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

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Message-ID: <YcFKH2kXFec9/pyn@casper.infradead.org>
Date:   Tue, 21 Dec 2021 03:29:35 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     David Hildenbrand <david@...hat.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>, Jan Kara <jack@...e.cz>,
        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 Mon, Dec 20, 2021 at 09:03:12PM -0400, Jason Gunthorpe wrote:
> That just leave the THP splitting.. I suppose we get the PTL, then
> compute the current value of the new bit based on refcount and diffuse
> it to all tail pages, then update the PMD and release the PTL. Safe
> against concurrent WP - don't need DoubleMap horrors because it isn't
> a counter.

One of the things I've been trying to figure out is how we do
can_split_huge_page().  Maybe an rmap walk to figure out how many
refcounts we would subtract if we did unmap it from everywhere it's
currently mapped?  (just to be clear, we call unmap_page() as the
next thing, so I don't mind warming up the rbtree cachelines
if it's mapped anywhere)