Re: [PATCH] sh: remove unneeded constructor.

On Wed, 01 Aug 2018 16:42:02 +0900,
Geert Uytterhoeven wrote:

Hi Sato-san,

CC Rob , Willy, linux-arch

On Tue, Jul 31, 2018 at 7:15 AM Yoshinori Sato
[off-list ref] wrote:

quoted

pgd_cache specifies __GFP_ZERO when allocating.
This constructor is meaningless.

Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>

Thanks for your patch!

quoted

--- a/arch/sh/mm/pgtable.c
+++ b/arch/sh/mm/pgtable.c

@@ -9,20 +9,11 @@ static struct kmem_cache *pgd_cachep;
 static struct kmem_cache *pmd_cachep;
 #endif

-void pgd_ctor(void *x)
-{
-       pgd_t *pgd = x;
-
-       memcpy(pgd + USER_PTRS_PER_PGD,
-              swapper_pg_dir + USER_PTRS_PER_PGD,
-              (PTRS_PER_PGD - USER_PTRS_PER_PGD) * sizeof(pgd_t));
-}
-
 void pgtable_cache_init(void)
 {
        pgd_cachep = kmem_cache_create("pgd_cache",
                                       PTRS_PER_PGD * (1<<PTE_MAGNITUDE),
-                                      PAGE_SIZE, SLAB_PANIC, pgd_ctor);
+                                      PAGE_SIZE, SLAB_PANIC, NULL);
 #if PAGETABLE_LEVELS > 2
        pmd_cachep = kmem_cache_create("pmd_cache",
                                       PTRS_PER_PMD * (1<<PTE_MAGNITUDE),

While I can confirm your patch fixes the

    WARNING: CPU: 0 PID: 1 at mm/slub.c:2412
___slab_alloc.constprop.34+0x196/0x288

seen since commit 128227e7fe4087b6 ("slab: __GFP_ZERO is incompatible
with a constructor"), I'm not 100% sure this is the correct fix.

I tried it in myself, but with this fix I will not get a warning.
I do not think that it is related.

swapper_pg_dir[] does have two non-zero entries (768 and 895), which were
copied by the constructor.  Perhaps SH does have a need for these two
entries, and thus for the constructor?

Unfortunately I'm too SH-illiterate to answer this myself.

I have not tested enough to impose mm on my part, so it may be
by chance this too.
Restore the constructor and modify it so that __ GFP_ZERO is not specified.
Because then I think that it is safer because it is exactly the same as
before the fix.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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-- 
Yosinori Sato