linux-kernel - Re: [PATCH 00/12] mm: free retracted page table by RCU

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Date:   Wed, 31 May 2023 19:59:20 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Yang Shi <shy828301@...il.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        Russell King <linux@...linux.org.uk>,
        "David S. Miller" <davem@...emloft.net>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 00/12] mm: free retracted page table by RCU

On Mon, May 29, 2023 at 8:11 AM Hugh Dickins <hughd@...gle.com> wrote:
> Here is the third series of patches to mm (and a few architectures), based
> on v6.4-rc3 with the preceding two series applied: in which khugepaged
> takes advantage of pte_offset_map[_lock]() allowing for pmd transitions.

To clarify: Part of the design here is that when you look up a user
page table with pte_offset_map_nolock() or pte_offset_map() without
holding mmap_lock in write mode, and you later lock the page table
yourself, you don't know whether you actually have the real page table
or a detached table that is currently in its RCU grace period, right?
And detached tables are supposed to consist of only zeroed entries,
and we assume that no relevant codepath will do anything bad if one of
these functions spuriously returns a pointer to a page table full of
zeroed entries?

So in particular, in handle_pte_fault() we can reach the "if
(unlikely(!pte_same(*vmf->pte, entry)))" with vmf->pte pointing to a
detached zeroed page table, but we're okay with that because in that
case we know that !pte_none(vmf->orig_pte)&&pte_none(*vmf->pte) ,
which implies !pte_same(*vmf->pte, entry) , which means we'll bail
out?

If that's the intent, it might be good to add some comments, because
at least to me that's not very obvious.