https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=274252
Bug ID: 274252
Summary: sys/vm: less-than-ideal handling of memory requests
that cannot be fulfilled
Product: Base System
Version: Unspecified
Hardware: Any
OS: Any
Status: New
Severity: Affects Only Me
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
Splitting off from PR 274237, because I haven't actually created a PR for this
previously, but it'd be nice to track.
With at least some ARM machines, it's possible to get stuck in a nice loop in
xhci attach because the VM bits don't handle some class of requests that cannot
be satisfied very well. In particular, consider this system:
Physical memory chunk(s):
0x000008010a8000 - 0x00000802313fff, 19316736 bytes (1179 pages)
0x000008023d8000 - 0x0000080389bfff, 21774336 bytes (1329 pages)
0x000008038b8000 - 0x00000808f97fff, 91095040 bytes (5560 pages)
0x00000808fb8000 - 0x0000080ba03fff, 44351488 bytes (2707 pages)
0x0000080c12c000 - 0x000009d036ffff, 7585677312 bytes (462993 pages)
0x000009d4f68000 - 0x000009db93bfff, 110968832 bytes (6773 pages)
0x000009db944000 - 0x000009e096ffff, 84066304 bytes (5131 pages)
0x000009e0980000 - 0x000009e0a37fff, 753664 bytes (46 pages)
avail memory = 7955300352 (7586 MB)
Note that there's absolutely no RAM in the lower 4G of the address space.
There's an XHCI controller that can only do 32-bit DMA (allegedly) and it has
an associated IOMMU that isn't hooked up just yet.
Right now, busdma will request some pages below 4G (IIRC, it's with
kmem_alloc_contig here[0]), but that request cannot be satisfied -- there's
absolutely no memory there. Instead, it ends up hanging in the VM layer trying
to fulfill an allocation that isn't physically possible.
I think it'd be better to fail the request and let busdma kick back an ENOMEM.
The XHCI controller will not be functional, but that's both expected and not a
deal-breaker for getting the machine into a usable state.
[0] https://cgit.freebsd.org/src/tree/sys/arm64/arm64/busdma_bounce.c#n572
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