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

[Pavel Tatashin]

On Tue, Jul 24, 2018 at 9:31 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, 24 Jul 2018 19:55:19 -0400 Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
>
> > update_defer_init() should be called only when struct page is about to be
> > initialized. Because it counts number of initialized struct pages, but
> > there we may skip struct pages if there is some mirrored memory.
> >
> > So move, update_defer_init() after checking for mirrored memory.
> >
> > Also, rename update_defer_init() to defer_init() and reverse the return
> > boolean to emphasize that this is a boolean function, that tells that the
> > reset of memmap initialization should be deferred.
> >
> > Make this function self-contained: do not pass number of already
> > initialized pages in this zone by using static counters.
> >
> > ...
> >
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -306,24 +306,28 @@ static inline bool __meminit early_page_uninitialised(unsigned long pfn)
> >  }
> >
> >  /*
> > - * Returns false when the remaining initialisation should be deferred until
> > + * Returns true when the remaining initialisation should be deferred until
> >   * later in the boot cycle when it can be parallelised.
> >   */
> > -static inline bool update_defer_init(pg_data_t *pgdat,
> > -                             unsigned long pfn, unsigned long zone_end,
> > -                             unsigned long *nr_initialised)
> > +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.

>
> --- 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().