On 17.10.22 11:09, Baolin Wang wrote:
On 10/17/2022 4:41 PM, David Hildenbrand wrote:
On 17.10.22 09:32, Baolin Wang wrote:
When creating a virtual machine, we will use memfd_create() to get
a file descriptor which can be used to create share memory mappings
using the mmap function, meanwhile the mmap() will set the MAP_POPULATE
flag to allocate physical pages for the virtual machine.
When allocating physical pages for the guest, the host can fallback to
allocate some CMA pages for the guest when over half of the zone's free
memory is in the CMA area.
In guest os, when the application wants to do some data transaction with
DMA, our QEMU will call VFIO_IOMMU_MAP_DMA ioctl to do longterm-pin and
create IOMMU mappings for the DMA pages. However, when calling
VFIO_IOMMU_MAP_DMA ioctl to pin the physical pages, we found it will be
failed to longterm-pin sometimes.
After some invetigation, we found the pages used to do DMA mapping can
contain some CMA pages, and these CMA pages will cause a possible
failure of the longterm-pin, due to failed to migrate the CMA pages.
The reason of migration failure may be temporary reference count or
memory allocation failure. So that will cause the VFIO_IOMMU_MAP_DMA
ioctl returns error, which makes the application failed to start.
To fix this issue, this patch introduces a new madvise behavior, named
as MADV_NOMOVABLE, to avoid allocating CMA pages and movable pages if
the users want to do longterm-pin, which can remove the possible failure
of movable or CMA pages migration.
Sorry to say, but that sounds like a hack to work around a kernel
implementation detail (how often we retry to migrate pages).
IMO, in our case one migration failure will make our application failed
to start, which is not a trival problem. So mitigate the failure of
migration can be important in this case.
The right thing to do is to understand why these migrations fail and see
if we can improve the migration code.
If there are CMA/ZONE_MOVABLE issue, please fix them instead, and avoid
leaking these details to user space.
Now we can not forbid the fallback to CMA allocation if there are enough
free CMA in the zone, right? So adding a hint to help to diable
ALLOC_CMA flag seems reasonable?
For CMA/ZONE_MOVABLE details, yes, not suitable to leak to user space.
so how about rename the madvise as MADV_PINNABLE, which means we will do
longterm-pin after allocation, and no CMA/ZONE_MOVABLE pages will be
allocated.
I really don't think any of these new user-visible madv modes with
questionable semantics to workaround kernel implementation issues are a
good idea.
Especially MADV_PINNABLE has a *very* misleading name.
I understand that something like "MADV_MIGHT_PIN" *might* be helpful to
minimize page migration. But IMHO that could only be a pure
optimization, but wouldn't stop us from allocating (or migrating to)
CMA/ZONE_MOVABLE in the kernel on all code paths. It would be best
effort only.
It's not user space decision how/where the kernel allocates memory. No
hacking around that.
Or do you have any good idea? Thanks.
Investigate why migration of these pages fails and how we can improve
the code to make migration of these pages work more reliably.
I am not completely against having a kernel parameter that would disable
allocating from CMA areas completely, even though it defeats the purpose
of CMA. But it wouldn't apply to ZONE_MOVABLE, so it would be just
another hackish approach.
ALSO, with MAP_POPULATE as described by you this madvise flag doesn't
make too much sense, because it will gets et after all memory already
was allocated ...
This is not a problem I think, we can change to use MADV_POPULATE_XXX to
preallocate the physical pages after MADV_NOMOVABLE madvise.
Yes, I know; I'm pointing out that your patch description is inconsistent.
--
Thanks,
David / dhildenb
