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SubjectRe: CONFIG_SLUB/USE_SPLIT_PTLOCKS compatibility
On Mon, Oct 14, 2013 at 01:12:47AM +0400, Max Filippov wrote:
> Hello,
>
> I'm reliably getting kernel crash on xtensa when CONFIG_SLUB
> is selected and USE_SPLIT_PTLOCKS appears to be true (SMP=y,
> NR_CPUS=4, DEBUG_SPINLOCK=n, DEBUG_LOCK_ALLOC=n).
> This happens because spinlock_t ptl and struct page *first_page overlap
> in the struct page. The following call chain makes allocation of order
> 3 and initializes first_page pointer in its 7 tail pages:
>
> do_page_fault
> handle_mm_fault
> __pte_alloc
> kmem_cache_alloc
> __slab_alloc
> new_slab
> __alloc_pages_nodemask
> get_page_from_freelist
> prep_compound_page
>
> Later pte_offset_map_lock is called with one of these tail pages
> overwriting its first_page pointer:
>
> do_fork
> copy_process
> dup_mm
> copy_page_range
> copy_pte_range
> pte_alloc_map_lock
> pte_offset_map_lock
>
> Finally kmem_cache_free is called for that tail page, which calls
> slab_free(s, virt_to_head_page(x),... but virt_to_head_page here
> returns NULL, because the page's first_page pointer was overwritten
> earlier:
>
> exit_mmap
> free_pgtables
> free_pgd_range
> free_pud_range
> free_pmd_range
> free_pte_range
> pte_free
> kmem_cache_free
> slab_free
> __slab_free
>
> __slab_free touches NULL struct page, that's it.
>
> Changing allocator to SLAB or enabling DEBUG_SPINLOCK
> fixes that crash.
>
> My question is, is CONFIG_SLUB supposed to work with
> USE_SPLIT_PTLOCKS (and if yes what's wrong in my case)?

Sure, CONFIG_SLUB && USE_SPLIT_PTLOCKS works fine. Unless you try use slab
to allocate pagetable.

Note: no other arch allocates PTE page tables from slab.
Some archs (sparc, power) uses slabs to allocate hihger page tables, but
not PTE. [ And these archs will have to avoid slab, if they wants to
support split ptl for PMD tables. ]

I don't see much sense in having separate slab for allocting PAGE_SIZE
objects aligned to PAGE_SIZE. What's wrong with plain buddy allocator?

Completely untested patch to use buddy allocator instead of slub for page
table allocation on xtensa is below. Please, try.

diff --git a/arch/xtensa/include/asm/pgalloc.h b/arch/xtensa/include/asm/pgalloc.h
index b8774f1e21..8e27d4200e 100644
--- a/arch/xtensa/include/asm/pgalloc.h
+++ b/arch/xtensa/include/asm/pgalloc.h
@@ -38,14 +38,16 @@ static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)
free_page((unsigned long)pgd);
}

-/* Use a slab cache for the pte pages (see also sparc64 implementation) */
-
-extern struct kmem_cache *pgtable_cache;
-
static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm,
unsigned long address)
{
- return kmem_cache_alloc(pgtable_cache, GFP_KERNEL|__GFP_REPEAT);
+ pte_t *ptep;
+
+ ptep = (pte_t *)__get_free_page(GFP_KERNEL|__GFP_REPEAT);
+ if (!ptep)
+ return NULL;
+ for (i = 0; i < 1024; i++, ptep++)
+ pte_clear(NULL, 0, ptep);
}

static inline pgtable_t pte_alloc_one(struct mm_struct *mm,
@@ -59,7 +61,7 @@ static inline pgtable_t pte_alloc_one(struct mm_struct *mm,
return NULL;
page = virt_to_page(pte);
if (!pgtable_page_ctor(page)) {
- kmem_cache_free(pgtable_cache, pte);
+ __free_page(page);
return NULL;
}
return page;
@@ -67,13 +69,13 @@ static inline pgtable_t pte_alloc_one(struct mm_struct *mm,

static inline void pte_free_kernel(struct mm_struct *mm, pte_t *pte)
{
- kmem_cache_free(pgtable_cache, pte);
+ free_page((unsigned long)pte);
}

static inline void pte_free(struct mm_struct *mm, pgtable_t pte)
{
pgtable_page_dtor(pte);
- kmem_cache_free(pgtable_cache, page_address(pte));
+ __free_page(page);
}
#define pmd_pgtable(pmd) pmd_page(pmd)

diff --git a/arch/xtensa/include/asm/pgtable.h b/arch/xtensa/include/asm/pgtable.h
index 0fdf5d043f..216446295a 100644
--- a/arch/xtensa/include/asm/pgtable.h
+++ b/arch/xtensa/include/asm/pgtable.h
@@ -220,12 +220,11 @@ extern unsigned long empty_zero_page[1024];
#ifdef CONFIG_MMU
extern pgd_t swapper_pg_dir[PAGE_SIZE/sizeof(pgd_t)];
extern void paging_init(void);
-extern void pgtable_cache_init(void);
#else
# define swapper_pg_dir NULL
static inline void paging_init(void) { }
-static inline void pgtable_cache_init(void) { }
#endif
+static inline void pgtable_cache_init(void) { }

/*
* The pmd contains the kernel virtual address of the pte page.
diff --git a/arch/xtensa/mm/mmu.c b/arch/xtensa/mm/mmu.c
index a1077570e3..c43771c974 100644
--- a/arch/xtensa/mm/mmu.c
+++ b/arch/xtensa/mm/mmu.c
@@ -50,23 +50,3 @@ void __init init_mmu(void)
*/
set_ptevaddr_register(PGTABLE_START);
}
-
-struct kmem_cache *pgtable_cache __read_mostly;
-
-static void pgd_ctor(void *addr)
-{
- pte_t *ptep = (pte_t *)addr;
- int i;
-
- for (i = 0; i < 1024; i++, ptep++)
- pte_clear(NULL, 0, ptep);
-
-}
-
-void __init pgtable_cache_init(void)
-{
- pgtable_cache = kmem_cache_create("pgd",
- PAGE_SIZE, PAGE_SIZE,
- SLAB_HWCACHE_ALIGN,
- pgd_ctor);
-}
--
Kirill A. Shutemov

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