Re: [PATCH v2 01/10] mm/hugetlb: add cache of descriptors to resv_map for region_add

[Mike Kravetz <mike.kravetz@oracle.com>]

On 07/08/2015 05:21 PM, Mike Kravetz 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@oracle.com>
> ---
>   include/linux/hugetlb.h |   3 +
>   mm/hugetlb.c            | 168 ++++++++++++++++++++++++++++++++++++++++++------
>   2 files changed, 152 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index d891f94..667cf44 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..de03374 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 exists which can be expanded to include the
> +		 * 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++;

Doh!  I noticed shortly after sending that this needs to go back and
check the condition (sufficient descriptors) after acquiring the lock.

Apologies for the obvious bug.  More comments are welcome.

-- 
Mike Kravetz

> +	}
> +
>   	/* 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 +619,44 @@ 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->rgn_cache;
> +	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);
> +		kfree(rg);
> +	}
> +
> +	VM_BUG_ON(resv_map->adds_in_progress);
> +
>   	kfree(resv_map);
>   }
>
> @@ -1473,16 +1570,18 @@ static void return_unused_surplus_pages(struct hstate *h,
>   	}
>   }
>
> +
>   /*
> - * vma_needs_reservation and vma_commit_reservation are used by the huge
> - * page allocation routines to manage reservations.
> + * vma_needs_reservation, vma_commit_reservation and vma_abort_reservation
> + * are used by the huge page allocation routines to manage reservations.
>    *
>    * vma_needs_reservation is called to determine if the huge page at addr
>    * within the vma has an associated reservation.  If a reservation is
>    * needed, the value 1 is returned.  The caller is then responsible for
>    * managing the global reservation and subpool usage counts.  After
>    * the huge page has been allocated, vma_commit_reservation is called
> - * to add the page to the reservation map.
> + * to add the page to the reservation map.  If the reservation must be
> + * aborted instead of committed, vma_abort_reservation is called.
>    *
>    * In the normal case, vma_commit_reservation returns the same value
>    * as the preceding vma_needs_reservation call.  The only time this
> @@ -1490,9 +1589,14 @@ static void return_unused_surplus_pages(struct hstate *h,
>    * is the responsibility of the caller to notice the difference and
>    * take appropriate action.
>    */
> +enum vma_resv_mode {
> +	VMA_NEEDS_RESV,
> +	VMA_COMMIT_RESV,
> +	VMA_ABORT_RESV,
> +};
>   static long __vma_reservation_common(struct hstate *h,
>   				struct vm_area_struct *vma, unsigned long addr,
> -				bool commit)
> +				enum vma_resv_mode mode)
>   {
>   	struct resv_map *resv;
>   	pgoff_t idx;
> @@ -1503,10 +1607,20 @@ static long __vma_reservation_common(struct hstate *h,
>   		return 1;
>
>   	idx = vma_hugecache_offset(h, vma, addr);
> -	if (commit)
> -		ret = region_add(resv, idx, idx + 1);
> -	else
> +	switch (mode) {
> +	case VMA_NEEDS_RESV:
>   		ret = region_chg(resv, idx, idx + 1);
> +		break;
> +	case VMA_COMMIT_RESV:
> +		ret = region_add(resv, idx, idx + 1);
> +		break;
> +	case VMA_ABORT_RESV:
> +		region_abort(resv, idx, idx + 1);
> +		ret = 0;
> +		break;
> +	default:
> +		BUG();
> +	}
>
>   	if (vma->vm_flags & VM_MAYSHARE)
>   		return ret;
> @@ -1517,13 +1631,19 @@ static long __vma_reservation_common(struct hstate *h,
>   static long vma_needs_reservation(struct hstate *h,
>   			struct vm_area_struct *vma, unsigned long addr)
>   {
> -	return __vma_reservation_common(h, vma, addr, false);
> +	return __vma_reservation_common(h, vma, addr, VMA_NEEDS_RESV);
>   }
>
>   static long vma_commit_reservation(struct hstate *h,
>   			struct vm_area_struct *vma, unsigned long addr)
>   {
> -	return __vma_reservation_common(h, vma, addr, true);
> +	return __vma_reservation_common(h, vma, addr, VMA_COMMIT_RESV);
> +}
> +
> +static void vma_abort_reservation(struct hstate *h,
> +			struct vm_area_struct *vma, unsigned long addr)
> +{
> +	(void)__vma_reservation_common(h, vma, addr, VMA_ABORT_RESV);
>   }
>
>   static struct page *alloc_huge_page(struct vm_area_struct *vma,
> @@ -1549,8 +1669,10 @@ static struct page *alloc_huge_page(struct vm_area_struct *vma,
>   	if (chg < 0)
>   		return ERR_PTR(-ENOMEM);
>   	if (chg || avoid_reserve)
> -		if (hugepage_subpool_get_pages(spool, 1) < 0)
> +		if (hugepage_subpool_get_pages(spool, 1) < 0) {
> +			vma_abort_reservation(h, vma, addr);
>   			return ERR_PTR(-ENOSPC);
> +		}
>
>   	ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h), &h_cg);
>   	if (ret)
> @@ -1596,6 +1718,7 @@ out_uncharge_cgroup:
>   out_subpool_put:
>   	if (chg || avoid_reserve)
>   		hugepage_subpool_put_pages(spool, 1);
> +	vma_abort_reservation(h, vma, addr);
>   	return ERR_PTR(-ENOSPC);
>   }
>
> @@ -3236,11 +3359,14 @@ retry:
>   	 * any allocations necessary to record that reservation occur outside
>   	 * the spinlock.
>   	 */
> -	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
> +	if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
>   		if (vma_needs_reservation(h, vma, address) < 0) {
>   			ret = VM_FAULT_OOM;
>   			goto backout_unlocked;
>   		}
> +		/* Just decrements count, does not deallocate */
> +		vma_abort_reservation(h, vma, address);
> +	}
>
>   	ptl = huge_pte_lockptr(h, mm, ptep);
>   	spin_lock(ptl);
> @@ -3387,6 +3513,8 @@ int hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>   			ret = VM_FAULT_OOM;
>   			goto out_mutex;
>   		}
> +		/* Just decrements count, does not deallocate */
> +		vma_abort_reservation(h, vma, address);
>
>   		if (!(vma->vm_flags & VM_MAYSHARE))
>   			pagecache_page = hugetlbfs_pagecache_page(h,
> @@ -3726,6 +3854,8 @@ int hugetlb_reserve_pages(struct inode *inode,
>   	}
>   	return 0;
>   out_err:
> +	if (!vma || vma->vm_flags & VM_MAYSHARE)
> +		region_abort(resv_map, from, to);
>   	if (vma && is_vma_resv_set(vma, HPAGE_RESV_OWNER))
>   		kref_put(&resv_map->refs, resv_map_release);
>   	return ret;
>

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