On 18.02.21 11:25, Michal Hocko wrote:
On Wed 17-02-21 16:48:44, David Hildenbrand wrote:
When we manage sparse memory mappings dynamically in user space - also
sometimes involving MADV_NORESERVE - we want to dynamically populate/
Just wondering what is MADV_NORESERVE? I do not see anything like that
in the Linus tree. Did you mean MAP_NORESERVE?
Most certainly, thanks :)
discard memory inside such a sparse memory region. Example users are
hypervisors (especially implementing memory ballooning or similar
technologies like virtio-mem) and memory allocators. In addition, we want
to fail in a nice way if populating does not succeed because we are out of
backend memory (which can happen easily with file-based mappings,
especially tmpfs and hugetlbfs).
by "fail in a nice way" you mean before a #PF would fail and SIGBUS
which would be harder to handle?
Yes.
[...]
Because we don't have a proper interface, what applications
(like QEMU and databases) end up doing is touching (i.e., writing) all
individual pages. However, it requires expensive signal handling (SIGBUS);
for example, this is problematic in hypervisors like QEMU where SIGBUS
handlers might already be used by other subsystems concurrently to e.g,
handle hardware errors. "Simply" doing preallocation from another thread
is not that easy.
OK, that clarifies my above question.
Let's introduce MADV_POPULATE with the following semantics
1. MADV_POPULATED does not work on PROT_NONE and special VMAs. It works
on everything else.
This would better clarify what "does not work" means. I assume those are
ignored and do not report any error?
I'm currently preparing the man page. "Fail with -ENOMEM" (like
MADV_DONTNEED or MADV_REMOVE)
2. Errors during MADV_POPULATED (especially OOM) are reported.
How do you want to achieve that? gup/page fault handler will allocate
memory and trigger the oom without caller noticing that. You would
somehow have to weaken the allocation context to GFP_RETRY_MAYFAIL or
NORETRY to achieve the error handling.
Okay, I should be more clear here (again, I'm realizing this as well
while I create the man page), OOM is confusing: avoid SIGBUS at runtime
- like we would get on actual file systems/shmem/hugetlbfs when
preallocating.
It cannot save us from the actual OOM killer. To handle anonymous memory
more reliable I'll need other means as well (dynamic swap space
allocation for sparse mappings).
If we hit
hardware errors on pages, ignore them - nothing we really can or
should do.
3. On errors during MADV_POPULATED, some memory might have been
populated. Callers have to clean up if they care.
How does caller find out? madvise reports 0 on success so how do you
find out how much has been populated?
If there is an error, something might have been populated. In my QEMU
implementation, I simply discard the range again, good enough. I don't
think we need to really indicate "error and populated" or "error and not
populated".
4. Concurrent changes to the virtual memory layour are tolerated - we
process each and every PFN only once, though.
I do not understand this. madvise is about virtual address space not a
physical address space.
What I wanted to express: if we detect a change in the mapping we don't
restart at the beginning, we always make forward progress. We process
each virtual address once (on a per-page basis, thus I accidentally used
"PFN").
5. If MADV_POPULATE succeeds, all memory in the range can be accessed
without SIGBUS. (of course, not if user space changed mappings in the
meantime or KSM kicked in on anonymous memory).
I do not see how KSM would change anything here and maybe it is not
really important to mention it. KSM should be really transparent from
the users space POV. Parallel and destructive virtual address space
operations are also expected to change the outcome and there is nothing
kernel do about at and provide any meaningful guarantees. I guess we
want to assume a reasonable userspace behavior here.
It's just a note that we cannot protect from someone interfering
(discard/ksm/whatever). I'm making that clearer in the cover letter.
Thanks!
--
Thanks,
David / dhildenb
