linux-kernel - Re: [PATCH v1 2/6] mm/rmap: move SetPageAnonExclusive out of __page_set_anon

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Date:   Wed, 13 Sep 2023 16:34:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <muchun.song@...ux.dev>
Subject: Re: [PATCH v1 2/6] mm/rmap: move SetPageAnonExclusive out of
 __page_set_anon_rmap()

On 13.09.23 16:32, Matthew Wilcox wrote:
> On Wed, Sep 13, 2023 at 02:51:09PM +0200, David Hildenbrand wrote:
>> @@ -1246,11 +1241,13 @@ void page_add_anon_rmap(struct page *page, struct vm_area_struct *vma,
>>   
>>   	if (likely(!folio_test_ksm(folio))) {
>>   		if (first)
>> -			__page_set_anon_rmap(folio, page, vma, address,
>> -					     !!(flags & RMAP_EXCLUSIVE));
>> +			__folio_set_anon(folio, vma, address,
>> +					 !!(flags & RMAP_EXCLUSIVE));
>>   		else
>>   			__page_check_anon_rmap(folio, page, vma, address);
>>   	}
>> +	if (flags & RMAP_EXCLUSIVE)
>> +		SetPageAnonExclusive(page);
> 
> Won't we end up setting AnonExclusive on ksm pages, or do we make sure
> to never pass RMAP_EXCLUSIVE for ksm pages?

Not if there is a bug and someone passes RMAP_EXCLUSIVE for these. :)

Fortunately, we do have

VM_BUG_ON_PGFLAGS(!PageAnon(page) || PageKsm(page), page);

in SetPageAnonExclusive() to catch such bugs.

> 
> Maybe better to move these last two lines inside the previous test,
> just to avoid the question.

That could end up hiding another BUG, so I'd rather let 
SetPageAnonExclusive() catch it.

-- 
Cheers,

David / dhildenb