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

[Pavel Tatashin <pasha.tatashin@oracle.com>]

On Wed, Jul 25, 2018 at 5:30 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Tue, 24 Jul 2018 21:46:25 -0400 Pavel Tatashin <pasha.tatashin@oracle.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.

Makes sense. I will do the change in the next version of the patches.

>
> > >
> > > --- 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.

Will add it.

Thank you,
Pavel