linux-kernel - Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of anonymous pages

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Date:   Mon, 21 Mar 2022 13:18:23 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Nadav Amit <namit@...are.com>, 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>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH v2 13/15] mm: support GUP-triggered unsharing of
 anonymous pages

On Mon, Mar 21, 2022 at 05:15:06PM +0100, David Hildenbrand wrote:
> On 19.03.22 00:30, Jason Gunthorpe wrote:
> > On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote:
> >> Whenever GUP currently ends up taking a R/O pin on an anonymous page that
> >> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault
> >> on the page table entry will end up replacing the mapped anonymous page
> >> due to COW, resulting in the GUP pin no longer being consistent with the
> >> page actually mapped into the page table.
> >>
> >> The possible ways to deal with this situation are:
> >>  (1) Ignore and pin -- what we do right now.
> >>  (2) Fail to pin -- which would be rather surprising to callers and
> >>      could break user space.
> >>  (3) Trigger unsharing and pin the now exclusive page -- reliable R/O
> >>      pins.
> >>
> >> We want to implement 3) because it provides the clearest semantics and
> >> allows for checking in unpin_user_pages() and friends for possible BUGs:
> >> when trying to unpin a page that's no longer exclusive, clearly
> >> something went very wrong and might result in memory corruptions that
> >> might be hard to debug. So we better have a nice way to spot such
> >> issues.
> >>
> >> To implement 3), we need a way for GUP to trigger unsharing:
> >> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped
> >> anonymous pages and resembles COW logic during a write fault. However, in
> >> contrast to a write fault, GUP-triggered unsharing will, for example, still
> >> maintain the write protection.
> > 
> > Given the way this series has developed you might want to call this
> > FAULT_FLAG_MAKE_ANON_EXCLUSIVE
> > 
> > Which strikes me as more directly connected to what it is trying to
> > do.
> 
> I thought about something similar along those lines, and I think it
> would apply even when extending that mechanism to anything !anon inside
> a MAP_PRIVATE mapping.
> 
> The whole
> 
> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;

I think the extra words are worthwhile, share makes me think about
MAP_SHARED as we don't really use shared anywhere else FWICT..

Jason