Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs | Date | Wed, 21 Jul 2021 18:19:42 +0200 |
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On 21.07.21 16:38, Ivan Teterevkov wrote: > On Mon, Jul 19, 2021 5:56 PM +0000, Peter Xu wrote: >> I'm also curious what would be the real use to have an accurate PM_SWAP >> accounting. To me current implementation may not provide accurate value but >> should be good enough for most cases. However not sure whether it's also true >> for your use case. > > We want the PM_SWAP bit implemented (for shared memory in the pagemap > interface) to enhance the live migration for some fraction of the guest > VMs that have their pages swapped out to the host swap. Once those pages > are paged in and transferred over network, we then want to release them > with madvise(MADV_PAGEOUT) and preserve the working set of the guest VMs > to reduce the thrashing of the host swap.
There are 3 possibilities I think (swap is just another variant of the page cache):
1) The page is not in the page cache, e.g., it resides on disk or in a swap file. pte_none(). 2) The page is in the page cache and is not mapped into the page table. pte_none(). 3) The page is in the page cache and mapped into the page table. !pte_none().
Do I understand correctly that you want to identify 1) and indicate it via PM_SWAP?
-- Thanks,
David / dhildenb
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