API for setting multiple PTEs at once

[Matthew Wilcox <willy@infradead.org>]

For those of you not subscribed, linux-mm is currently discussing
how best to handle page faults on large folios.  I simply made it work
when adding large folio support.  Now Yin Fengwei is working on
making it fast.

https://lore.kernel.org/linux-mm/Y9qjn0Y+1ir787nc@casper.infradead.org/
is perhaps the best place to start as it pertains to what the
architecture will see.

At the bottom of that function, I propose

+       for (i = 0; i < nr; i++) {
+               set_pte_at(vma->vm_mm, addr, vmf->pte + i, entry);
+               /* no need to invalidate: a not-present page won't be cached */
+               update_mmu_cache(vma, addr, vmf->pte + i);
+               addr += PAGE_SIZE;
+		entry = pte_next(entry);
+	}

(or I would have, had I not forgotten that pte_t isn't an integral type)

But I think that some architectures want to mark PTEs specially for
"This is part of a contiguous range" -- ARM, perhaps?  So would you like
an API like:

	arch_set_ptes(mm, addr, vmf->pte, entry, nr);
	update_mmu_cache_range(vma, addr, vmf->pte, nr);

There are some challenges here.  For example, folios may be mapped
askew (ie not naturally aligned).  Another problem is that folios may
be unmapped in part (eg mmap(), fault, followed by munmap() of one of
the pages in the folio), and I presume you'd need to go and unmark the
other PTEs in that case.  So it's not as simple as just checking whether
'addr' and 'nr' are in some way compatible.