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mm/hmm: Simplify hmm_get_or_create and make it reliable 

As coded this function can false-fail in various racy situations. Make it
reliable and simpler by running under the write side of the mmap_sem and
avoiding the false-failing compare/exchange pattern. Due to the mmap_sem
this no longer has to avoid racing with a 2nd parallel
hmm_get_or_create().

Unfortunately this still has to use the page_table_lock as the
non-sleeping lock protecting mm->hmm, since the contexts where we free the
hmm are incompatible with mmap_sem.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Philip Yang <Philip.Yang@amd.com>
This commit is contained in:
Jason Gunthorpe 2019-05-23 10:24:13 -03:00
parent c8a53b2db0
commit 8a9320b7ec
1 changed files with 31 additions and 48 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

79
mm/hmm.c
View File

@ -31,16 +31,6 @@
#if IS_ENABLED(CONFIG_HMM_MIRROR)
static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
{
struct hmm *hmm = READ_ONCE(mm->hmm);
if (hmm && kref_get_unless_zero(&hmm->kref))
return hmm;
return NULL;
}
/**
* hmm_get_or_create - register HMM against an mm (HMM internal)
*
@ -55,11 +45,16 @@ static inline struct hmm *mm_get_hmm(struct mm_struct *mm)
*/
static struct hmm *hmm_get_or_create(struct mm_struct *mm)
{
struct hmm *hmm = mm_get_hmm(mm);
bool cleanup = false;
struct hmm *hmm;
if (hmm)
return hmm;
lockdep_assert_held_exclusive(&mm->mmap_sem);
/* Abuse the page_table_lock to also protect mm->hmm. */
spin_lock(&mm->page_table_lock);
hmm = mm->hmm;
if (mm->hmm && kref_get_unless_zero(&mm->hmm->kref))
goto out_unlock;
spin_unlock(&mm->page_table_lock);
hmm = kmalloc(sizeof(*hmm), GFP_KERNEL);
if (!hmm)
@ -74,57 +69,45 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
hmm->notifiers = 0;
hmm->dead = false;
hmm->mm = mm;
hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
if (__mmu_notifier_register(&hmm->mmu_notifier, mm)) {
kfree(hmm);
return NULL;
}
mmgrab(hmm->mm);
spin_lock(&mm->page_table_lock);
if (!mm->hmm)
mm->hmm = hmm;
else
cleanup = true;
spin_unlock(&mm->page_table_lock);
if (cleanup)
goto error;
/*
* We should only get here if hold the mmap_sem in write mode ie on
* registration of first mirror through hmm_mirror_register()
* We hold the exclusive mmap_sem here so we know that mm->hmm is
* still NULL or 0 kref, and is safe to update.
*/
hmm->mmu_notifier.ops = &hmm_mmu_notifier_ops;
if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
goto error_mm;
return hmm;
error_mm:
spin_lock(&mm->page_table_lock);
if (mm->hmm == hmm)
mm->hmm = NULL;
mm->hmm = hmm;
out_unlock:
spin_unlock(&mm->page_table_lock);
error:
mmdrop(hmm->mm);
kfree(hmm);
return NULL;
return hmm;
}
static void hmm_free_rcu(struct rcu_head *rcu)
{
kfree(container_of(rcu, struct hmm, rcu));
struct hmm *hmm = container_of(rcu, struct hmm, rcu);
mmdrop(hmm->mm);
kfree(hmm);
}
static void hmm_free(struct kref *kref)
{
struct hmm *hmm = container_of(kref, struct hmm, kref);
struct mm_struct *mm = hmm->mm;
mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
spin_lock(&hmm->mm->page_table_lock);
if (hmm->mm->hmm == hmm)
hmm->mm->hmm = NULL;
spin_unlock(&hmm->mm->page_table_lock);
spin_lock(&mm->page_table_lock);
if (mm->hmm == hmm)
mm->hmm = NULL;
spin_unlock(&mm->page_table_lock);
mmdrop(hmm->mm);
mmu_notifier_unregister_no_release(&hmm->mmu_notifier, hmm->mm);
mmu_notifier_call_srcu(&hmm->rcu, hmm_free_rcu);
}