Re: [PATCH v5 2/7] slob: Respect list_head abstraction layer

[Tobin C. Harding]

On Wed, Apr 03, 2019 at 06:00:30PM +0000, Roman Gushchin wrote:
> On Wed, Apr 03, 2019 at 10:05:40AM +1100, Tobin C. Harding wrote:
> > Currently we reach inside the list_head.  This is a violation of the
> > layer of abstraction provided by the list_head.  It makes the code
> > fragile.  More importantly it makes the code wicked hard to understand.
> > 
> > The code reaches into the list_head structure to counteract the fact
> > that the list _may_ have been changed during slob_page_alloc().  Instead
> > of this we can add a return parameter to slob_page_alloc() to signal
> > that the list was modified (list_del() called with page->lru to remove
> > page from the freelist).
> > 
> > This code is concerned with an optimisation that counters the tendency
> > for first fit allocation algorithm to fragment memory into many small
> > chunks at the front of the memory pool.  Since the page is only removed
> > from the list when an allocation uses _all_ the remaining memory in the
> > page then in this special case fragmentation does not occur and we
> > therefore do not need the optimisation.
> > 
> > Add a return parameter to slob_page_alloc() to signal that the
> > allocation used up the whole page and that the page was removed from the
> > free list.  After calling slob_page_alloc() check the return value just
> > added and only attempt optimisation if the page is still on the list.
> > 
> > Use list_head API instead of reaching into the list_head structure to
> > check if sp is at the front of the list.
> > 
> > Signed-off-by: Tobin C. Harding <tobin@xxxxxxxxxx>
> > ---
> >  mm/slob.c | 51 +++++++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 37 insertions(+), 14 deletions(-)
> > 
> > diff --git a/mm/slob.c b/mm/slob.c
> > index 307c2c9feb44..07356e9feaaa 100644
> > --- a/mm/slob.c
> > +++ b/mm/slob.c
> > @@ -213,13 +213,26 @@ static void slob_free_pages(void *b, int order)
> >  }
> >  
> >  /*
> > - * Allocate a slob block within a given slob_page sp.
> > + * slob_page_alloc() - Allocate a slob block within a given slob_page sp.
> > + * @sp: Page to look in.
> > + * @size: Size of the allocation.
> > + * @align: Allocation alignment.
> > + * @page_removed_from_list: Return parameter.
> > + *
> > + * Tries to find a chunk of memory at least @size bytes big within @page.
> > + *
> > + * Return: Pointer to memory if allocated, %NULL otherwise.  If the
> > + *         allocation fills up @page then the page is removed from the
> > + *         freelist, in this case @page_removed_from_list will be set to
> > + *         true (set to false otherwise).
> >   */
> > -static void *slob_page_alloc(struct page *sp, size_t size, int align)
> > +static void *slob_page_alloc(struct page *sp, size_t size, int align,
> > +			     bool *page_removed_from_list)
> 
> Hi Tobin!
> 
> Isn't it better to make slob_page_alloc() return a bool value?
> Then it's easier to ignore the returned value, no need to introduce "_unused".

We need a pointer to the memory allocated also so AFAICS its either a
return parameter for the memory pointer or a return parameter to
indicate the boolean value?  Open to any other ideas I'm missing.

In a previous crack at this I used a double pointer to the page struct
then set that to null to indicate the boolean value.  I think the
explicit boolean parameter is cleaner.

thanks,
Tobin.