a5979cc 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio)
> > }
> >
> > struct page_frag_cache {
> > - void * va;
> > -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> > - __u16 offset;
> > - __u16 size;
> > -#else
> > - __u32 offset;
> > -#endif
> > + struct folio *folio;
> > + unsigned int offset;
> > /* we maintain a pagecount bias, so that we dont dirty cache line
> > * containing page->_refcount every time we allocate a fragment.
> > */
> > - unsigned int pagecnt_bias;
> > - bool pfmemalloc;
> > + unsigned int pagecnt_bias;
> > + bool pfmemalloc;
> > };
>
> It seems 'va' and 'size' field is used to avoid touching 'stuct page' to
> avoid possible cache bouncing when there is more frag can be allocated
> from the page while other frags is freed at the same time before this patch?
> It might be worth calling that out in the commit log or split it into another
> patch to make it clearer and easier to review?

Yes, there is a cost for going from page to virtual address. That is
why we only use the page when we finally get to freeing or resetting
the pagecnt_bias.

Also I have some concerns about going from page to folio as it seems
like the folio_alloc setups the transparent hugepage destructor instead
of using the compound page destructor. I would think that would slow
down most users as it looks like there is a spinlock that is taken in
the hugepage destructor that isn't there in the compound page
destructor.