Re: [PATCH] mm/vmalloc: map contiguous pages in batches for vmap() whenever possible

[Barry Song <21cnbao@gmail.com>]

[...]
> >
> > +static inline int get_vmap_batch_order(struct page **pages,
> > +             unsigned int stride, unsigned int max_steps, unsigned int idx)
> > +{
> > +     int nr_pages = 1;
> > +
> > +     /*
> > +      * Currently, batching is only supported in vmap_pages_range
> > +      * when page_shift == PAGE_SHIFT.
> > +      */
> > +     if (stride != 1)
> > +             return 0;
> > +
> > +     nr_pages = compound_nr(pages[idx]);
> > +     if (nr_pages == 1)
> > +             return 0;
> > +     if (max_steps < nr_pages)
> > +             return 0;
> > +
> > +     if (num_pages_contiguous(&pages[idx], nr_pages) == nr_pages)
> > +             return compound_order(pages[idx]);
> > +     return 0;
> > +}
> > +
> Can we instead look at this as: it can be that we have continues
> set of pages let's find out. I mean if we do not stick just to
> compound pages.

We use PageCompound(pages[0]) and compound_nr() as quick
filters to skip checking the contiguous count, and this is
now the intended use case. Always checking contiguity might
cause a slight regression, I guess.

BTW, do we have a strong use case where GFP_COMP or folio is
not used, yet the pages are physically contiguous?

Thanks
Barry