Re: [RFC v3 PATCH 03/10] mm/hugetlb: add region_del() to delete a specific range of entries

[Naoya Horiguchi]

On Thu, May 21, 2015 at 08:47:37AM -0700, Mike Kravetz wrote:
> fallocate hole punch will want to remove a specific range of pages.
> The existing region_truncate() routine deletes all region/reserve
> map entries after a specified offset.  region_del() will provide
> this same functionality if the end of region is specified as -1.
> Hence, region_del() can replace region_truncate().
> 
> Unlike region_truncate(), region_del() can return an error in the
> rare case where it can not allocate memory for a region descriptor.
> This ONLY happens in the case where an existing region must be split.
> Current callers passing -1 as end of range will never experience
> this error and do not need to deal with error handling.  Future
> callers of region_del() (such as fallocate hole punch) will need to
> handle this error.  A routine hugetlb_fix_reserve_counts() is added
> to assist in cleaning up if fallocate hole punch experiences this
> type of error in region_del().
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  include/linux/hugetlb.h |  1 +
>  mm/hugetlb.c            | 99 ++++++++++++++++++++++++++++++++++++++-----------
>  2 files changed, 79 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 7b57850..fd337f2 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -81,6 +81,7 @@ bool isolate_huge_page(struct page *page, struct list_head *list);
>  void putback_active_hugepage(struct page *page);
>  bool is_hugepage_active(struct page *page);
>  void free_huge_page(struct page *page);
> +void hugetlb_fix_reserve_counts(struct inode *inode, bool restore_reserve);

This function is used in patch 6/10 for the first time,
so is it better to move the definition to that patch?
(this temporarily introduces "defined but not used" warning...)

>  #ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
>  pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud);
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 63f6d43..620cc9e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -261,38 +261,74 @@ out_nrg:
>  	return chg;
>  }
>  
> -static long region_truncate(struct resv_map *resv, long end)
> +static long region_del(struct resv_map *resv, long f, long t)
>  {
>  	struct list_head *head = &resv->regions;
>  	struct file_region *rg, *trg;
> +	struct file_region *nrg = NULL;
>  	long chg = 0;
>  
> +	/*
> +	 * Locate segments we overlap and etiher split, remove or
> +	 * trim the existing regions.  The end of region (t) == -1
> +	 * indicates all remaining regions.  Special case t == -1 as
> +	 * all comparisons are signed.  Also, when t == -1 it is not
> +	 * possible to return an error (-ENOMEM) as this only happens
> +	 * when splitting a region.  Callers take advantage of this
> +	 * when calling with -1.
> +	 */
> +	if (t == -1)
> +		t = LONG_MAX;
> +retry:
>  	spin_lock(&resv->lock);
> -	/* Locate the region we are either in or before. */
> -	list_for_each_entry(rg, head, link)
> -		if (end <= rg->to)
> +	list_for_each_entry_safe(rg, trg, head, link) {
> +		if (rg->to <= f)
> +			continue;
> +		if (rg->from >= t)
>  			break;
> -	if (&rg->link == head)
> -		goto out;
>  
> -	/* If we are in the middle of a region then adjust it. */
> -	if (end > rg->from) {
> -		chg = rg->to - end;
> -		rg->to = end;
> -		rg = list_entry(rg->link.next, typeof(*rg), link);
> -	}
> +		if (f > rg->from && t < rg->to) { /* must split region */
> +			if (!nrg) {
> +				spin_unlock(&resv->lock);
> +				nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> +				if (!nrg)
> +					return -ENOMEM;
> +				goto retry;
> +			}
>  
> -	/* Drop any remaining regions. */
> -	list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
> -		if (&rg->link == head)
> +			chg += t - f;
> +
> +			/* new entry for end of split region */
> +			nrg->from = t;
> +			nrg->to = rg->to;
> +			INIT_LIST_HEAD(&nrg->link);
> +
> +			/* original entry is trimmed */
> +			rg->to = f;
> +
> +			list_add(&nrg->link, &rg->link);
> +			nrg = NULL;
>  			break;
> -		chg += rg->to - rg->from;
> -		list_del(&rg->link);
> -		kfree(rg);
> +		}
> +
> +		if (f <= rg->from && t >= rg->to) { /* remove entire region */
> +			chg += rg->to - rg->from;
> +			list_del(&rg->link);
> +			kfree(rg);
> +			continue;
> +		}
> +
> +		if (f <= rg->from) {	/* trim beginning of region */
> +			chg += t - rg->from;
> +			rg->from = t;
> +		} else {		/* trim end of region */
> +			chg += rg->to - f;
> +			rg->to = f;

Is it better to put "break" here?

Thanks,
Naoya Horiguchi

> +		}
>  	}
>  
> -out:
>  	spin_unlock(&resv->lock);
> +	kfree(nrg);
>  	return chg;
>  }
>  
> @@ -324,6 +360,27 @@ static long region_count(struct resv_map *resv, long f, long t)
>  }
>  
>  /*
> + * A rare out of memory error was encountered which prevented removal of
> + * the reserve map region for a page.  The huge page itself was free''ed
> + * and removed from the page cache.  This routine will adjust the global
> + * reserve count if needed, and the subpool usage count.  By incrementing
> + * these counts, the reserve map entry which could not be deleted will
> + * appear as a "reserved" entry instead of simply dangling with incorrect
> + * counts.
> + */
> +void hugetlb_fix_reserve_counts(struct inode *inode, bool restore_reserve)
> +{
> +	struct hugepage_subpool *spool = subpool_inode(inode);
> +
> +	if (restore_reserve) {
> +		struct hstate *h = hstate_inode(inode);
> +
> +		h->resv_huge_pages++;
> +	}
> +	hugepage_subpool_get_pages(spool, 1);
> +}
> +
> +/*
>   * Convert the address within this vma to the page offset within
>   * the mapping, in pagecache page units; huge pages here.
>   */
> @@ -427,7 +484,7 @@ void resv_map_release(struct kref *ref)
>  	struct resv_map *resv_map = container_of(ref, struct resv_map, refs);
>  
>  	/* Clear out any active regions before we release the map. */
> -	region_truncate(resv_map, 0);
> +	region_del(resv_map, 0, -1);
>  	kfree(resv_map);
>  }
>  
> @@ -3558,7 +3615,7 @@ void hugetlb_unreserve_pages(struct inode *inode, long offset, long freed)
>  	struct hugepage_subpool *spool = subpool_inode(inode);
>  
>  	if (resv_map)
> -		chg = region_truncate(resv_map, offset);
> +		chg = region_del(resv_map, offset, -1);
>  	spin_lock(&inode->i_lock);
>  	inode->i_blocks -= (blocks_per_huge_page(h) * freed);
>  	spin_unlock(&inode->i_lock);
> -- 
> 2.1.0
> --
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