linux-kernel - Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using folio_pte

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Message-ID: <c0309ab6-8bae-42b7-8d27-1df895689fb8@redhat.com>
Date: Mon, 3 Jun 2024 16:56:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, Lance Yang <ioworker0@...il.com>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com, 21cnbao@...il.com,
 baolin.wang@...ux.alibaba.com, ziy@...dia.com, fengwei.yin@...el.com,
 ying.huang@...el.com, libang.li@...group.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using
 folio_pte_batch()

On 03.06.24 16:43, Matthew Wilcox wrote:
> On Mon, Jun 03, 2024 at 10:07:45PM +0800, Lance Yang wrote:
>> +++ b/mm/mlock.c
>> @@ -307,26 +307,15 @@ void munlock_folio(struct folio *folio)
>>   static inline unsigned int folio_mlock_step(struct folio *folio,
>>   		pte_t *pte, unsigned long addr, unsigned long end)
>>   {
>> -	unsigned int count, i, nr = folio_nr_pages(folio);
>> -	unsigned long pfn = folio_pfn(folio);
>> +	const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
>> +	unsigned int count = (end - addr) >> PAGE_SHIFT;
> 
> This is a pre-existing bug, but ... what happens if you're on a 64-bit
> system and you mlock() a range that is exactly 2^44 bytes?  Seems to me
> that count becomes 0.  Why not use an unsigned long here and avoid the
> problem entirely?
> 
> folio_pte_batch() also needs to take an unsigned long max_nr in that
> case, because you aren't restricting it to folio_nr_pages().

Yeah, likely we should also take a look at other folio_pte_batch() users 
like copy_present_ptes() that pass the count as an int. Nothing should 
really be broken, but we might not batch as much as we could, which is 
unfortunate.

-- 
Cheers,

David / dhildenb