On 25.04.24 17:19, Guillaume Morin wrote:
On 24 Apr 23:00, David Hildenbrand wrote:
One issue here is that FOLL_FORCE|FOLL_WRITE is not implemented for
hugetlb mappings. However this was also on my TODO and I have a draft
patch that implements it.
Yes, I documented it back then and added sanity checks in GUP code to fence
it off. Shouldn't be too hard to implement (famous last words) and would be
the cleaner thing to use here once I manage to switch over to
FOLL_WRITE|FOLL_FORCE to break COW.
Yes, my patch seems to be working. The hugetlb code is pretty simple.
And it allows ptrace and the proc pid mem file to work on the executable
private hugetlb mappings.
There is one thing I am unclear about though. hugetlb enforces that
huge_pte_write() is true on FOLL_WRITE in both the fault and
follow_page_mask paths. I am not sure if we can simply assume in the
hugetlb code that if the pte is not writable and this is a write fault
then we're in the FOLL_FORCE|FOLL_WRITE case. Or do we want to keep the
checks simply not enforce it for FOLL_FORCE|FOLL_WRITE?
The latter is more complicated in the fault path because there is no
FAULT_FLAG_FORCE flag.
I just pushed something to
https://github.com/davidhildenbrand/linux/tree/uprobes_cow
Only very lightly tested so far. Expect the worst :)
I still detest having the zapping logic there, but to get it all right I
don't see a clean way around that.
For hugetlb, we'd primarily have to implement the
mm_walk_ops->hugetlb_entry() callback (well, and FOLL_FORCE).
Likely vaddr and PAGE_SIZE in uprobe_write_opcode() would have to be
expanded to cover the full hugetlb page.
--
Cheers,
David / dhildenb
