archive mirror
 help / color / mirror / Atom feed
From: Peter Xu <>
To: Hugh Dickins <>
Cc: Andrea Arcangeli <>,
	Matthew Wilcox <>,,,
	Andrew Morton <>,
	Mike Rapoport <>,
	David Hildenbrand <>
Subject: Re: [PATCH v2] mm: Don't fault around userfaultfd-registered regions on reads
Date: Wed, 2 Dec 2020 18:41:17 -0500	[thread overview]
Message-ID: <20201202234117.GD108496@xz-x1> (raw)
In-Reply-To: <alpine.LSU.2.11.2012021410260.4989@eggly.anvils>

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


Peter Xu

  reply	other threads:[~2020-12-02 23:42 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 [this message]
2020-12-03  5:36           ` Hugh Dickins
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201202234117.GD108496@xz-x1 \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).