On 11.08.23 18:54, Peter Xu wrote:
On Fri, Aug 11, 2023 at 06:25:14PM +0200, David Hildenbrand wrote:
On 11.08.23 18:22, Peter Xu wrote:
On Fri, Aug 11, 2023 at 06:17:05PM +0200, David Hildenbrand wrote:
We wouldn't touch "-mem-path".
But still the same issue when someone uses -object memory-backend-file for
hugetlb, mapping privately, expecting ram discard to work?
Basically I see that example as, "hugetlb" in general made the private
mapping over RW file usable, so forbidden that anywhere may take a risk.
These users can be directed to using hugetlb
a) using MAP_SHARED
b) using memory-backend-memfd, if MAP_PRIVATE is desired
Am I missing any important use case? Are we being a bit to careful about
virtio-balloon and postcopy simply not being available for these corner
cases?
The current immediate issue is not really mem=rw + fd=rw + private case
(which was a known issue), but how to make mem=rw + fd=ro + private work
for ThinnerBloger, iiuc.
I'd just think it safer to expose that cap to solve problem A (vm
templating) without affecting problem B (fallcate-over-private not working
right), when B is uncertain.
Right, and I'm thinking about if B is worth the effort.
I'm also copy Daniel & libvirt list in case there's quick comment from
there. Say, maybe libvirt never use private mapping on hugetlb files over
memory-backend-file at all, then it's probably fine.
libvirt certainly allows setting <access mode="shared"/> with <source
type="file">.
Could be that they also end up mapping "<hugepages>" to
memory-backend-file instead of memory-backend-memfd (e.g., compatibility
with older kernels?).
In all cases, you and Igor should have the final grasp; no stand on a
strong opinon from my side.
I do value your opinion, so I'm still trying to figure out if there are
sane use cases that really need a new parameter. Let's recap:
When opening the file R/O, resulting in fallocate() refusing to work:
* virtio-balloon will fail to discard RAM but continue to "be alive"
* virtio-mem will discard any private pages, but cannot free up disk
blocks using fallocate.
* postcopy would fail early
Postcopy:
* Works on shmem (MAP_SHARED / MAP_PRIVATE)
* Works on hugetlb (MAP_SHARED / MAP_PRIVATE)
* Does not work on file-backed memory (including MAP_PRIVATE)
We can ignore virtio-mem for now. What remains is postcopy and
virtio-balloon.
memory-backend-file with MAP_PRIVATE on shmem/tmpfs results in a double
memory consumption, so we can mostly cross that out as "sane use case".
Rather make such users aware of that :D
memory-backend-file with MAP_PRIVATE on hugetlb works. virtio-balloon is
not really compatible with hugetlb, free-page-reporting might work
(although quite non-nonsensical). So postcopy as the most important use
case remains.
memory-backend-file with MAP_PRIVATE on file-backed memory works.
postcopy does not apply. virtio-balloon should work I guess.
So the two use cases that are left are:
* postcopy with hugetlb would fail
* virtio-balloon with file-backed memory cannot free up disk blocks
Am I missing a case?
--
Cheers,
David / dhildenb
