linux-kernel - Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG

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Message-ID: <YcPA8gJ0OBPTdCdB@casper.infradead.org>
Date:   Thu, 23 Dec 2021 00:21:06 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Jason Gunthorpe <jgg@...dia.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Nadav Amit <namit@...are.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via
 FAULT_FLAG_UNSHARE (!hugetlb)

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.