linux-kernel - Re: [PATCH v3 3/7] slob: Use slab

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Date:   Fri, 15 Mar 2019 07:42:20 +1100
From:   "Tobin C. Harding" <me@...in.cc>
To:     Roman Gushchin <guro@...com>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/7] slob: Use slab_list instead of lru

On Fri, Mar 15, 2019 at 07:38:09AM +1100, Tobin C. Harding wrote:
> On Thu, Mar 14, 2019 at 06:52:25PM +0000, Roman Gushchin wrote:
> > On Thu, Mar 14, 2019 at 04:31:31PM +1100, Tobin C. Harding wrote:
> > > Currently we use the page->lru list for maintaining lists of slabs.  We
> > > have a list_head in the page structure (slab_list) that can be used for
> > > this purpose.  Doing so makes the code cleaner since we are not
> > > overloading the lru list.
> > > 
> > > The slab_list is part of a union within the page struct (included here
> > > stripped down):
> > > 
> > > 	union {
> > > 		struct {	/* Page cache and anonymous pages */
> > > 			struct list_head lru;
> > > 			...
> > > 		};
> > > 		struct {
> > > 			dma_addr_t dma_addr;
> > > 		};
> > > 		struct {	/* slab, slob and slub */
> > > 			union {
> > > 				struct list_head slab_list;
> > > 				struct {	/* Partial pages */
> > > 					struct page *next;
> > > 					int pages;	/* Nr of pages left */
> > > 					int pobjects;	/* Approximate count */
> > > 				};
> > > 			};
> > > 		...
> > > 
> > > Here we see that slab_list and lru are the same bits.  We can verify
> > > that this change is safe to do by examining the object file produced from
> > > slob.c before and after this patch is applied.
> > > 
> > > Steps taken to verify:
> > > 
> > >  1. checkout current tip of Linus' tree
> > > 
> > >     commit a667cb7a94d4 ("Merge branch 'akpm' (patches from Andrew)")
> > > 
> > >  2. configure and build (select SLOB allocator)
> > > 
> > >     CONFIG_SLOB=y
> > >     CONFIG_SLAB_MERGE_DEFAULT=y
> > > 
> > >  3. dissasemble object file `objdump -dr mm/slub.o > before.s
> > >  4. apply patch
> > >  5. build
> > >  6. dissasemble object file `objdump -dr mm/slub.o > after.s
> > >  7. diff before.s after.s
> > > 
> > > Use slab_list list_head instead of the lru list_head for maintaining
> > > lists of slabs.
> > > 
> > > Reviewed-by: Roman Gushchin <guro@...com>
> > > Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> > > ---
> > >  mm/slob.c | 8 ++++----
> > >  1 file changed, 4 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/mm/slob.c b/mm/slob.c
> > > index 39ad9217ffea..94486c32e0ff 100644
> > > --- a/mm/slob.c
> > > +++ b/mm/slob.c
> > > @@ -112,13 +112,13 @@ static inline int slob_page_free(struct page *sp)
> > >  
> > >  static void set_slob_page_free(struct page *sp, struct list_head *list)
> > >  {
> > > -	list_add(&sp->lru, list);
> > > +	list_add(&sp->slab_list, list);
> > >  	__SetPageSlobFree(sp);
> > >  }
> > >  
> > >  static inline void clear_slob_page_free(struct page *sp)
> > >  {
> > > -	list_del(&sp->lru);
> > > +	list_del(&sp->slab_list);
> > >  	__ClearPageSlobFree(sp);
> > >  }
> > >  
> > > @@ -282,7 +282,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
> > >  
> > >  	spin_lock_irqsave(&slob_lock, flags);
> > >  	/* Iterate through each partially free page, try to find room */
> > > -	list_for_each_entry(sp, slob_list, lru) {
> > > +	list_for_each_entry(sp, slob_list, slab_list) {
> > >  #ifdef CONFIG_NUMA
> > >  		/*
> > >  		 * If there's a node specification, search for a partial
> > 
> > 
> > Hi Tobin!
> > 
> > How about list_rotate_to_front(&next->lru, slob_list) from the previous patch?
> > Shouldn't it use slab_list instead of lru too?
> 
> Thanks Roman, my mistake - one too many rebases.  I hate when I drop the
> ball like this.

Oh that's right, its a union so it still builds and boots - I was
thinking that I had rebased and not built.  I guess that's just a fumble
instead of a complete ball drop.

Thanks for the careful review all the same.

	Tobin