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Message-ID: <7495754c-9267-74af-b943-9b0f86619b5d@redhat.com>
Date:   Fri, 13 Oct 2023 10:04:12 +0200
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Subject: Re: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma
 in folio_move_anon_rmap()
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To:     Peter Xu <peterx@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
        brauner@kernel.org, shuah@kernel.org, aarcange@redhat.com,
        lokeshgidra@google.com, hughd@google.com, mhocko@suse.com,
        axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org,
        Liam.Howlett@oracle.com, jannh@google.com, zhangpeng362@huawei.com,
        bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com,
        jdduke@google.com, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, kernel-team@android.com
References: <20231009064230.2952396-1-surenb@google.com>
 <20231009064230.2952396-2-surenb@google.com> <ZShswW2rkKTwnrV3@x1n>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZShswW2rkKTwnrV3@x1n>
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On 13.10.23 00:01, Peter Xu wrote:
> On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote:
>> From: Andrea Arcangeli <aarcange@redhat.com>
>>
>> For now, folio_move_anon_rmap() was only used to move a folio to a
>> different anon_vma after fork(), whereby the root anon_vma stayed
>> unchanged. For that, it was sufficient to hold the folio lock when
>> calling folio_move_anon_rmap().
>>
>> However, we want to make use of folio_move_anon_rmap() to move folios
>> between VMAs that have a different root anon_vma. As folio_referenced()
>> performs an RMAP walk without holding the folio lock but only holding the
>> anon_vma in read mode, holding the folio lock is insufficient.
>>
>> When moving to an anon_vma with a different root anon_vma, we'll have to
>> hold both, the folio lock and the anon_vma lock in write mode.
>> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to
>> read-lock the anon_vma, we have to re-check if the mapping was changed
>> in the meantime. If that was the case, we have to retry.
>>
>> Note that folio_move_anon_rmap() must only be called if the anon page is
>> exclusive to a process, and must not be called on KSM folios.
>>
>> This is a preparation for UFFDIO_MOVE, which will hold the folio lock,
>> the anon_vma lock in write mode, and the mmap_lock in read mode.
>>
>> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>> ---
>>   mm/rmap.c | 24 ++++++++++++++++++++++++
>>   1 file changed, 24 insertions(+)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index c1f11c9dbe61..f9ddc50269d2 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>>   	struct anon_vma *root_anon_vma;
>>   	unsigned long anon_mapping;
>>   
>> +retry:
>>   	rcu_read_lock();
>> +retry_under_rcu:
>>   	anon_mapping = (unsigned long)READ_ONCE(folio->mapping);
>>   	if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>>   		goto out;
>> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio,
>>   	anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
>>   	root_anon_vma = READ_ONCE(anon_vma->root);
>>   	if (down_read_trylock(&root_anon_vma->rwsem)) {
>> +		/*
>> +		 * folio_move_anon_rmap() might have changed the anon_vma as we
>> +		 * might not hold the folio lock here.
>> +		 */
>> +		if (unlikely((unsigned long)READ_ONCE(folio->mapping) !=
>> +			     anon_mapping)) {
>> +			up_read(&root_anon_vma->rwsem);
>> +			goto retry_under_rcu;
> 
> Is adding this specific label worthwhile?  How about rcu unlock and goto
> retry (then it'll also be clear that we won't hold rcu read lock for
> unpredictable time)?

+1, sounds good to me

-- 
Cheers,

David / dhildenb