linux-kernel - Re: [PATCH 02/10] mm/page_alloc: use unsigned int for "order" in __rmqueue

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Date:   Sat, 27 Jul 2019 10:34:04 +0800
From:   Pengfei Li <lpf.vector@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        mgorman@...hsingularity.net, mhocko@...e.com, vbabka@...e.cz,
        Qian Cai <cai@....pw>, aryabinin@...tuozzo.com,
        osalvador@...e.de, rostedt@...dmis.org, mingo@...hat.com,
        pavel.tatashin@...rosoft.com, rppt@...ux.ibm.com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 02/10] mm/page_alloc: use unsigned int for "order" in __rmqueue_fallback()

On Fri, Jul 26, 2019 at 5:36 PM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> On 25/07/2019 20.42, Pengfei Li wrote:
> > Because "order" will never be negative in __rmqueue_fallback(),
> > so just make "order" unsigned int.
> > And modify trace_mm_page_alloc_extfrag() accordingly.
> >
>
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 75c18f4fd66a..1432cbcd87cd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2631,8 +2631,8 @@ static bool unreserve_highatomic_pageblock(const struct alloc_context *ac,
> >   * condition simpler.
> >   */
> >  static __always_inline bool
> > -__rmqueue_fallback(struct zone *zone, int order, int start_migratetype,
> > -                                             unsigned int alloc_flags)
> > +__rmqueue_fallback(struct zone *zone, unsigned int order,
> > +             int start_migratetype, unsigned int alloc_flags)
> >  {
>
> Please read the last paragraph of the comment above this function, run
> git blame to figure out when that was introduced, and then read the full
> commit description.

Thanks for your comments.

I have read the commit info of commit b002529d2563 ("mm/page_alloc.c:
eliminate unsigned confusion in __rmqueue_fallback").

And I looked at the discussion at https://lkml.org/lkml/2017/6/21/684 in detail.

> Here be dragons. At the very least, this patch is
> wrong in that it makes that comment inaccurate.

I wonder if you noticed the commit 6bb154504f8b ("mm, page_alloc: spread
allocations across zones before introducing fragmentation").

Commit 6bb154504f8b introduces a local variable min_order in
__rmqueue_fallback().

And you can see

        for (current_order = MAX_ORDER - 1; current_order >= min_order;
                                --current_order) {

The “current_order” and "min_order"  are int, so here is ok.

Since __rmqueue_fallback() is only called by __rmqueue() and "order" is unsigned
int in __rmqueue(), then I think that making "order" is also unsigned
int is good.

Maybe I should also modify the comments here?

>
> Rasmus

Thank you again for your review.

--
Pengfei