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From:   "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
To:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>
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        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[_lock]() to fail
In-Reply-To: <8218ffdc-8be-54e5-0a8-83f5542af283@google.com>
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com>
 <8218ffdc-8be-54e5-0a8-83f5542af283@google.com>
Date:   Wed, 05 Jul 2023 20:18:12 +0530
Message-ID: <87y1juxvmb.fsf@linux.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk


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@google.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