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

[Roman Gushchin <guro@fb.com>]

On Thu, Apr 04, 2019 at 08:03:27AM +1100, Tobin C. Harding wrote:
> 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@kernel.org>
> > > ---
> > >  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.

Yeah, sorry, it's my fault. Please, ignore this comment.
Bool* argument is perfectly fine here.

Thanks!