linux-kernel - Re: [PATCH v3 04/13] powerpc: assert_pte_locked() use pte_offset_map

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Message-ID: <20230721131341.w5abuxcbohofpzwa@patel>
Date:   Fri, 21 Jul 2023 18:43:41 +0530
From:   Jay Patel <jaypatel@...ux.ibm.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
Cc:     Hugh Dickins <hughd@...gle.com>, Miaohe Lin <linmiaohe@...wei.com>,
        David Hildenbrand <david@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Yang Shi <shy828301@...il.com>, Peter Xu <peterx@...hat.com>,
        linux-kernel@...r.kernel.org, Song Liu <song@...nel.org>,
        sparclinux@...r.kernel.org,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Will Deacon <will@...nel.org>, linux-s390@...r.kernel.org,
        Yu Zhao <yuzhao@...gle.com>, Ira Weiny <ira.weiny@...el.com>,
        Alistair Popple <apopple@...dia.com>,
        Russell King <linux@...linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Steven Price <steven.price@....com>,
        Christoph Hellwig <hch@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        linux-arm-kernel@...ts.infradead.org, Zi Yan <ziy@...dia.com>,
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        Axel Rasmussen <axelrasmussen@...gle.com>,
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        Christian Borntraeger <borntraeger@...ux.ibm.com>,
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        linuxppc-dev@...ts.ozlabs.org,
        Naoya Horiguchi <naoya.horiguchi@....com>,
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        "David S. Miller" <davem@...emloft.net>,
        Mike Rapoport <rppt@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3 04/13] powerpc: assert_pte_locked() use
 pte_offset_map_nolock()

On Jul 19 2023, Aneesh Kumar K V wrote:
> On 7/19/23 10:34 AM, Hugh Dickins wrote:
> > On Tue, 18 Jul 2023, Aneesh Kumar K.V wrote:
> >> Hugh Dickins <hughd@...gle.com> writes:
> >>
> >>> Instead of pte_lockptr(), use the recently added pte_offset_map_nolock()
> >>> in assert_pte_locked().  BUG if pte_offset_map_nolock() fails: this is
> >>> stricter than the previous implementation, which skipped when pmd_none()
> >>> (with a comment on khugepaged collapse transitions): but wouldn't we want
> >>> to know, if an assert_pte_locked() caller can be racing such transitions?
> >>>
> >>
> >> The reason we had that pmd_none check there was to handle khugpaged. In
> >> case of khugepaged we do pmdp_collapse_flush and then do a ptep_clear.
> >> ppc64 had the assert_pte_locked check inside that ptep_clear.
> >>
> >> _pmd = pmdp_collapse_flush(vma, address, pmd);
> >> ..
> >> ptep_clear()
> >> -> asset_ptep_locked()
> >> ---> pmd_none
> >> -----> BUG
> >>
> >>
> >> The problem is how assert_pte_locked() verify whether we are holding
> >> ptl. It does that by walking the page table again and in this specific
> >> case by the time we call the function we already had cleared pmd .
> > 
> > Aneesh, please clarify, I've spent hours on this.
> > 
> > From all your use of past tense ("had"), I thought you were Acking my
> > patch; but now, after looking again at v3.11 source and today's,
> > I think you are NAKing my patch in its present form.
> > 
> 
> Sorry for the confusion my reply created. 
> 
> > You are pointing out that anon THP's __collapse_huge_page_copy_succeeded()
> > uses ptep_clear() at a point after pmdp_collapse_flush() already cleared
> > *pmd, so my patch now leads that one use of assert_pte_locked() to BUG.
> > Is that your point?
> > 
> 
> Yes. I haven't tested this yet to verify that it is indeed hitting that BUG.
> But a code inspection tells me we will hit that BUG on powerpc because of
> the above details.
>
Hi Aneesh,

After testing it, I can confirm that it encountered a BUG on powerpc.
Log report as attached

Thanks,
Jay Patel 
> > I can easily restore that khugepaged comment (which had appeared to me
> > out of date at the time, but now looks still relevant) and pmd_none(*pmd)
> > check: but please clarify.
> > 
> 
> That is correct. if we add that pmd_none check back we should be good here.
> 
> 
> -aneesh

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