Re: VM_BUG_ON(!tlb->end) on munmap() with CONT hugetlb pages

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Mar 23, 2022 at 11:51:25AM +0000, Steve Capper wrote:
> On 22/03/2022 17:56, Catalin Marinas wrote:
> > At a quick look, we wouldn't have a problem with missing TLB flushing
> > since huge_ptep_get_and_clear() does this for contiguous PTEs. Not sure
> > why it needs this though, Steve added it in commit d8bdcff28764. I think
> > we can defer this flushing to tlb_remove_page_size().
> 
> The TLB flush in huge_ptep_get_and_clear() was added because it was called
> by hugetlb_change_protection() without any flushing. The concern was that,
> without the flush, it would be possible to get to different views of the
> same contiguous huge page. (Being contiguous they were not changed en masse
> atomically).

Maybe the code paths have been changed since but looking at
hugetlb_change_protection(), we have huge_ptep_modify_prot_start()
calling huge_ptep_get_and_clear() which AFAICT only needs to clear the
ptes. huge_ptep_modify_prot_commit() calls set_huge_pte_at() which does
another pte clearing + TLBI (clear_flush()) before setting the new ptes.
So we do the pte clearing and TLBI twice already.

-- 
Catalin