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Message-ID: <06996a88-4248-d909-dc06-5af1ba580ef3@linux.dev>
Date:   Wed, 24 May 2023 11:11:48 +0800
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Subject: Re: [PATCH 04/31] mm/pgtable: allow pte_offset_map[_lock]() to fail
Content-Language: en-US
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Yang Shi <shy828301@gmail.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
References: <68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com>
 <8218ffdc-8be-54e5-0a8-83f5542af283@google.com>
 <9dc72654-79db-e988-54a8-488550d235ac@linux.dev>
 <1efc993b-5b41-4895-a4d-20d38eb95de5@google.com>
From:   Qi Zheng <qi.zheng@linux.dev>
In-Reply-To: <1efc993b-5b41-4895-a4d-20d38eb95de5@google.com>
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On 2023/5/24 10:22, Hugh Dickins wrote:
> On Mon, 22 May 2023, Qi Zheng wrote:
>> On 2023/5/22 12:53, Hugh Dickins wrote:
>>
>> [...]
>>
>>>    @@ -229,3 +231,57 @@ pmd_t pmdp_collapse_flush(struct vm_area_struct *vma,
>>> unsigned long address,
>>>    }
>>>    #endif
>>>    #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>>> +
>>> +pte_t *__pte_offset_map(pmd_t *pmd, unsigned long addr, pmd_t *pmdvalp)
>>> +{
>>> +	pmd_t pmdval;
>>> +
>>> +	/* rcu_read_lock() to be added later */
>>> +	pmdval = pmdp_get_lockless(pmd);
>>> +	if (pmdvalp)
>>> +		*pmdvalp = pmdval;
>>> +	if (unlikely(pmd_none(pmdval) || is_pmd_migration_entry(pmdval)))
>>> +		goto nomap;
>>> +	if (unlikely(pmd_trans_huge(pmdval) || pmd_devmap(pmdval)))
>>> +		goto nomap;
>>
>> Will the follow-up patch deal with the above situation specially?
> 
> No, the follow-up patch will only insert the rcu_read_lock() and unlock();
> and do something (something!) about the PAE mismatched halves case.
> 
>> Otherwise, maybe it can be changed to the following check method?
>>
>> 	if (unlikely(pmd_none(pmdval) || pmd_leaf(pmdval)))
>> 		goto nomap;
> 
> Maybe, but I'm not keen.  Partly just because pmd_leaf() is quite a
> (good) new invention (I came across a few instances in updating to
> the current tree), whereas here I'm just following the old examples,
> from zap_pmd_range() etc.  I'd have to spend a while getting to know
> pmd_leaf(), and its interaction with strange gotchas like pmd_present().
> 
> And partly because I do want as many corrupt cases as possible to
> reach the pmd_bad() check below, so generating a warning (and clear).
> I might be wrong, I haven't checked through the architectures and how
> pmd_leaf() is implemented in each, but my guess is that pmd_leaf()
> will tend to miss the pmd_bad() check.

IIUC, pmd_leaf() is just for checking a leaf mapped PMD, and will
not cover pmd_bad() case. Can see the examples in vmalloc_to_page()
and apply_to_pmd_range().

> 
> But if you can demonstrate a performance improvement from using
> pmd_leaf() there, I expect many people would prefer that improvement
> to badness catching: send a patch later to make that change if it's
> justified.

Probably not a lot of performance gain, just makes the check more
concise.

Thanks,
Qi

> 
> Thanks a lot for all your attention to these.
> 
> Hugh
> 
>>
>>> +	if (unlikely(pmd_bad(pmdval))) {
>>> +		pmd_clear_bad(pmd);
>>> +		goto nomap;
>>> +	}
>>> +	return __pte_map(&pmdval, addr);
>>> +nomap:
>>> +	/* rcu_read_unlock() to be added later */
>>> +	return NULL;
>>> +}

-- 
Thanks,
Qi