On Mon, Jan 06, 2014 at 12:43:00PM -0800, Dave Hansen wrote:
>
>From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
>There used to be only one path out of __slab_alloc(), and
>ALLOC_SLOWPATH got bumped in that exit path. Now there are two,
>and a bunch of gotos. ALLOC_SLOWPATH can now get set more than once
>during a single call to __slab_alloc() which is pretty bogus.
>Here's the sequence:
>
>1. Enter __slab_alloc(), fall through all the way to the
> stat(s, ALLOC_SLOWPATH);
>2. hit 'if (!freelist)', and bump DEACTIVATE_BYPASS, jump to
> new_slab (goto #1)
>3. Hit 'if (c->partial)', bump CPU_PARTIAL_ALLOC, goto redo
> (goto #2)
>4. Fall through in the same path we did before all the way to
> stat(s, ALLOC_SLOWPATH)
>5. bump ALLOC_REFILL stat, then return
>
>Doing this is obviously bogus. It keeps us from being able to
>accurately compare ALLOC_SLOWPATH vs. ALLOC_FASTPATH. It also
>means that the total number of allocs always exceeds the total
>number of frees.
>
>This patch moves stat(s, ALLOC_SLOWPATH) to be called from the
>same place that __slab_alloc() is. This makes it much less
>likely that ALLOC_SLOWPATH will get botched again in the
>spaghetti-code inside __slab_alloc().
>
>Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
>---
>
> linux.git-davehans/mm/slub.c | 8 +++-----
> 1 file changed, 3 insertions(+), 5 deletions(-)
>
>diff -puN mm/slub.c~slub-ALLOC_SLOWPATH-stat mm/slub.c
>--- linux.git/mm/slub.c~slub-ALLOC_SLOWPATH-stat 2014-01-06 12:39:28.148072544 -0800
>+++ linux.git-davehans/mm/slub.c 2014-01-06 12:39:28.155072860 -0800
>@@ -2301,8 +2301,6 @@ redo:
> if (freelist)
> goto load_freelist;
>
>- stat(s, ALLOC_SLOWPATH);
>-
> freelist = get_freelist(s, page);
>
> if (!freelist) {
>@@ -2409,10 +2407,10 @@ redo:
>
> object = c->freelist;
> page = c->page;
>- if (unlikely(!object || !node_match(page, node)))
>+ if (unlikely(!object || !node_match(page, node))) {
> object = __slab_alloc(s, gfpflags, node, addr, c);
>-
>- else {
>+ stat(s, ALLOC_SLOWPATH);
>+ } else {
> void *next_object = get_freepointer_safe(s, object);
>
> /*
>_
>
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