Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI only to CPUs in kernel mode

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On 06.04.23 16:04, Peter Zijlstra wrote:
On Thu, Apr 06, 2023 at 03:29:28PM +0200, Peter Zijlstra wrote:
On Thu, Apr 06, 2023 at 09:38:50AM -0300, Marcelo Tosatti wrote:

To actually hit this path you're doing something really dodgy.

Apparently khugepaged is using the same infrastructure:

$ grep tlb_remove_table khugepaged.c
	tlb_remove_table_sync_one();
	tlb_remove_table_sync_one();

So just enabling khugepaged will hit that path.

Urgh, WTF..

Let me go read that stuff :/

At the very least the one on collapse_and_free_pmd() could easily become
a call_rcu() based free.

I'm not sure I'm following what collapse_huge_page() does just yet.

It wants to replace a leaf page table by a THP (Transparent Huge Page mapped by a PMD). So we want to rip out a leaf page table while other code (GUP-fast) might still be walking it. In contrast to freeing the page table, we put it into a list where it can be reuse when having to PTE-map a THP again.

Now, similar to after freeing the page table, someone else could reuse that page table and modify it.

If we have GUP-fast walking the page table while that is happening, we're in trouble. So we have to make sure GUP-fast is done before enqueuing the now-free page table.

That's why the tlb_remove_table_sync_one() was recently added (by Jann IIRC).

--
Thanks,

David / dhildenb




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