Hello Andrea,
On 11.08.2016 20:17, Andrea Arcangeli wrote:
Hello Evgeny,
On Thu, Aug 11, 2016 at 04:51:30PM +0300, Evgeny Yakovlev wrote:
* 1. First fault is expected UFFD_PAGEFAULT_FLAG_WRITE set which we
resolve
* with zeropage
What if you resolve it with bzero(4096);UFFDIO_COPY? Does the problem
go away?
Yes, i don't see additional WP fault now, only expected missing write fault.
If the zeropage is mapped by UFFDIO_ZEROPAGE, there's no way to turn
that into a writable zeropage ever again because
userfaultfd_writeprotect is basically a no-vma-mangling mmap_sem-read
mprotect and it can't trigger faults. Instead a fault in do_wp_page is
required to get rid of the zeropage and copy it off.
Maybe i am missing something but why do we then get WP faults on that
page right after we UFFDIO_ZEROPAGE it? We never call writeprotect on
zeropaged page and still get a WP fault on it which we can't resolve
properly.
If the problem goes away if you s/UFFDIO_ZEROPAGE/bzero(4096);
UFFDIO_COPY/ as I would expect, there would be two ways to solve it:
1) forbid UFFDIO_ZEROPAGE and not return the UFFDIO_ZEROPAGE ioctl in
uffdio_register.ioctls, if UFFDIO_REGISTER is called with
uffdio_register.mode = ...WP|..MISSING so userland is aware it
can't use that.
2) teach UFFDIO_WRITEPROTECT not just to mangle pagetables but also
trigger a write fault on any zeropage if it's called with
uffdio_writeprotect.mode without UFFDIO_WRITEPROTECT_MODE_WP being
set. This will require a bit more work to fix.
The latter would increase performance if not all zeropages needs to be
turned writable.
Feedback welcome on what solution would you prefer.
Our use case is as follows. We have a huge region and most of it we need
to be writable. Most of the time we just gradually resolve missing
faults as they appear. We only enable write protection on some selective
already present pages to have a way to track attempted page modification
for a short period of time. We register initial region as MISSING | WP
so that we don't have to register a new page-sized region each time we
need to write-protect a single page inside a region.
Thanks,
Andrea
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