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

Roman Gushchin
2019-03-14T18:52:57Z

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!