Date:	Fri, 03 Jul 2015 10:12:09 -0700
From:	Mike Kravetz <mike.kravetz@...cle.com>
To:	Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
CC:	"'Dave Hansen'" <dave.hansen@...ux.intel.com>,
	"'Naoya Horiguchi'" <n-horiguchi@...jp.nec.com>,
	"'David Rientjes'" <rientjes@...gle.com>,
	"'Hugh Dickins'" <hughd@...gle.com>,
	"'Davidlohr Bueso'" <dave@...olabs.net>,
	"'Aneesh Kumar'" <aneesh.kumar@...ux.vnet.ibm.com>,
	"'Christoph Hellwig'" <hch@...radead.org>
Subject: Re: [PATCH 01/10] mm/hugetlb: add cache of descriptors to resv_map
 for region_add

On 07/02/2015 09:21 PM, Hillf Danton wrote:
>>
>> fallocate hole punch will want to remove a specific range of
>> pages.  When pages are removed, their associated entries in
>> the region/reserve map will also be removed.  This will break
>> an assumption in the region_chg/region_add calling sequence.
>> If a new region descriptor must be allocated, it is done as
>> part of the region_chg processing.  In this way, region_add
>> can not fail because it does not need to attempt an allocation.
>>
>> To prepare for fallocate hole punch, create a "cache" of
>> descriptors that can be used by region_add if necessary.
>> region_chg will ensure there are sufficient entries in the
>> cache.  It will be necessary to track the number of in progress
>> add operations to know a sufficient number of descriptors
>> reside in the cache.  A new routine region_abort is added to
>> adjust this in progress count when add operations are aborted.
>> vma_abort_reservation is also added for callers creating
>> reservations with vma_needs_reservation/vma_commit_reservation.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
>> ---
>>   include/linux/hugetlb.h |   3 +
>>   mm/hugetlb.c            | 166 ++++++++++++++++++++++++++++++++++++++++++------
>>   2 files changed, 150 insertions(+), 19 deletions(-)
>>
>> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
>> index 2050261..f9c8cb2 100644
>> --- a/include/linux/hugetlb.h
>> +++ b/include/linux/hugetlb.h
>> @@ -35,6 +35,9 @@ struct resv_map {
>>   	struct kref refs;
>>   	spinlock_t lock;
>>   	struct list_head regions;
>> +	long adds_in_progress;
>> +	struct list_head rgn_cache;
>> +	long rgn_cache_count;
>>   };
>>   extern struct resv_map *resv_map_alloc(void);
>>   void resv_map_release(struct kref *ref);
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index a8c3087..189c0d7 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -240,11 +240,14 @@ struct file_region {
>>
>>   /*
>>    * Add the huge page range represented by [f, t) to the reserve
>> - * map.  Existing regions will be expanded to accommodate the
>> - * specified range.  We know only existing regions need to be
>> - * expanded, because region_add is only called after region_chg
>> - * with the same range.  If a new file_region structure must
>> - * be allocated, it is done in region_chg.
>> + * map.  In the normal case, existing regions will be expanded
>> + * to accommodate the specified range.  Sufficient regions should
>> + * exist for expansion due to the previous call to region_chg
>> + * with the same range.  However, it is possible that region_del
>> + * could have been called after region_chg and modifed the map
>> + * in such a way that no region exists to be expanded.  In this
>> + * case, pull a region descriptor from the cache associated with
>> + * the map and use that for the new range.
>>    *
>>    * Return the number of new huge pages added to the map.  This
>>    * number is greater than or equal to zero.
>> @@ -261,6 +264,27 @@ static long region_add(struct resv_map *resv, long f, long t)
>>   		if (f <= rg->to)
>>   			break;
>>
>> +	if (&rg->link == head || t < rg->from) {
>> +		/*
>> +		 * No region exits which can be expanded to include the
>
> s/exits/exists/ ?

Yes, that is a typo and should be exists.

>> +		 * specified range.  Pull a region descriptor from the
>> +		 * cache, and use it for this range.
>> +		 */
>> +		VM_BUG_ON(!resv->rgn_cache_count);
>> +
>> +		resv->rgn_cache_count--;
>> +		nrg = list_first_entry(&resv->rgn_cache, struct file_region,
>> +					link);
>> +		list_del(&nrg->link);
>> +
>> +		nrg->from = f;
>> +		nrg->to = t;
>> +		list_add(&nrg->link, rg->link.prev);
>> +
>> +		add += t - f;
>> +		goto out_locked;
>> +	}
>> +
>>   	/* Round our left edge to the current segment if it encloses us. */
>>   	if (f > rg->from)
>>   		f = rg->from;
>> @@ -294,6 +318,8 @@ static long region_add(struct resv_map *resv, long f, long t)
>>   	add += t - nrg->to;		/* Added to end of region */
>>   	nrg->to = t;
>>
>> +out_locked:
>> +	resv->adds_in_progress--;
>>   	spin_unlock(&resv->lock);
>>   	VM_BUG_ON(add < 0);
>>   	return add;
>> @@ -312,11 +338,16 @@ static long region_add(struct resv_map *resv, long f, long t)
>>    * so that the subsequent region_add call will have all the
>>    * regions it needs and will not fail.
>>    *
>> + * Upon entry, region_chg will also examine the cache of
>> + * region descriptors associated with the map.  If there
>> + * not enough descriptors cached, one will be allocated
>> + * for the in progress add operation.
>> + *
>>    * Returns the number of huge pages that need to be added
>>    * to the existing reservation map for the range [f, t).
>>    * This number is greater or equal to zero.  -ENOMEM is
>> - * returned if a new file_region structure is needed and can
>> - * not be allocated.
>> + * returned if a new file_region structure or cache entry
>> + * is needed and can not be allocated.
>>    */
>>   static long region_chg(struct resv_map *resv, long f, long t)
>>   {
>> @@ -326,6 +357,30 @@ static long region_chg(struct resv_map *resv, long f, long t)
>>
>>   retry:
>>   	spin_lock(&resv->lock);
>> +	resv->adds_in_progress++;
>> +
>> +	/*
>> +	 * Check for sufficient descriptors in the cache to accommodate
>> +	 * the number of in progress add operations.
>> +	 */
>> +	if (resv->adds_in_progress > resv->rgn_cache_count) {
>> +		struct file_region *trg;
>> +
>> +		VM_BUG_ON(resv->adds_in_progress - resv->rgn_cache_count > 1);
>> +		/* Must drop lock to allocate a new descriptor. */
>> +		resv->adds_in_progress--;
>> +		spin_unlock(&resv->lock);
>> +
>> +		trg = kmalloc(sizeof(*trg), GFP_KERNEL);
>> +		if (!trg)
>> +			return -ENOMEM;
>> +
>> +		spin_lock(&resv->lock);
>> +		resv->adds_in_progress++;
>> +		list_add(&trg->link, &resv->rgn_cache);
>> +		resv->rgn_cache_count++;
>> +	}
>> +
>>   	/* Locate the region we are before or in. */
>>   	list_for_each_entry(rg, head, link)
>>   		if (f <= rg->to)
>> @@ -336,6 +391,7 @@ retry:
>>   	 * size such that we can guarantee to record the reservation. */
>>   	if (&rg->link == head || t < rg->from) {
>>   		if (!nrg) {
>> +			resv->adds_in_progress--;
>>   			spin_unlock(&resv->lock);
>>   			nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
>>   			if (!nrg)
>> @@ -385,6 +441,25 @@ out_nrg:
>>   }
>>
>>   /*
>> + * Abort the in progress add operation.  The adds_in_progress field
>> + * of the resv_map keeps track of the operations in progress between
>> + * calls to region_chg and region_add.  Operations are sometimes
>> + * aborted after the call to region_chg.  In such cases, region_abort
>> + * is called to decrement the adds_in_progress counter.
>> + *
>> + * NOTE: The range arguments [f, t) are not needed or used in this
>> + * routine.  They are kept to make reading the calling code easier as
>> + * arguments will match the associated region_chg call.
>> + */
>> +static void region_abort(struct resv_map *resv, long f, long t)
>> +{
>> +	spin_lock(&resv->lock);
>> +	VM_BUG_ON(!resv->rgn_cache_count);
>> +	resv->adds_in_progress--;
>> +	spin_unlock(&resv->lock);
>> +}
>> +
>> +/*
>>    * Truncate the reserve map at index 'end'.  Modify/truncate any
>>    * region which contains end.  Delete any regions past end.
>>    * Return the number of huge pages removed from the map.
>> @@ -544,22 +619,42 @@ static void set_vma_private_data(struct vm_area_struct *vma,
>>   struct resv_map *resv_map_alloc(void)
>>   {
>>   	struct resv_map *resv_map = kmalloc(sizeof(*resv_map), GFP_KERNEL);
>> -	if (!resv_map)
>> +	struct file_region *rg = kmalloc(sizeof(*rg), GFP_KERNEL);
>> +
>> +	if (!resv_map || !rg) {
>> +		kfree(resv_map);
>> +		kfree(rg);
>>   		return NULL;
>> +	}
>>
>>   	kref_init(&resv_map->refs);
>>   	spin_lock_init(&resv_map->lock);
>>   	INIT_LIST_HEAD(&resv_map->regions);
>>
>> +	resv_map->adds_in_progress = 0;
>> +
>> +	INIT_LIST_HEAD(&resv_map->rgn_cache);
>> +	list_add(&rg->link, &resv_map->rgn_cache);
>> +	resv_map->rgn_cache_count = 1;
>> +
>>   	return resv_map;
>>   }
>>
>>   void resv_map_release(struct kref *ref)
>>   {
>>   	struct resv_map *resv_map = container_of(ref, struct resv_map, refs);
>> +	struct list_head *head = &resv_map->regions;
>> +	struct file_region *rg, *trg;
>>
>>   	/* Clear out any active regions before we release the map. */
>>   	region_truncate(resv_map, 0);
>> +
>> +	/* ... and any entries left in the cache */
>> +	list_for_each_entry_safe(rg, trg, head, link)
>> +		list_del(&rg->link);
>> +
>
> I am wondering if `cache' is rgn_cache.
> And rg should be freed if yes.

Good catch.

It should be:

	struct list_head *head = &resv_map->rgn_cache;

and

	list_for_each_entry_safe(rg, trg, head, link) {
		list_del(&rg->link);
		kfree(rg);
	}

As is, it will leak memory and not free entries in the cache.

I will fix this in the next revision.

-- 
Mike Kravetz

>> +	VM_BUG_ON(resv_map->adds_in_progress);
>> +
>>   	kfree(resv_map);
>>   }
>>
--
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