From: Hugh Dickins <firstname.lastname@example.org>
To: Peter Xu <email@example.com>
Cc: Hugh Dickins <firstname.lastname@example.org>,
Andrea Arcangeli <email@example.com>,
Matthew Wilcox <firstname.lastname@example.org>,
Andrew Morton <email@example.com>,
Mike Rapoport <firstname.lastname@example.org>,
David Hildenbrand <email@example.com>
Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads
Date: Wed, 2 Dec 2020 21:36:45 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.firstname.lastname@example.org> (raw)
On Wed, 2 Dec 2020, Peter Xu wrote:
> On Wed, Dec 02, 2020 at 02:37:33PM -0800, Hugh Dickins wrote:
> > On Tue, 1 Dec 2020, Andrea Arcangeli wrote:
> > >
> > > Any suggestions on how to have the per-vaddr per-mm _PAGE_UFFD_WP bit
> > > survive the pte invalidates in a way that remains associated to a
> > > certain vaddr in a single mm (so it can shoot itself in the foot if it
> > > wants, but it can't interfere with all other mm sharing the shmem
> > > file) would be welcome...
> > I think it has to be a new variety of swap-like non_swap_entry() pte,
> > see include/linux/swapops.h. Anything else would be more troublesome.
> > Search for non_swap_entry and for migration_entry, to find places that
> > might need to learn about this new variety.
> > IIUC you only need a single value, no need to carve out another whole
> > swp_type: could probably be swp_offset 0 of any swp_type other than 0.
> > Note that fork's copy_page_range() does not "copy ptes where a page
> > fault will fill them correctly", so would in effect put a pte_none
> > into the child where the parent has this uffd_wp entry. I don't know
> > anything about uffd versus fork, whether that would pose a problem.
> Thanks for the idea, Hugh!
> I thought about something similar today, but instead of swap entries, I was
> thinking about constantly filling in a pte with a value of "_PAGE_PROTNONE |
> _PAGE_UFFD_WP" when e.g. we'd like to zap a page with shmem+uffd-wp. I feel
> like the fundamental idea is similar - we can somehow keep the pte with uffd-wp
> information even if zapped/swapped-out, so as long as the shmem access will
> fruther trap into the fault handler, then we can operate on that pte and read
> that information out, like recover that pte into a normal pte (with swap/page
> cache, and vma/addr information, we'll be able to) and then we can retry the
Yes, I think that should work too: I can't predict which way would cause
We usually tend to keep away from protnone games, because NUMA balancing
use of protnone is already confusing enough.
But those ptes will be pte_present(), so you must provide a pfn, and I
think if you use the zero_pfn, vm_normal_page() will return false on it,
and avoid special casing (and reference counting) it in various places.
next prev parent reply other threads:[~2020-12-03 5:37 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-30 23:06 [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads Peter Xu
2020-12-01 9:30 ` David Hildenbrand
2020-12-01 12:59 ` Matthew Wilcox
2020-12-01 22:30 ` Peter Xu
2020-12-02 0:02 ` Andrea Arcangeli
2020-12-02 22:37 ` Hugh Dickins
2020-12-02 23:41 ` Peter Xu
2020-12-03 5:36 ` Hugh Dickins [this message]
2020-12-03 18:02 ` Peter Xu
2020-12-03 19:44 ` Andrea Arcangeli
2020-12-04 2:30 ` Peter Xu
2020-12-04 4:10 ` Andrea Arcangeli
2020-12-04 5:59 ` Hugh Dickins
2020-12-04 16:50 ` Peter Xu
2020-12-04 18:12 ` Andrea Arcangeli
2020-12-04 19:23 ` Peter Xu
2020-12-04 19:37 ` Andrea Arcangeli
2020-12-04 20:21 ` Peter Xu
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