Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for shmem/hugetlbfs

[David Hildenbrand]

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