Re: [Xen-devel] [PATCH v5 3/8] mm: Scrub pages in alloc_heap_pages() if needed

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 06/27/2017 02:00 PM, Jan Beulich wrote:
> > > > Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 06/22/17 8:55 PM >>>
> > @@ -862,10 +879,19 @@ static struct page_info *alloc_heap_pages(
> >       if ( d != NULL )
> >           d->last_alloc_node = node;
> >   
> > +    need_scrub = !!first_dirty_pg && !(memflags & MEMF_no_scrub);
> No need for !! here. But I wonder whether that part of the check is really
> useful anyway, considering the sole use ...
>
> >       for ( i = 0; i < (1 << order); i++ )
> >       {
> >           /* Reference count must continuously be zero for free pages. */
> > -        BUG_ON(pg[i].count_info != PGC_state_free);
> > +        BUG_ON((pg[i].count_info & ~PGC_need_scrub) != PGC_state_free);
> > +
> > +        if ( test_bit(_PGC_need_scrub, &pg[i].count_info) )
> > +        {
> > +            if ( need_scrub )
> > +                scrub_one_page(&pg[i]);
> ... here. If it isn't, I think the local variable isn't warranted either.
> If you agree, the thus adjusted patch can have
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> (otherwise I'll wait with it to understand the reason first).


first_dirty_pg is indeed unnecessary but I think local variable is 
useful to avoid ANDing memflags inside the loop on each iteration 
(unless you think compiler is smart enough to realize that memflags is 
not changing).

-boris

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