On 05.01.23 04:10, Nadav Amit wrote:
On Jan 4, 2023, at 2:52 PM, Peter Xu <peterx@xxxxxxxxxx> wrote:
Before this patch, when there's any pgtable allocation issues happened
during change_protection(), the error will be ignored from the syscall.
For shmem, there will be an error dumped into the host dmesg. Two issues
with that:
(1) Doing a trace dump when allocation fails is not anything close to
grace..
(2) The user should be notified with any kind of such error, so the user
can trap it and decide what to do next, either by retrying, or stop
the process properly, or anything else.
For userfault users, this will change the API of UFFDIO_WRITEPROTECT when
pgtable allocation failure happened. It should not normally break anyone,
though. If it breaks, then in good ways.
One man-page update will be on the way to introduce the new -ENOMEM for
UFFDIO_WRITEPROTECT. Not marking stable so we keep the old behavior on the
5.19-till-now kernels.
I understand that the current assumption is that change_protection() should
fully succeed or fail, and I guess this is the current behavior.
However, to be more “future-proof” perhaps this needs to be revisited.
For instance, UFFDIO_WRITEPROTECT can benefit from the ability to (based on
userspace request) prevent write-protection of pages that are pinned. This is
necessary to allow userspace uffd monitor to avoid write-protection of
O_DIRECT’d memory, for instance, that might change even if a uffd monitor
considers it write-protected.
Just a note that this is pretty tricky IMHO, because:
a) We cannot distinguished "pinned readable" from "pinned writable"
b) We can have false positives ("pinned") even for compound pages due to
concurrent GUP-fast.
c) Synchronizing against GUP-fast is pretty tricky ... as we learned.
Concurrent pinning is usually problematic.
d) O_DIRECT still uses FOLL_GET and we cannot identify that. (at least
that should be figured out at one point)
I have a patch lying around for a very long time that removes that
special-pinned handling from softdirty code, because of the above
reasons (and because it forgets THP). For now I didn't send it because
for softdirty, it's acceptable to over-indicate and it hasn't been
reported to be an actual problem so far.
For existing UFFDIO_WRITEPROTECT users, however, it might be very
harmful (especially for existing users) to get false protection errors.
Failing due to ENOMEM is different to failing due to some temporary
concurrency issues.
Having that said, I started thinking about alternative ways of detecting
that in that past, without much outcome so far: that latest idea was
indicating "this MM has had pinned pages at one point, be careful
because any techniques that use write-protection (softdirty, mprotect,
uffd-wp) won't be able to catch writes via pinned pages reliably".
Hm.
--
Thanks,
David / dhildenb
