Re: [patch] hugetlb strict commit accounting

['David Gibson']

On Thu, Mar 09, 2006 at 02:55:23AM -0800, Chen, Kenneth W wrote:
> Here is a competing implementation of hugetlb strict commit accounting.
> A back port of what was done about two years ago by Andy Whitcroft, Ray
> Bryant, and myself.
> 
> Essentially for the same purpose of this patch currently sit in -mm:
> 
> http://kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.16-rc5/2.6.16-rc5-mm3/broken-out/hugepage-strict-page-reservation-for
> -hugepage-inodes.patch
> 
> Except it is BETTER and more robust :-)  because:
> 
> (1) it does arbitrary variable length and arbitrary variable offset
> (2) doesn't need to perform linear traverse of page cache
> 
> It is more flexible that it will handle any arbitrary mmap offset
> versus the one in -mm that always reserve entire hugetlb file size.
> I've heard numerous complains from application developers that
> hugetlb is difficult to use in current state of affair. Having
> another peculiar behavior like "reservation would only work if
> mmap offset is zero" adds another horrendous factor.

Um... as far as I can tell, this patch doesn't actually reserve
anything.  There are no changes to the fault handler ot
alloc_huge_page(), so there's nothing to stop PRIVATE mappings dipping
into the supposedly reserved pool.

More comments below.

>  fs/hugetlbfs/inode.c    |  188 ++++++++++++++++++++++++++++++++++++++----------
>  include/linux/hugetlb.h |    1 
>  mm/hugetlb.c            |    3 
>  3 files changed, 156 insertions(+), 36 deletions(-)
> 
> --- ./fs/hugetlbfs/inode.c.orig	2006-03-09 02:29:28.166820138 -0800
> +++ ./fs/hugetlbfs/inode.c	2006-03-09 03:20:29.311313889 -0800
> @@ -56,48 +56,160 @@ static void huge_pagevec_release(struct 
>  	pagevec_reinit(pvec);
>  }
>  
> -/*
> - * huge_pages_needed tries to determine the number of new huge pages that
> - * will be required to fully populate this VMA.  This will be equal to
> - * the size of the VMA in huge pages minus the number of huge pages
> - * (covered by this VMA) that are found in the page cache.
> - *
> - * Result is in bytes to be compatible with is_hugepage_mem_enough()
> - */
> -static unsigned long
> -huge_pages_needed(struct address_space *mapping, struct vm_area_struct *vma)
> +struct file_region {
> +	struct list_head link;
> +	int from;
> +	int to;
> +};
> +
> +static int region_add(struct list_head *head, int f, int t)
>  {
> -	int i;
> -	struct pagevec pvec;
> -	unsigned long start = vma->vm_start;
> -	unsigned long end = vma->vm_end;
> -	unsigned long hugepages = (end - start) >> HPAGE_SHIFT;
> -	pgoff_t next = vma->vm_pgoff >> (HPAGE_SHIFT - PAGE_SHIFT);
> -	pgoff_t endpg = next + hugepages;
> +	struct file_region *rg;
> +	struct file_region *nrg;
> +	struct file_region *trg;
> +
> +	/* Locate the region we are either in or before. */
> +	list_for_each_entry(rg, head, link)
> +		if (f <= rg->to)
> +			break;
>  
> -	pagevec_init(&pvec, 0);
> -	while (next < endpg) {
> -		if (!pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE))
> +	/* Round our left edge to the current segment if it encloses us. */
> +	if (f > rg->from)
> +		f = rg->from;
> +
> +	/* Check for and consume any regions we now overlap with. */
> +	nrg = rg;
> +	list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
> +		if (&rg->link == head)
>  			break;
> -		for (i = 0; i < pagevec_count(&pvec); i++) {
> -			struct page *page = pvec.pages[i];
> -			if (page->index > next)
> -				next = page->index;
> -			if (page->index >= endpg)
> -				break;
> -			next++;
> -			hugepages--;
> +		if (rg->from > t)
> +			break;
> +
> +		/* If this area reaches higher then extend our area to
> +		 * include it completely.  If this is not the first area
> +		 * which we intend to reuse, free it. */
> +		if (rg->to > t)
> +			t = rg->to;
> +		if (rg != nrg) {
> +			list_del(&rg->link);
> +			kfree(rg);
>  		}
> -		huge_pagevec_release(&pvec);
>  	}
> -	return hugepages << HPAGE_SHIFT;
> +	nrg->from = f;
> +	nrg->to = t;
> +	return 0;
> +}
> +
> +static int region_chg(struct list_head *head, int f, int t)
> +{
> +	struct file_region *rg;
> +	struct file_region *nrg;
> +	loff_t chg = 0;
> +
> +	/* Locate the region we are before or in. */
> +	list_for_each_entry(rg, head, link)
> +		if (f <= rg->to)
> +			break;
> +
> +	/* If we are below the current region then a new region is required.
> +	 * Subtle, allocate a new region at the position but make it zero
> +	 * size such that we can guarentee to record the reservation. */
> +	if (&rg->link == head || t < rg->from) {
> +		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
> +		if (nrg == 0)
> +			return -ENOMEM;
> +		nrg->from = f;
> +		nrg->to   = f;
> +		INIT_LIST_HEAD(&nrg->link);
> +		list_add(&nrg->link, rg->link.prev);
> +
> +		return t - f;
> +	}
> +
> +	/* Round our left edge to the current segment if it encloses us. */
> +	if (f > rg->from)
> +		f = rg->from;
> +	chg = t - f;
> +
> +	/* Check for and consume any regions we now overlap with. */

Looks like we have this test code duplicated from region_add().  Any
way to avoid that?

> +	list_for_each_entry(rg, rg->link.prev, link) {
> +		if (&rg->link == head)
> +			break;
> +		if (rg->from > t)
> +			return chg;
> +
> +		/* We overlap with this area, if it extends futher than
> +		 * us then we must extend ourselves.  Account for its
> +		 * existing reservation. */
> +		if (rg->to > t) {
> +			chg += rg->to - t;
> +			t = rg->to;
> +		}
> +		chg -= rg->to - rg->from;
> +	}
> +	return chg;
> +}
> +
> +static int region_truncate(struct list_head *head, int end)
> +{
> +	struct file_region *rg;
> +	struct file_region *trg;
> +	int chg = 0;
> +
> +	/* Locate the region we are either in or before. */
> +	list_for_each_entry(rg, head, link)
> +		if (end <= rg->to)
> +			break;
> +	if (&rg->link == head)
> +		return 0;
> +
> +	/* 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);
> +	}
> +
> +	/* Drop any remaining regions. */
> +	list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
> +		if (&rg->link == head)
> +			break;
> +		chg += rg->to - rg->from;
> +		list_del(&rg->link);
> +		kfree(rg);
> +	}
> +	return chg;
> +}
> +
> +#define VMACCTPG(x) ((x) >> (HPAGE_SHIFT - PAGE_SHIFT))
> +int hugetlb_acct_memory(long delta)
> +{
> +	atomic_add(delta, &resv_huge_pages);
> +	if (delta > 0 && atomic_read(&resv_huge_pages) >
> +			VMACCTPG(hugetlb_total_pages())) {
> +		atomic_add(-delta, &resv_huge_pages);
> +		return -ENOMEM;
> +	}
> +	return 0;
> +}

This looks a bit like a case of "let's make it an atomic_t to sprinkle
it with magic atomicity dust" without thinking about what operations
are and need to be atomic.  I think resv_huge_pages should be an
ordinary int, but protected by a lock (exactly which lock is not
immediately obvious).

> +static int hugetlb_reserve_pages(struct inode *inode, int from, int to)
> +{
> +	int ret, chg;
> +
> +	chg = region_chg(&inode->i_mapping->private_list, from, to);

What is the list of regions (mapping->private_list) protected by?
mmap_sem (the only thing I can think of off hand that's already taken)
doesn't cut it, because the mapping can be accessed by multiple mms.

> +	if (chg < 0)
> +		return chg;
> +	ret = hugetlb_acct_memory(chg);
> +	if (ret < 0)
> +		return ret;
> +	region_add(&inode->i_mapping->private_list, from, to);
> +	return 0;
>  }
>  
>  static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
>  {
>  	struct inode *inode = file->f_dentry->d_inode;
> -	struct address_space *mapping = inode->i_mapping;
> -	unsigned long bytes;
>  	loff_t len, vma_len;
>  	int ret;
>  
> @@ -113,10 +225,6 @@ static int hugetlbfs_file_mmap(struct fi
>  	if (vma->vm_end - vma->vm_start < HPAGE_SIZE)
>  		return -EINVAL;
>  
> -	bytes = huge_pages_needed(mapping, vma);
> -	if (!is_hugepage_mem_enough(bytes))
> -		return -ENOMEM;
> -
>  	vma_len = (loff_t)(vma->vm_end - vma->vm_start);
>  
>  	mutex_lock(&inode->i_mutex);
> @@ -129,6 +237,11 @@ static int hugetlbfs_file_mmap(struct fi
>  	if (!(vma->vm_flags & VM_WRITE) && len > inode->i_size)
>  		goto out;
>  
> +	if (vma->vm_flags & VM_MAYSHARE)
> +		if (hugetlb_reserve_pages(inode, VMACCTPG(vma->vm_pgoff),
> +			VMACCTPG(vma->vm_pgoff + (vma_len >> PAGE_SHIFT))))
> +			goto out;
> +
>  	ret = 0;
>  	hugetlb_prefault_arch_hook(vma->vm_mm);
>  	if (inode->i_size < len)
> @@ -258,6 +371,8 @@ static void truncate_hugepages(struct ad
>  		huge_pagevec_release(&pvec);
>  	}
>  	BUG_ON(!lstart && mapping->nrpages);
> +	i = region_truncate(&mapping->private_list, start);
> +	hugetlb_acct_memory(-i);
>  }
>  
>  static void hugetlbfs_delete_inode(struct inode *inode)
> @@ -401,6 +516,7 @@ static struct inode *hugetlbfs_get_inode
>  		inode->i_mapping->a_ops = &hugetlbfs_aops;
>  		inode->i_mapping->backing_dev_info =&hugetlbfs_backing_dev_info;
>  		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
> +		INIT_LIST_HEAD(&inode->i_mapping->private_list);
>  		info = HUGETLBFS_I(inode);
>  		mpol_shared_policy_init(&info->policy, MPOL_DEFAULT, NULL);
>  		switch (mode & S_IFMT) {
> --- ./include/linux/hugetlb.h.orig	2006-03-09 02:29:28.943187316 -0800
> +++ ./include/linux/hugetlb.h	2006-03-09 03:11:45.820109364 -0800
> @@ -30,6 +30,7 @@ int hugetlb_fault(struct mm_struct *mm, 
>  extern unsigned long max_huge_pages;
>  extern const unsigned long hugetlb_zero, hugetlb_infinity;
>  extern int sysctl_hugetlb_shm_group;
> +extern atomic_t resv_huge_pages;
>  
>  /* arch callbacks */
>  
> --- ./mm/hugetlb.c.orig	2006-03-09 02:29:29.314281061 -0800
> +++ ./mm/hugetlb.c	2006-03-09 03:24:34.546662447 -0800
> @@ -21,6 +21,7 @@
>  
>  const unsigned long hugetlb_zero = 0, hugetlb_infinity = ~0UL;
>  static unsigned long nr_huge_pages, free_huge_pages;
> +atomic_t resv_huge_pages;
>  unsigned long max_huge_pages;
>  static struct list_head hugepage_freelists[MAX_NUMNODES];
>  static unsigned int nr_huge_pages_node[MAX_NUMNODES];
> @@ -225,9 +226,11 @@ int hugetlb_report_meminfo(char *buf)
>  	return sprintf(buf,
>  			"HugePages_Total: %5lu\n"
>  			"HugePages_Free:  %5lu\n"
> +			"HugePages_Resv:  %5u\n"
>  			"Hugepagesize:    %5lu kB\n",
>  			nr_huge_pages,
>  			free_huge_pages,
> +			atomic_read(&resv_huge_pages),
>  			HPAGE_SIZE/1024);
>  }

Again, there are no changes to the fault handler.  Including the
promised changes which would mean my instantiation serialization path
isn't necessary ;-).

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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