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

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Message-ID: <Yb8XzTRX8TZOYIQ/@casper.infradead.org>
Date:   Sun, 19 Dec 2021 11:30:21 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Nadav Amit <namit@...are.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.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>,
        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 Sun, Dec 19, 2021 at 12:01:59AM -0800, John Hubbard wrote:
> On 12/18/21 22:02, Nadav Amit wrote:
> > I found my old messy code for the software-PTE thing.
> > 
> > I see that eventually I decided to hold a pointer to the “extra PTEs”
> > of each page in the PMD-page-struct. [ I also implemented the 2-adjacent
> > pages approach but this code is long gone. ]
>
> a) The PMD-page-struct approach won't help as much, because (assuming
> that we're using it in an attempt to get a true, perfect pin count), you
> are combining the pin counts of a PMD's worth of pages. OTOH...maybe
> that actually *is* OK, assuming you don't overflow--except that you can
> only answer the "is it dma-pinned?" question at a PMD level. That's a
> contradiction of your stated desire above to have very granular control.
> 
> Also, because of not having bit 0 available in page._pt_pad_1, I think
> the count would have to be implemented as adding and subtracting 2,
> instead of 1 (in order to keep the value even), further reducing the
> counter range.

I think you misunderstood Nadav's approach.  He's talking about making
an extra side-allocation per PMD if you're using uffd, and storing
extra information in it.  I think it's a worthwile approach.