IMHO, we should rip out that code here and enforce page alignment in
vmemmap_populate()/vmemmap_free().
Am I missing something?
Thanks David for bringing this up, I must say I was not aware that this
topic was ever discussed.
Yeah, last time I raised it was in
https://lkml.kernel.org/r/20200703013435.GA11340@L-31X9LVDL-1304.local
but I never got to clean it up myself.
Ok, I've been having a look into this.
At first I was concerced because of a pure SPARSEMEM configuration, but I
see that those allocations are done in a very diferent way so it does not
bother us.
So we have the following enforcements during hotplug:
add_memory_resource
check_hotplug_memory_range : Checks range aligned to memory_block_size_bytes,
: which means it must be section-size aligned
populate_section_memmap
__populate_section_memmap : Checks range aligned to sub-section size
So, IIRC we have two cases during hotplug:
1) the ones that want memory blocks
2) the ones that do not want them (pmem stuff)
For #1, we always enforce section alignment in add_memory_resource, and for
#2 we always make sure the range is at least sub-section aligned.
And the important stuff is that boot memory is no longer to be hot-removed
(boot memory had some strange layout sometimes).
The vmemmap of boot mem sections is always fully populated, even with
strange memory layouts (e.g., see comment in pfn_valid()). In addition,
we can only offline+remove whole sections, so that should be fine.
So, given the above, I think it should be safe to drop that check in
remote_pte_table.
But do we really need to force page alignment in vmemmap_populate/vmemmap_free?
vmemmap_populate should already receive a page-aligned chunk because
__populate_section_memmap made sure of that, and vmemmap_free() should be ok
as we already filtered out at hot-adding stage.
Of course, this will hold as long as struct page size of multiple of 8.
Should that change we might get trouble, but I do not think that can ever
happened (tm).
But anyway, I am fine with placing a couple of checks in vmemmap_{populate,free}
just to double check.
What do you think?
I'd just throw in 1 or 2 VM_BUG_ON() to self-document what we expect and
that we thought about these conditions. It's then easy to identify the
relevant commit where we explain the rationale.
I don't have a strong opinion, the other archs also don't seem to care
about documenting/enforcing it.
--
Thanks,
David / dhildenb
