linux-kernel - Re: [PATCH 2/3] mm: calculate deferred pages after skipping mirrored memory

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Date:   Wed, 25 Jul 2018 14:30:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kirill.shutemov@...ux.intel.com, Michal Hocko <mhocko@...e.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        dan.j.williams@...el.com, jack@...e.cz, jglisse@...hat.com,
        Souptick Joarder <jrdr.linux@...il.com>, bhe@...hat.com,
        gregkh@...uxfoundation.org, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...il.com>, dave.hansen@...el.com,
        rientjes@...gle.com, mingo@...nel.org,
        osalvador@...hadventures.net, abdhalee@...ux.vnet.ibm.com,
        mpe@...erman.id.au
Subject: Re: [PATCH 2/3] mm: calculate deferred pages after skipping
 mirrored memory

On Tue, 24 Jul 2018 21:46:25 -0400 Pavel Tatashin <pasha.tatashin@...cle.com> wrote:

> > > +static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> > >  {
> > > +     static unsigned long prev_end_pfn, nr_initialised;
> >
> > So answer me quick, what happens with a static variable in an inlined
> > function?  Is there one copy kernel-wide?  One copy per invocation
> > site?  One copy per compilation unit?
> >
> > Well I didn't know so I wrote a little test.  One copy per compilation
> > unit (.o file), it appears.
> >
> > It's OK in this case because the function is in .c (and has only one
> > call site).  But if someone moves it into a header and uses it from a
> > different .c file, they have problems.
> >
> > So it's dangerous, and poor practice.  I'll make this non-static
> > __meminit.
> 
> I agree, it should not be moved to header it is dangerous.
> 
> But, on the other hand this is a hot-path. memmap_init_zone() might
> need to go through billions of struct pages early in boot, and I did
> not want us to waste time on function calls. With defer_init() this is
> not a problem, because if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set
> memmap_init_zone() won't have much work to do, but for
> overlap_memmap_init() this is a problem, especially because I expect
> compiler to optimize the pfn dereference usage in inline function.

Well.  The compiler will just go and inline defer_init() anwyay - it
has a single callsite and is in the same __meminint section as its
calling function.  My gcc-7.2.0 does this.  Marking it noninline
__meminit is basically syntactic fluff designed to encourage people to
think twice.

> >
> > --- a/mm/page_alloc.c~mm-calculate-deferred-pages-after-skipping-mirrored-memory-fix
> > +++ a/mm/page_alloc.c
> > @@ -309,7 +309,8 @@ static inline bool __meminit early_page_
> >   * Returns true when the remaining initialisation should be deferred until
> >   * later in the boot cycle when it can be parallelised.
> >   */
> > -static inline bool defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> > +static bool __meminit
> > +defer_init(int nid, unsigned long pfn, unsigned long end_pfn)
> >  {
> >         static unsigned long prev_end_pfn, nr_initialised;
> >
> >
> > Also, what locking protects these statics?  Our knowledge that this
> > code is single-threaded, presumably?
> 
> Correct, this is called only from "context == MEMMAP_EARLY", way
> before smp_init().

Might be worth a little comment to put readers minds at ease.