Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb)

[Matthew Wilcox]

On Wed, Dec 22, 2021 at 02:09:41PM +0100, David Hildenbrand wrote:
> Right, from an API perspective we really want people to use FOLL_PIN.
> 
> To optimize this case in particular it would help if we would have the
> FOLL flags on the unpin path. Then we could just decide internally
> "well, short-term R/O FOLL_PIN can be really lightweight, we can treat
> this like a FOLL_GET instead". And we would need that as well if we were
> to keep different counters for R/O vs. R/W pinned.

FYI, in my current tree, there's a gup_put_folio() which replaces
put_compound_head:

static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
{
        if (flags & FOLL_PIN) {
                node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
                if (hpage_pincount_available(&folio->page))
                        hpage_pincount_sub(&folio->page, refs);
                else
                        refs *= GUP_PIN_COUNTING_BIAS;
        }

        folio_put_refs(folio, refs);
}

That can become non-static if it's needed.  I'm still working on that
series, because I'd like to get it to a point where we return one
folio pointer instead of N page pointers.  Not quite there yet.