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

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Date:   Tue, 21 Dec 2021 19:07:05 +0100
From:   Jan Kara <jack@...e.cz>
To:     David Hildenbrand <david@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        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>,
        Matthew Wilcox <willy@...radead.org>,
        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 Tue 21-12-21 18:40:30, David Hildenbrand wrote:
> On 21.12.21 18:05, Linus Torvalds wrote:
> > On Tue, Dec 21, 2021 at 12:58 AM David Hildenbrand <david@...hat.com> wrote:
> >> Note that I am trying to make also any kind of R/O pins on an anonymous
> >> page work as expected as well, to fix any kind of GUP after fork() and
> >> GUP before fork(). So taking a R/O pin on an !PageAnonExclusive() page
> >> similarly has to make sure that the page is exclusive -- even if it's
> >> mapped R/O (!).
> > 
> > I do think the existing "maybe_pinned()" logic is fine for that. The
> > "exclusive to this VM" bit can be used to *help* that decision -
> > because only an exclusive page can be pinned - bit I don't think it
> > should _replace_ that logic.
> 
> The issue is that O_DIRECT uses FOLL_GET and cannot easily be changed to
> FOLL_PIN unfortunately. So I'm *trying* to make it more generic such
> that such corner cases can be handled as well correctly. But yeah, I'll
> see where this goes ... O_DIRECT has to be fixed one way or the other.
> 
> John H. mentioned that he wants to look into converting that to
> FOLL_PIN. So maybe that will work eventually.

For record we always intended (and still intend) to make O_DIRECT use
FOLL_PIN. Just it is tricky because some users mix pages pinned with GUP
and pages acquired through get_page() in a single bio (such as zero page)
and thus it is non-trivial to do the right thing on IO completion (unpin or
just put_page).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR