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

Barry Song
2025-12-18T20:05:55Z

[...]
> >
> > +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