Re: [PATCH v3 3/7] slob: Use slab_list instead of lru

[Roman Gushchin <guro@fb.com>]

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@fb.com>
> Signed-off-by: Tobin C. Harding <tobin@kernel.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!