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

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Date: Mon, 3 Jun 2024 17:26:32 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Lance Yang <ioworker0@...il.com>, 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 17:01, Matthew Wilcox wrote:
> On Mon, Jun 03, 2024 at 04:56:05PM +0200, David Hildenbrand wrote:
>> 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.
> 
> You did include:
> 
>          VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio);
> 
> so at the least we have a userspace-triggerable warning.

Yes, and max_nr == 0 would likely not be healthy to the system.

But

For copy_pte_range(), zap_pte_range() and the madvise users, we should 
always have:
	next = pmd_addr_end(addr, end);

and use "next" as the actual "end" -- not the VMA end. So "end - addr" = 
"next - addr" should never exceed a single PMD size.


mlock_pte_range() is also called from walk_page_range(), which uses
	next = pmd_addr_end(addr, end);

So likely exceeding PMD size is not possible here and all is working as 
expected.

Will double check later.

-- 
Cheers,

David / dhildenb