Received-SPF: pass (google.com: domain of linux-kernel+bounces-119429-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Message-ID: <cf16a19d-cc3b-4f19-a46f-83554a472368@redhat.com>
Date: Tue, 26 Mar 2024 17:10:43 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCH v2] mm: hold PTL from the first PTE while reclaiming a
 large folio
Content-Language: en-US
To: Barry Song <21cnbao@gmail.com>, akpm@linux-foundation.org,
 linux-mm@kvack.org
Cc: chrisl@kernel.org, hanchuanhua@oppo.com, hughd@google.com,
 linux-kernel@vger.kernel.org, mhocko@suse.com, ryan.roberts@arm.com,
 shy828301@gmail.com, v-songbaohua@oppo.com, wangkefeng.wang@huawei.com,
 willy@infradead.org, xiang@kernel.org, ying.huang@intel.com,
 yuzhao@google.com
References: <20240306095219.71086-1-21cnbao@gmail.com>
From: David Hildenbrand <david@redhat.com>
Autocrypt: addr=david@redhat.com; keydata=
 xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ
 dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL
 QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp
 XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK
 Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9
 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt
 WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc
 UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv
 jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb
 B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk
 ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW
 AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q
 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp
 rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf
 wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4
 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l
 pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd
 KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE
 BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs
 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF
 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9
 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz
 Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb
 T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A
 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk
 CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G
 NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75
 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx
 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS
 lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv
 AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa
 N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3
 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB
 boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq
 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f
 XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ
 a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq
 Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6
 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8
 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E
 th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr
 jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt
 WNyWQQ==
Organization: Red Hat
In-Reply-To: <20240306095219.71086-1-21cnbao@gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

On 06.03.24 10:52, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.com>
> 
> Within try_to_unmap_one(), page_vma_mapped_walk() races with other
> PTE modifications preceded by pte clear. While iterating over PTEs
> of a large folio, it only starts acquiring PTL from the first valid
> (present) PTE. PTE modifications can temporarily set PTEs to
> pte_none. Consequently, the initial PTEs of a large folio might
> be skipped in try_to_unmap_one().
> For example, for an anon folio, if we skip PTE0, we may have PTE0
> which is still present, while PTE1 ~ PTE(nr_pages - 1) are swap
> entries after try_to_unmap_one().
> So folio will be still mapped, the folio fails to be reclaimed
> and is put back to LRU in this round.
> This also breaks up PTEs optimization such as CONT-PTE on this
> large folio and may lead to accident folio_split() afterwards.
> And since a part of PTEs are now swap entries, accessing those
> parts will introduce overhead - do_swap_page.
> Although the kernel can withstand all of the above issues, the
> situation still seems quite awkward and warrants making it more
> ideal.
> The same race also occurs with small folios, but they have only
> one PTE, thus, it won't be possible for them to be partially
> unmapped.
> This patch holds PTL from PTE0, allowing us to avoid reading PTE
> values that are in the process of being transformed. With stable
> PTE values, we can ensure that this large folio is either
> completely reclaimed or that all PTEs remain untouched in this
> round.
> A corner case is that if we hold PTL from PTE0 and most initial
> PTEs have been really unmapped before that, we may increase the
> duration of holding PTL. Thus we only apply this optimization to
> folios which are still entirely mapped (not in deferred_split
> list).
> 
> Cc: Hugh Dickins <hughd@google.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
> ---
>   v2:
>    * Refine commit message and code comment after reading all comments
>      from Ryan and David, thanks!
>    * Avoid increasing the duration of PTL by applying optimization
>      on folios not in deferred_split_list with respect to Ying's
>      comment, thanks!
> 
>   mm/vmscan.c | 12 ++++++++++++
>   1 file changed, 12 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 0b888a2afa58..7106741387e8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1270,6 +1270,18 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>   
>   			if (folio_test_pmd_mappable(folio))
>   				flags |= TTU_SPLIT_HUGE_PMD;
> +			/*
> +			 * Without TTU_SYNC, try_to_unmap will only begin to hold PTL
> +			 * from the first present PTE within a large folio. Some initial
> +			 * PTEs might be skipped due to races with parallel PTE writes
> +			 * in which PTEs can be cleared temporarily before being written
> +			 * new present values. This will lead to a large folio is still
> +			 * mapped while some subpages have been partially unmapped after
> +			 * try_to_unmap; TTU_SYNC helps try_to_unmap acquire PTL from the
> +			 * first PTE, eliminating the influence of temporary PTE values.
> +			 */
> +			if (folio_test_large(folio) && list_empty(&folio->_deferred_list))
> +				flags |= TTU_SYNC;
>   
>   			try_to_unmap(folio, flags);
>   			if (folio_mapped(folio)) {

Hopefully this won't have unexpected performance "surprises".

I do wonder if we should really care about the "_deferred_list" 
optimization here, though, I'd just drop it.

In any case

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb