Revision e2093926a098a8ccf0f1d10f6df8dad452cb28d3 authored by Ross Zwisler on 02 June 2017, 21:46:37 UTC, committed by Linus Torvalds on 02 June 2017, 22:07:37 UTC
We currently have two related PMD vs PTE races in the DAX code. These can both be easily triggered by having two threads reading and writing simultaneously to the same private mapping, with the key being that private mapping reads can be handled with PMDs but private mapping writes are always handled with PTEs so that we can COW. Here is the first race: CPU 0 CPU 1 (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK handle_pte_fault() passes check for pmd_devmap() (private mapping read) __handle_mm_fault() create_huge_pmd() dax_iomap_pmd_fault() inserts PMD dax_iomap_pte_fault() does a PTE fault, but we already have a DAX PMD installed in our page tables at this spot. Here's the second race: CPU 0 CPU 1 (private mapping read) __handle_mm_fault() passes check for pmd_none() create_huge_pmd() dax_iomap_pmd_fault() inserts PMD (private mapping write) __handle_mm_fault() create_huge_pmd() - FALLBACK (private mapping read) __handle_mm_fault() passes check for pmd_none() create_huge_pmd() handle_pte_fault() dax_iomap_pte_fault() inserts PTE dax_iomap_pmd_fault() inserts PMD, but we already have a PTE at this spot. The core of the issue is that while there is isolation between faults to the same range in the DAX fault handlers via our DAX entry locking, there is no isolation between faults in the code in mm/memory.c. This means for instance that this code in __handle_mm_fault() can run: if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) { ret = create_huge_pmd(&vmf); But by the time we actually get to run the fault handler called by create_huge_pmd(), the PMD is no longer pmd_none() because a racing PTE fault has installed a normal PMD here as a parent. This is the cause of the 2nd race. The first race is similar - there is the following check in handle_pte_fault(): } else { /* See comment in pte_alloc_one_map() */ if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd)) return 0; So if a pmd_devmap() PMD (a DAX PMD) has been installed at vmf->pmd, we will bail and retry the fault. This is correct, but there is nothing preventing the PMD from being installed after this check but before we actually get to the DAX PTE fault handlers. In my testing these races result in the following types of errors: BUG: Bad rss-counter state mm:ffff8800a817d280 idx:1 val:1 BUG: non-zero nr_ptes on freeing mm: 15 Fix this issue by having the DAX fault handlers verify that it is safe to continue their fault after they have taken an entry lock to block other racing faults. [ross.zwisler@linux.intel.com: improve fix for colliding PMD & PTE entries] Link: http://lkml.kernel.org/r/20170526195932.32178-1-ross.zwisler@linux.intel.com Link: http://lkml.kernel.org/r/20170522215749.23516-2-ross.zwisler@linux.intel.com Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com> Reported-by: Pawel Lebioda <pawel.lebioda@intel.com> Reviewed-by: Jan Kara <jack@suse.cz> Cc: "Darrick J. Wong" <darrick.wong@oracle.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Christoph Hellwig <hch@lst.de> Cc: Dan Williams <dan.j.williams@intel.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Matthew Wilcox <mawilcox@microsoft.com> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com> Cc: Pawel Lebioda <pawel.lebioda@intel.com> Cc: Dave Jiang <dave.jiang@intel.com> Cc: Xiong Zhou <xzhou@redhat.com> Cc: Eryu Guan <eguan@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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highmem.h
#ifndef _LINUX_HIGHMEM_H
#define _LINUX_HIGHMEM_H
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/bug.h>
#include <linux/mm.h>
#include <linux/uaccess.h>
#include <linux/hardirq.h>
#include <asm/cacheflush.h>
#ifndef ARCH_HAS_FLUSH_ANON_PAGE
static inline void flush_anon_page(struct vm_area_struct *vma, struct page *page, unsigned long vmaddr)
{
}
#endif
#ifndef ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE
static inline void flush_kernel_dcache_page(struct page *page)
{
}
static inline void flush_kernel_vmap_range(void *vaddr, int size)
{
}
static inline void invalidate_kernel_vmap_range(void *vaddr, int size)
{
}
#endif
#include <asm/kmap_types.h>
#ifdef CONFIG_HIGHMEM
#include <asm/highmem.h>
/* declarations for linux/mm/highmem.c */
unsigned int nr_free_highpages(void);
extern unsigned long totalhigh_pages;
void kmap_flush_unused(void);
struct page *kmap_to_page(void *addr);
#else /* CONFIG_HIGHMEM */
static inline unsigned int nr_free_highpages(void) { return 0; }
static inline struct page *kmap_to_page(void *addr)
{
return virt_to_page(addr);
}
#define totalhigh_pages 0UL
#ifndef ARCH_HAS_KMAP
static inline void *kmap(struct page *page)
{
might_sleep();
return page_address(page);
}
static inline void kunmap(struct page *page)
{
}
static inline void *kmap_atomic(struct page *page)
{
preempt_disable();
pagefault_disable();
return page_address(page);
}
#define kmap_atomic_prot(page, prot) kmap_atomic(page)
static inline void __kunmap_atomic(void *addr)
{
pagefault_enable();
preempt_enable();
}
#define kmap_atomic_pfn(pfn) kmap_atomic(pfn_to_page(pfn))
#define kmap_flush_unused() do {} while(0)
#endif
#endif /* CONFIG_HIGHMEM */
#if defined(CONFIG_HIGHMEM) || defined(CONFIG_X86_32)
DECLARE_PER_CPU(int, __kmap_atomic_idx);
static inline int kmap_atomic_idx_push(void)
{
int idx = __this_cpu_inc_return(__kmap_atomic_idx) - 1;
#ifdef CONFIG_DEBUG_HIGHMEM
WARN_ON_ONCE(in_irq() && !irqs_disabled());
BUG_ON(idx >= KM_TYPE_NR);
#endif
return idx;
}
static inline int kmap_atomic_idx(void)
{
return __this_cpu_read(__kmap_atomic_idx) - 1;
}
static inline void kmap_atomic_idx_pop(void)
{
#ifdef CONFIG_DEBUG_HIGHMEM
int idx = __this_cpu_dec_return(__kmap_atomic_idx);
BUG_ON(idx < 0);
#else
__this_cpu_dec(__kmap_atomic_idx);
#endif
}
#endif
/*
* Prevent people trying to call kunmap_atomic() as if it were kunmap()
* kunmap_atomic() should get the return value of kmap_atomic, not the page.
*/
#define kunmap_atomic(addr) \
do { \
BUILD_BUG_ON(__same_type((addr), struct page *)); \
__kunmap_atomic(addr); \
} while (0)
/* when CONFIG_HIGHMEM is not set these will be plain clear/copy_page */
#ifndef clear_user_highpage
static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
{
void *addr = kmap_atomic(page);
clear_user_page(addr, vaddr, page);
kunmap_atomic(addr);
}
#endif
#ifndef __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE
/**
* __alloc_zeroed_user_highpage - Allocate a zeroed HIGHMEM page for a VMA with caller-specified movable GFP flags
* @movableflags: The GFP flags related to the pages future ability to move like __GFP_MOVABLE
* @vma: The VMA the page is to be allocated for
* @vaddr: The virtual address the page will be inserted into
*
* This function will allocate a page for a VMA but the caller is expected
* to specify via movableflags whether the page will be movable in the
* future or not
*
* An architecture may override this function by defining
* __HAVE_ARCH_ALLOC_ZEROED_USER_HIGHPAGE and providing their own
* implementation.
*/
static inline struct page *
__alloc_zeroed_user_highpage(gfp_t movableflags,
struct vm_area_struct *vma,
unsigned long vaddr)
{
struct page *page = alloc_page_vma(GFP_HIGHUSER | movableflags,
vma, vaddr);
if (page)
clear_user_highpage(page, vaddr);
return page;
}
#endif
/**
* alloc_zeroed_user_highpage_movable - Allocate a zeroed HIGHMEM page for a VMA that the caller knows can move
* @vma: The VMA the page is to be allocated for
* @vaddr: The virtual address the page will be inserted into
*
* This function will allocate a page for a VMA that the caller knows will
* be able to migrate in the future using move_pages() or reclaimed
*/
static inline struct page *
alloc_zeroed_user_highpage_movable(struct vm_area_struct *vma,
unsigned long vaddr)
{
return __alloc_zeroed_user_highpage(__GFP_MOVABLE, vma, vaddr);
}
static inline void clear_highpage(struct page *page)
{
void *kaddr = kmap_atomic(page);
clear_page(kaddr);
kunmap_atomic(kaddr);
}
static inline void zero_user_segments(struct page *page,
unsigned start1, unsigned end1,
unsigned start2, unsigned end2)
{
void *kaddr = kmap_atomic(page);
BUG_ON(end1 > PAGE_SIZE || end2 > PAGE_SIZE);
if (end1 > start1)
memset(kaddr + start1, 0, end1 - start1);
if (end2 > start2)
memset(kaddr + start2, 0, end2 - start2);
kunmap_atomic(kaddr);
flush_dcache_page(page);
}
static inline void zero_user_segment(struct page *page,
unsigned start, unsigned end)
{
zero_user_segments(page, start, end, 0, 0);
}
static inline void zero_user(struct page *page,
unsigned start, unsigned size)
{
zero_user_segments(page, start, start + size, 0, 0);
}
#ifndef __HAVE_ARCH_COPY_USER_HIGHPAGE
static inline void copy_user_highpage(struct page *to, struct page *from,
unsigned long vaddr, struct vm_area_struct *vma)
{
char *vfrom, *vto;
vfrom = kmap_atomic(from);
vto = kmap_atomic(to);
copy_user_page(vto, vfrom, vaddr, to);
kunmap_atomic(vto);
kunmap_atomic(vfrom);
}
#endif
static inline void copy_highpage(struct page *to, struct page *from)
{
char *vfrom, *vto;
vfrom = kmap_atomic(from);
vto = kmap_atomic(to);
copy_page(vto, vfrom);
kunmap_atomic(vto);
kunmap_atomic(vfrom);
}
#endif /* _LINUX_HIGHMEM_H */
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