linux-kernel - Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[

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Date:   Wed, 05 Jul 2023 20:18:12 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[_lock]() to fail


Hi Hugh,

Sorry for not checking about this before. I am looking at a kernel
crash (BUG_ON()) on ppc64 with 4K page size. The reason we hit
BUG_ON() is beause we have pmd_same calling BUG_ON on 4K with hash
translation. We don't support THP with 4k page size and hash
translation.

Hugh Dickins <hughd@...gle.com> writes:

....

 +
> +pte_t *pte_offset_map_nolock(struct mm_struct *mm, pmd_t *pmd,
> +			     unsigned long addr, spinlock_t **ptlp)
> +{
> +	pmd_t pmdval;
> +	pte_t *pte;
> +
> +	pte = __pte_offset_map(pmd, addr, &pmdval);
> +	if (likely(pte))
> +		*ptlp = pte_lockptr(mm, &pmdval);
> +	return pte;
> +}
> +
> +pte_t *__pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> +			     unsigned long addr, spinlock_t **ptlp)
> +{
> +	spinlock_t *ptl;
> +	pmd_t pmdval;
> +	pte_t *pte;
> +again:
> +	pte = __pte_offset_map(pmd, addr, &pmdval);
> +	if (unlikely(!pte))
> +		return pte;
> +	ptl = pte_lockptr(mm, &pmdval);
> +	spin_lock(ptl);
> +	if (likely(pmd_same(pmdval, pmdp_get_lockless(pmd)))) {
> +		*ptlp = ptl;
> +		return pte;
> +	}
> +	pte_unmap_unlock(pte, ptl);
> +	goto again;
> +}

What is expected by that pmd_same check? We are holding pte lock
and not pmd lock. So contents of pmd can change.

-aneesh