[PATCH v3 02/12] mm/hmm: use reference counting for HMM struct v3

From: jglisse
Date: Wed Apr 03 2019 - 15:34:25 EST

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From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>

Every time i read the code to check that the HMM structure does not
vanish before it should thanks to the many lock protecting its removal
i get a headache. Switch to reference counting instead it is much
easier to follow and harder to break. This also remove some code that
is no longer needed with refcounting.

Changes since v2:
- Renamed hmm_register() to hmm_get_or_create() updated comments
accordingly
Changes since v1:
- removed bunch of useless check (if API is use with bogus argument
better to fail loudly so user fix their code)
- s/hmm_get/mm_get_hmm/

Signed-off-by: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: John Hubbard <jhubbard@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
---
include/linux/hmm.h | 2 +
mm/hmm.c | 190 ++++++++++++++++++++++++++++----------------
2 files changed, 124 insertions(+), 68 deletions(-)

diff --git a/include/linux/hmm.h b/include/linux/hmm.h
index ad50b7b4f141..716fc61fa6d4 100644
--- a/include/linux/hmm.h
+++ b/include/linux/hmm.h
@@ -131,6 +131,7 @@ enum hmm_pfn_value_e {
/*
* struct hmm_range - track invalidation lock on virtual address range
*
+ * @hmm: the core HMM structure this range is active against
* @vma: the vm area struct for the range
* @list: all range lock are on a list
* @start: range virtual start address (inclusive)
@@ -142,6 +143,7 @@ enum hmm_pfn_value_e {
* @valid: pfns array did not change since it has been fill by an HMM function
*/
struct hmm_range {
+ struct hmm *hmm;
struct vm_area_struct *vma;
struct list_head list;
unsigned long start;
diff --git a/mm/hmm.c b/mm/hmm.c
index fe1cd87e49ac..919d78fd21c5 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -50,6 +50,7 @@ static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
*/
struct hmm {
struct mm_struct *mm;
+ struct kref kref;
spinlock_t lock;
struct list_head ranges;
struct list_head mirrors;
@@ -57,24 +58,33 @@ struct hmm {
struct rw_semaphore mirrors_sem;
};

-/*
- * hmm_register - register HMM against an mm (HMM internal)
+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)
*
* @mm: mm struct to attach to
+ * Returns: returns an HMM object, either by referencing the existing
+ * (per-process) object, or by creating a new one.
*
- * This is not intended to be used directly by device drivers. It allocates an
- * HMM struct if mm does not have one, and initializes it.
+ * This is not intended to be used directly by device drivers. If mm already
+ * has an HMM struct then it get a reference on it and returns it. Otherwise
+ * it allocates an HMM struct, initializes it, associate it with the mm and
+ * returns it.
*/
-static struct hmm *hmm_register(struct mm_struct *mm)
+static struct hmm *hmm_get_or_create(struct mm_struct *mm)
{
- struct hmm *hmm = READ_ONCE(mm->hmm);
+ struct hmm *hmm = mm_get_hmm(mm);
bool cleanup = false;

- /*
- * The hmm struct can only be freed once the mm_struct goes away,
- * hence we should always have pre-allocated an new hmm struct
- * above.
- */
if (hmm)
return hmm;

@@ -86,6 +96,7 @@ static struct hmm *hmm_register(struct mm_struct *mm)
hmm->mmu_notifier.ops = NULL;
INIT_LIST_HEAD(&hmm->ranges);
spin_lock_init(&hmm->lock);
+ kref_init(&hmm->kref);
hmm->mm = mm;

spin_lock(&mm->page_table_lock);
@@ -106,7 +117,7 @@ static struct hmm *hmm_register(struct mm_struct *mm)
if (__mmu_notifier_register(&hmm->mmu_notifier, mm))
goto error_mm;

- return mm->hmm;
+ return hmm;

error_mm:
spin_lock(&mm->page_table_lock);
@@ -118,9 +129,41 @@ static struct hmm *hmm_register(struct mm_struct *mm)
return NULL;
}

+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(&mm->page_table_lock);
+ if (mm->hmm == hmm)
+ mm->hmm = NULL;
+ spin_unlock(&mm->page_table_lock);
+
+ kfree(hmm);
+}
+
+static inline void hmm_put(struct hmm *hmm)
+{
+ kref_put(&hmm->kref, hmm_free);
+}
+
void hmm_mm_destroy(struct mm_struct *mm)
{
- kfree(mm->hmm);
+ struct hmm *hmm;
+
+ spin_lock(&mm->page_table_lock);
+ hmm = mm_get_hmm(mm);
+ mm->hmm = NULL;
+ if (hmm) {
+ hmm->mm = NULL;
+ spin_unlock(&mm->page_table_lock);
+ hmm_put(hmm);
+ return;
+ }
+
+ spin_unlock(&mm->page_table_lock);
}

static int hmm_invalidate_range(struct hmm *hmm, bool device,
@@ -165,7 +208,7 @@ static int hmm_invalidate_range(struct hmm *hmm, bool device,
static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
{
struct hmm_mirror *mirror;
- struct hmm *hmm = mm->hmm;
+ struct hmm *hmm = mm_get_hmm(mm);

down_write(&hmm->mirrors_sem);
mirror = list_first_entry_or_null(&hmm->mirrors, struct hmm_mirror,
@@ -186,13 +229,16 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
struct hmm_mirror, list);
}
up_write(&hmm->mirrors_sem);
+
+ hmm_put(hmm);
}

static int hmm_invalidate_range_start(struct mmu_notifier *mn,
const struct mmu_notifier_range *range)
{
+ struct hmm *hmm = mm_get_hmm(range->mm);
struct hmm_update update;
- struct hmm *hmm = range->mm->hmm;
+ int ret;

VM_BUG_ON(!hmm);

@@ -200,14 +246,16 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
update.end = range->end;
update.event = HMM_UPDATE_INVALIDATE;
update.blockable = range->blockable;
- return hmm_invalidate_range(hmm, true, &update);
+ ret = hmm_invalidate_range(hmm, true, &update);
+ hmm_put(hmm);
+ return ret;
}

static void hmm_invalidate_range_end(struct mmu_notifier *mn,
const struct mmu_notifier_range *range)
{
+ struct hmm *hmm = mm_get_hmm(range->mm);
struct hmm_update update;
- struct hmm *hmm = range->mm->hmm;

VM_BUG_ON(!hmm);

@@ -216,6 +264,7 @@ static void hmm_invalidate_range_end(struct mmu_notifier *mn,
update.event = HMM_UPDATE_INVALIDATE;
update.blockable = true;
hmm_invalidate_range(hmm, false, &update);
+ hmm_put(hmm);
}

static const struct mmu_notifier_ops hmm_mmu_notifier_ops = {
@@ -241,24 +290,13 @@ int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
if (!mm || !mirror || !mirror->ops)
return -EINVAL;

-again:
- mirror->hmm = hmm_register(mm);
+ mirror->hmm = hmm_get_or_create(mm);
if (!mirror->hmm)
return -ENOMEM;

down_write(&mirror->hmm->mirrors_sem);
- if (mirror->hmm->mm == NULL) {
- /*
- * A racing hmm_mirror_unregister() is about to destroy the hmm
- * struct. Try again to allocate a new one.
- */
- up_write(&mirror->hmm->mirrors_sem);
- mirror->hmm = NULL;
- goto again;
- } else {
- list_add(&mirror->list, &mirror->hmm->mirrors);
- up_write(&mirror->hmm->mirrors_sem);
- }
+ list_add(&mirror->list, &mirror->hmm->mirrors);
+ up_write(&mirror->hmm->mirrors_sem);

return 0;
}
@@ -273,33 +311,18 @@ EXPORT_SYMBOL(hmm_mirror_register);
*/
void hmm_mirror_unregister(struct hmm_mirror *mirror)
{
- bool should_unregister = false;
- struct mm_struct *mm;
- struct hmm *hmm;
+ struct hmm *hmm = READ_ONCE(mirror->hmm);

- if (mirror->hmm == NULL)
+ if (hmm == NULL)
return;

- hmm = mirror->hmm;
down_write(&hmm->mirrors_sem);
list_del_init(&mirror->list);
- should_unregister = list_empty(&hmm->mirrors);
+ /* To protect us against double unregister ... */
mirror->hmm = NULL;
- mm = hmm->mm;
- hmm->mm = NULL;
up_write(&hmm->mirrors_sem);

- if (!should_unregister || mm == NULL)
- return;
-
- mmu_notifier_unregister_no_release(&hmm->mmu_notifier, mm);
-
- spin_lock(&mm->page_table_lock);
- if (mm->hmm == hmm)
- mm->hmm = NULL;
- spin_unlock(&mm->page_table_lock);
-
- kfree(hmm);
+ hmm_put(hmm);
}
EXPORT_SYMBOL(hmm_mirror_unregister);

@@ -708,23 +731,29 @@ int hmm_vma_get_pfns(struct hmm_range *range)
struct mm_walk mm_walk;
struct hmm *hmm;

+ range->hmm = NULL;
+
/* Sanity check, this really should not happen ! */
if (range->start < vma->vm_start || range->start >= vma->vm_end)
return -EINVAL;
if (range->end < vma->vm_start || range->end > vma->vm_end)
return -EINVAL;

- hmm = hmm_register(vma->vm_mm);
+ hmm = hmm_get_or_create(vma->vm_mm);
if (!hmm)
return -ENOMEM;
- /* Caller must have registered a mirror, via hmm_mirror_register() ! */
- if (!hmm->mmu_notifier.ops)
+
+ /* Check if hmm_mm_destroy() was call. */
+ if (hmm->mm == NULL) {
+ hmm_put(hmm);
return -EINVAL;
+ }

/* FIXME support hugetlb fs */
if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL) ||
vma_is_dax(vma)) {
hmm_pfns_special(range);
+ hmm_put(hmm);
return -EINVAL;
}

@@ -736,6 +765,7 @@ int hmm_vma_get_pfns(struct hmm_range *range)
* operations such has atomic access would not work.
*/
hmm_pfns_clear(range, range->pfns, range->start, range->end);
+ hmm_put(hmm);
return -EPERM;
}

@@ -758,6 +788,12 @@ int hmm_vma_get_pfns(struct hmm_range *range)
mm_walk.pte_hole = hmm_vma_walk_hole;

walk_page_range(range->start, range->end, &mm_walk);
+ /*
+ * Transfer hmm reference to the range struct it will be drop inside
+ * the hmm_vma_range_done() function (which _must_ be call if this
+ * function return 0).
+ */
+ range->hmm = hmm;
return 0;
}
EXPORT_SYMBOL(hmm_vma_get_pfns);
@@ -802,25 +838,27 @@ EXPORT_SYMBOL(hmm_vma_get_pfns);
*/
bool hmm_vma_range_done(struct hmm_range *range)
{
- unsigned long npages = (range->end - range->start) >> PAGE_SHIFT;
- struct hmm *hmm;
+ bool ret = false;

- if (range->end <= range->start) {
+ /* Sanity check this really should not happen. */
+ if (range->hmm == NULL || range->end <= range->start) {
BUG();
return false;
}

- hmm = hmm_register(range->vma->vm_mm);
- if (!hmm) {
- memset(range->pfns, 0, sizeof(*range->pfns) * npages);
- return false;
- }
-
- spin_lock(&hmm->lock);
+ spin_lock(&range->hmm->lock);
list_del_rcu(&range->list);
- spin_unlock(&hmm->lock);
+ ret = range->valid;
+ spin_unlock(&range->hmm->lock);

- return range->valid;
+ /* Is the mm still alive ? */
+ if (range->hmm->mm == NULL)
+ ret = false;
+
+ /* Drop reference taken by hmm_vma_fault() or hmm_vma_get_pfns() */
+ hmm_put(range->hmm);
+ range->hmm = NULL;
+ return ret;
}
EXPORT_SYMBOL(hmm_vma_range_done);

@@ -880,25 +918,31 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
struct hmm *hmm;
int ret;

+ range->hmm = NULL;
+
/* Sanity check, this really should not happen ! */
if (range->start < vma->vm_start || range->start >= vma->vm_end)
return -EINVAL;
if (range->end < vma->vm_start || range->end > vma->vm_end)
return -EINVAL;

- hmm = hmm_register(vma->vm_mm);
+ hmm = hmm_get_or_create(vma->vm_mm);
if (!hmm) {
hmm_pfns_clear(range, range->pfns, range->start, range->end);
return -ENOMEM;
}
- /* Caller must have registered a mirror using hmm_mirror_register() */
- if (!hmm->mmu_notifier.ops)
+
+ /* Check if hmm_mm_destroy() was call. */
+ if (hmm->mm == NULL) {
+ hmm_put(hmm);
return -EINVAL;
+ }

/* FIXME support hugetlb fs */
if (is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_SPECIAL) ||
vma_is_dax(vma)) {
hmm_pfns_special(range);
+ hmm_put(hmm);
return -EINVAL;
}

@@ -910,6 +954,7 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
* operations such has atomic access would not work.
*/
hmm_pfns_clear(range, range->pfns, range->start, range->end);
+ hmm_put(hmm);
return -EPERM;
}

@@ -945,7 +990,16 @@ int hmm_vma_fault(struct hmm_range *range, bool block)
hmm_pfns_clear(range, &range->pfns[i], hmm_vma_walk.last,
range->end);
hmm_vma_range_done(range);
+ hmm_put(hmm);
+ } else {
+ /*
+ * Transfer hmm reference to the range struct it will be drop
+ * inside the hmm_vma_range_done() function (which _must_ be
+ * call if this function return 0).
+ */
+ range->hmm = hmm;
}
+
return ret;
}
EXPORT_SYMBOL(hmm_vma_fault);
--
2.17.2

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