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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cpuset.h
#ifndef _LINUX_CPUSET_H
#define _LINUX_CPUSET_H
/*
* cpuset interface
*
* Copyright (C) 2003 BULL SA
* Copyright (C) 2004-2006 Silicon Graphics, Inc.
*
*/
#include <linux/sched.h>
#include <linux/sched/topology.h>
#include <linux/sched/task.h>
#include <linux/cpumask.h>
#include <linux/nodemask.h>
#include <linux/mm.h>
#include <linux/jump_label.h>
#ifdef CONFIG_CPUSETS
extern struct static_key_false cpusets_enabled_key;
static inline bool cpusets_enabled(void)
{
return static_branch_unlikely(&cpusets_enabled_key);
}
static inline int nr_cpusets(void)
{
/* jump label reference count + the top-level cpuset */
return static_key_count(&cpusets_enabled_key.key) + 1;
}
static inline void cpuset_inc(void)
{
static_branch_inc(&cpusets_enabled_key);
}
static inline void cpuset_dec(void)
{
static_branch_dec(&cpusets_enabled_key);
}
extern int cpuset_init(void);
extern void cpuset_init_smp(void);
extern void cpuset_update_active_cpus(void);
extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
extern void cpuset_cpus_allowed_fallback(struct task_struct *p);
extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
#define cpuset_current_mems_allowed (current->mems_allowed)
void cpuset_init_current_mems_allowed(void);
int cpuset_nodemask_valid_mems_allowed(nodemask_t *nodemask);
extern bool __cpuset_node_allowed(int node, gfp_t gfp_mask);
static inline bool cpuset_node_allowed(int node, gfp_t gfp_mask)
{
if (cpusets_enabled())
return __cpuset_node_allowed(node, gfp_mask);
return true;
}
static inline bool __cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
{
return __cpuset_node_allowed(zone_to_nid(z), gfp_mask);
}
static inline bool cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
{
if (cpusets_enabled())
return __cpuset_zone_allowed(z, gfp_mask);
return true;
}
extern int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
const struct task_struct *tsk2);
#define cpuset_memory_pressure_bump() \
do { \
if (cpuset_memory_pressure_enabled) \
__cpuset_memory_pressure_bump(); \
} while (0)
extern int cpuset_memory_pressure_enabled;
extern void __cpuset_memory_pressure_bump(void);
extern void cpuset_task_status_allowed(struct seq_file *m,
struct task_struct *task);
extern int proc_cpuset_show(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *tsk);
extern int cpuset_mem_spread_node(void);
extern int cpuset_slab_spread_node(void);
static inline int cpuset_do_page_mem_spread(void)
{
return task_spread_page(current);
}
static inline int cpuset_do_slab_mem_spread(void)
{
return task_spread_slab(current);
}
extern int current_cpuset_is_being_rebound(void);
extern void rebuild_sched_domains(void);
extern void cpuset_print_current_mems_allowed(void);
/*
* read_mems_allowed_begin is required when making decisions involving
* mems_allowed such as during page allocation. mems_allowed can be updated in
* parallel and depending on the new value an operation can fail potentially
* causing process failure. A retry loop with read_mems_allowed_begin and
* read_mems_allowed_retry prevents these artificial failures.
*/
static inline unsigned int read_mems_allowed_begin(void)
{
if (!cpusets_enabled())
return 0;
return read_seqcount_begin(¤t->mems_allowed_seq);
}
/*
* If this returns true, the operation that took place after
* read_mems_allowed_begin may have failed artificially due to a concurrent
* update of mems_allowed. It is up to the caller to retry the operation if
* appropriate.
*/
static inline bool read_mems_allowed_retry(unsigned int seq)
{
if (!cpusets_enabled())
return false;
return read_seqcount_retry(¤t->mems_allowed_seq, seq);
}
static inline void set_mems_allowed(nodemask_t nodemask)
{
unsigned long flags;
task_lock(current);
local_irq_save(flags);
write_seqcount_begin(¤t->mems_allowed_seq);
current->mems_allowed = nodemask;
write_seqcount_end(¤t->mems_allowed_seq);
local_irq_restore(flags);
task_unlock(current);
}
#else /* !CONFIG_CPUSETS */
static inline bool cpusets_enabled(void) { return false; }
static inline int cpuset_init(void) { return 0; }
static inline void cpuset_init_smp(void) {}
static inline void cpuset_update_active_cpus(void)
{
partition_sched_domains(1, NULL, NULL);
}
static inline void cpuset_cpus_allowed(struct task_struct *p,
struct cpumask *mask)
{
cpumask_copy(mask, cpu_possible_mask);
}
static inline void cpuset_cpus_allowed_fallback(struct task_struct *p)
{
}
static inline nodemask_t cpuset_mems_allowed(struct task_struct *p)
{
return node_possible_map;
}
#define cpuset_current_mems_allowed (node_states[N_MEMORY])
static inline void cpuset_init_current_mems_allowed(void) {}
static inline int cpuset_nodemask_valid_mems_allowed(nodemask_t *nodemask)
{
return 1;
}
static inline bool cpuset_node_allowed(int node, gfp_t gfp_mask)
{
return true;
}
static inline bool __cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
{
return true;
}
static inline bool cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)
{
return true;
}
static inline int cpuset_mems_allowed_intersects(const struct task_struct *tsk1,
const struct task_struct *tsk2)
{
return 1;
}
static inline void cpuset_memory_pressure_bump(void) {}
static inline void cpuset_task_status_allowed(struct seq_file *m,
struct task_struct *task)
{
}
static inline int cpuset_mem_spread_node(void)
{
return 0;
}
static inline int cpuset_slab_spread_node(void)
{
return 0;
}
static inline int cpuset_do_page_mem_spread(void)
{
return 0;
}
static inline int cpuset_do_slab_mem_spread(void)
{
return 0;
}
static inline int current_cpuset_is_being_rebound(void)
{
return 0;
}
static inline void rebuild_sched_domains(void)
{
partition_sched_domains(1, NULL, NULL);
}
static inline void cpuset_print_current_mems_allowed(void)
{
}
static inline void set_mems_allowed(nodemask_t nodemask)
{
}
static inline unsigned int read_mems_allowed_begin(void)
{
return 0;
}
static inline bool read_mems_allowed_retry(unsigned int seq)
{
return false;
}
#endif /* !CONFIG_CPUSETS */
#endif /* _LINUX_CPUSET_H */
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