linux-kernel - Re: [PATCH v2 12/32] mm/vmalloc: vmalloc_to_page() use pte_offset

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Message-ID: <b479b946-f052-eb75-295d-6fa7c2d8ce8e@google.com>
Date:   Tue, 11 Jul 2023 09:13:18 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Mark Brown <broonie@...nel.org>
cc:     Hugh Dickins <hughd@...gle.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        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>,
        Huang Ying <ying.huang@...el.com>,
        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>,
        Ryan Roberts <ryan.roberts@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        regressions@...mhuis.info, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 12/32] mm/vmalloc: vmalloc_to_page() use
 pte_offset_kernel()

On Tue, 11 Jul 2023, Mark Brown wrote:
> On Mon, Jul 10, 2023 at 09:34:42PM -0700, Hugh Dickins wrote:
> 
> > I suppose I should ask you to try reverting this 0d1c81edc61e alone
> > from 6.5-rc1: the consistency of your bisection implies that it will
> > "fix" the issues, and it is a commit which we could drop.  It makes
> > me a little nervous, applying userspace-pagetable validation to kernel
> > pagetables, so I don't want to drop it; and it would really be cargo-
> > culting to drop it without understanding.  But we could drop it.
> 
> I did look at that, it doesn't revert cleanly by itself. ...

Right, that ptep_get() wrapper on the next line came in on top.
The patch to revert just 0d1c81edc61e is this:

--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -703,10 +703,11 @@ struct page *vmalloc_to_page(const void
 	if (WARN_ON_ONCE(pmd_bad(*pmd)))
 		return NULL;
 
-	ptep = pte_offset_kernel(pmd, addr);
+	ptep = pte_offset_map(pmd, addr);
 	pte = ptep_get(ptep);
 	if (pte_present(pte))
 		page = pte_page(pte);
+	pte_unmap(ptep);
 
 	return page;
 }