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

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Date:   Wed, 21 Jul 2021 18:19:42 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        Peter Xu <peterx@...hat.com>,
        Tiberiu Georgescu <tiberiu.georgescu@...anix.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Alistair Popple <apopple@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "Carl Waldspurger [C]" <carl.waldspurger@...anix.com>,
        Florian Schmidt <flosch@...anix.com>,
        "ovzxemul@...il.com" <ovzxemul@...il.com>
Subject: Re: [PATCH v5 24/26] mm/pagemap: Recognize uffd-wp bit for
 shmem/hugetlbfs

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