linux-kernel - Re: [PATCH v14 011/138] mm/lru: Add folio LRU functions

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Date:   Wed, 11 Aug 2021 10:58:21 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>,
        Christoph Hellwig <hch@....de>,
        David Howells <dhowells@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v14 011/138] mm/lru: Add folio LRU functions

On 8/10/21 7:43 PM, Matthew Wilcox wrote:
> On Tue, Aug 10, 2021 at 06:01:16PM +0200, Vlastimil Babka wrote:
>> Actually looking at the git version, which has also this:
>> 
>>  static __always_inline void update_lru_size(struct lruvec *lruvec,
>>                                 enum lru_list lru, enum zone_type zid,
>> -                               int nr_pages)
>> +                               long nr_pages)
>>  {
>> 
>> Why now and here? Some of the functions called from update_lru_size()
>> still take int so this looks arbitrary?
> 
> I'm still a little freaked out about the lack of warning for:
> 
> void f(long n);
> void g(unsigned int n) { f(-n); }
> 
> so I've decided that the count of pages in a folio is always of type
> long.  The actual number is positive, and currently it's between 1 and
> 1024 (inclusive on both bounds), so it's always going to be
> representable in an int.  Narrowing it doesn't cause a bug, so we don't
> need to change nr_pages anywhere, but it does no harm to make functions
> take a long instead of an int (it may even cause slightly better code
> generation, based on the sample of functions I've looked at).
> 
> Maybe changing update_lru_size() in this patch is wrong.  I can drop it
> if you like.

It's fine, knowing it wasn't some rebasing error.