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

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Date:   Tue, 10 Aug 2021 18:43:39 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
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 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.