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

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Message-ID: <CY4PR0201MB3460E372956C0E1B8D33F904E9E39@CY4PR0201MB3460.namprd02.prod.outlook.com>
Date:   Wed, 21 Jul 2021 14:38:30 +0000
From:   Ivan Teterevkov <ivan.teterevkov@...anix.com>
To:     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>,
        David Hildenbrand <david@...hat.com>,
        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 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.

At this point, we don't really need the PM_UFFD_WP or PM_SOFT_DIRTY bits
in the pagemap report and were considering them only if they were easy to
retrieve. The latter one seems to require some plumbing through the variety
of use cases in the kernel, so our intention at the moment is to capture it
in the pagemap docs as the known issue, presumably to handle by CRIU users.

(Cc Pavel Emelyanov CRIU chief maintainer)

Thanks,
Ivan