Re: [PATCH] z3fold: add inter-page compaction

[Vitaly Wool]

On Sun, May 26, 2019 at 12:09 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

<snip>
> Forward-declaring inline functions is peculiar, but it does appear to work.
>
> z3fold is quite inline-happy.  Fortunately the compiler will ignore the
> inline hint if it seems a bad idea.  Even then, the below shrinks
> z3fold.o text from 30k to 27k.  Which might even make it faster....

It is faster with inlines, I'll try to find a better balance between
size and performance in the next version of the patch though.

<snip>
> >
> > ...
> >
> > +static inline struct z3fold_header *__get_z3fold_header(unsigned long handle,
> > +                                                     bool lock)
> > +{
> > +     struct z3fold_buddy_slots *slots;
> > +     struct z3fold_header *zhdr;
> > +     unsigned int seq;
> > +     bool is_valid;
> > +
> > +     if (!(handle & (1 << PAGE_HEADLESS))) {
> > +             slots = handle_to_slots(handle);
> > +             do {
> > +                     unsigned long addr;
> > +
> > +                     seq = read_seqbegin(&slots->seqlock);
> > +                     addr = *(unsigned long *)handle;
> > +                     zhdr = (struct z3fold_header *)(addr & PAGE_MASK);
> > +                     preempt_disable();
>
> Why is this done?
>
> > +                     is_valid = !read_seqretry(&slots->seqlock, seq);
> > +                     if (!is_valid) {
> > +                             preempt_enable();
> > +                             continue;
> > +                     }
> > +                     /*
> > +                      * if we are here, zhdr is a pointer to a valid z3fold
> > +                      * header. Lock it! And then re-check if someone has
> > +                      * changed which z3fold page this handle points to
> > +                      */
> > +                     if (lock)
> > +                             z3fold_page_lock(zhdr);
> > +                     preempt_enable();
> > +                     /*
> > +                      * we use is_valid as a "cached" value: if it's false,
> > +                      * no other checks needed, have to go one more round
> > +                      */
> > +             } while (!is_valid || (read_seqretry(&slots->seqlock, seq) &&
> > +                     (lock ? ({ z3fold_page_unlock(zhdr); 1; }) : 1)));
> > +     } else {
> > +             zhdr = (struct z3fold_header *)(handle & PAGE_MASK);
> > +     }
> > +
> > +     return zhdr;
> > +}
> >
> > ...
> >
> >  static unsigned short handle_to_chunks(unsigned long handle)
> >  {
> > -     unsigned long addr = *(unsigned long *)handle;
> > +     unsigned long addr;
> > +     struct z3fold_buddy_slots *slots = handle_to_slots(handle);
> > +     unsigned int seq;
> > +
> > +     do {
> > +             seq = read_seqbegin(&slots->seqlock);
> > +             addr = *(unsigned long *)handle;
> > +     } while (read_seqretry(&slots->seqlock, seq));
>
> It isn't done here (I think).

handle_to_chunks() is always called with z3fold header locked which
makes it a lot easier in this case. I'll add some comments in V2.

Thanks,
   Vitaly