On 26.04.24 23:58, Peter Xu wrote:
On Fri, Apr 26, 2024 at 11:33:08PM +0200, David Hildenbrand wrote:
I raised this topic in the past, and IMHO we either (a) never should have
added COW support; or (b) added COW support by using ordinary anonymous
memory (hey, partial mappings of hugetlb pages! ;) ).
After all, COW is an optimization to speed up fork and defer copying. It
relies on memory overcommit, but that doesn't really apply to hugetlb, so we
fake it ...
Good summary.
One easy ABI break I had in mind was to simply *not* allow COW-sharing of
anon hugetlb folios; for example, simply don't copy the page into the child.
Chances are there are not really a lot of child processes that would fail
... but likely we would break *something*. So there is no easy way out :(
Right, not easy. The thing is this is one spot out of many of the
specialties, it also may or may not be worthwhile to have dedicated time
while nobody yet has a problem with it. It might be easier to start with
v2, even though that's also hard to nail everything properly - the
challenge can come from different angles.
Thanks for the sharings, helpful. I'll go ahead with the Power fix on
hugepd putting this aside.
Yes, hopefully we already do have a test case for that. When writing
gup_longterm.c I was more focusing on memfd vs. ordinary file systems
("filesystem type") than how it's mapped into the page tables.
I hope that before the end of this year, whatever I'll fix can go away, by
removing hugepd completely from Linux. For now that may or may not be as
smooth, so we'd better still fix it.
Crossing fingers, I'm annoyed whenever I stumble over it :)
--
Cheers,
David / dhildenb
