Re: [RFC 0/1] memfd: Support mapping to zero page on reading

[Yang Shi <shy828301@gmail.com>]

On Tue, Jan 11, 2022 at 6:30 PM Hugh Dickins <hughd@google.com> wrote:
>
> On Wed, 22 Dec 2021, Peng Liang wrote:
>
> > Hi all,
> >
> > Recently we are working on implementing CRIU [1] for QEMU based on
> > Steven's work [2].  It will use memfd to allocate guest memory in order
> > to restore (inherit) it in the new QEMU process.  However, memfd will
> > allocate a new page for reading while anonymous memory will map to zero
> > page for reading.  For QEMU, memfd may cause that all memory are
> > allocated during the migration because QEMU will read all pages in
> > migration.  It may lead to OOM if over-committed memory is enabled,
> > which is usually enabled in public cloud.
> >
> > In this patch I try to add support mapping to zero pages on reading
> > memfd.  On reading, memfd will map to zero page instead of allocating a
> > new page.  Then COW it when a write occurs.
> >
> > For now it's just a demo for discussion.  There are lots of work to do,
> > e.g.:
> > 1. don't support THP;
> > 2. don't support shared reading and writing, only for inherit.  For
> >    example:
> >      task1                        | task2
> >        1) read from addr          |
> >                                   |   2) write to addr
> >        3) read from addr again    |
> >    then 3) will read 0 instead of the data task2 writed in 2).
> >
> > Would something similar be welcome in the Linux?
>
> David has made good suggestions on better avoiding the need for
> such a change, for the use case you have in mind.
>
> And I don't care for the particular RFC patch that you posted.
>
> But I have to say that use of ZERO_PAGE for shmem/memfd/tmpfs read-fault
> might (potentially) be very welcome.  Not as some MFD_ZEROPAGE special
> case, but as how it would always work.  Deleting the shmem_recalc_inode()
> cruft, which is there to correct accounting for the unmodified read-only
> pages, after page reclaim has got around to freeing them later.

I'm wondering if we could use ZERO_PAGE for shmem_getpage() too so
that we have simpler return value? Currently shmem_getpage() returns:
#1. errno and NULL *pagep
#2. 0 and valid *pagep
#3. 0 and NULL *pagep if SGP_READ

Using ZERO_PAGE should be able to consolidate #2 and #3 so that we
know there must be valid *pagep if 0 is returned. This should make
read-fault use ZERO_PAGE automatically.

>
> It does require more work than you gave it in 1/1: mainly, as you call
> out above, there's a need to note in the mapping's XArray when ZERO_PAGE
> has been used at an offset, and do an rmap walk to unmap those ptes when
> a writable page is substituted - see __xip_unmap() in Linux 3.19's
> mm/filemap_xip.c for such an rmap walk.
>
> Though when this came up before (in the "no-fault mmap" MAP_NOSIGBUS
> thread last year - which then got forgotten), Linus was wary of that
> unmapping, and it was dropped for a simple MAP_PRIVATE implementation.
>
> And I've never scoped out what is needed to protect the page from
> writing in all circumstances: in principle, it ought to be easy by
> giving shmem_vm_ops a page_mkwrite; but that's likely to come with
> a performance penalty, which may not be justified for this case.
>
> I didn't check what you did for write protection: maybe what you
> did was enough, but one has to be very careful about that.
>
> Making this change to ZERO_PAGE has never quite justified the effort
> so far: temporarily allocated pages have worked well enough in most
> circumstances.
>
> Hugh
>
> >
> > Thanks,
> > Peng
> >
> > [1] https://criu.org/Checkpoint/Restore
> > [2] https://patchwork.kernel.org/project/qemu-devel/cover/1628286241-217457-1-git-send-email-steven.sistare@oracle.com/
> >
> > Peng Liang (1):
> >   memfd: Support mapping to zero page on reading memfd
> >
> >  include/linux/fs.h         |  2 ++
> >  include/uapi/linux/memfd.h |  1 +
> >  mm/memfd.c                 |  8 ++++++--
> >  mm/memory.c                | 37 ++++++++++++++++++++++++++++++++++---
> >  mm/shmem.c                 | 10 ++++++++--
> >  5 files changed, 51 insertions(+), 7 deletions(-)
> >
> > --
> > 2.33.1
>