Re: [RFC v3 PATCH 09/10] hugetlbfs: add hugetlbfs_fallocate()

[Naoya Horiguchi]

On Thu, May 21, 2015 at 08:47:43AM -0700, Mike Kravetz wrote:
> This is based on the shmem version, but it has diverged quite
> a bit.  We have no swap to worry about, nor the new file sealing.
> Add synchronication via the fault mutex table to coordinate
> page faults,  fallocate allocation and fallocate hole punch.
> 
> What this allows us to do is move physical memory in and out of
> a hugetlbfs file without having it mapped.  This also gives us
> the ability to support MADV_REMOVE since it is currently
> implemented using fallocate().  MADV_REMOVE lets madvise() remove
> pages from the middle of a hugetlbfs file, which wasn't possible
> before.
> 
> hugetlbfs fallocate only operates on whole huge pages.
> 
> Based-on code-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

This patch changes the behavior of user API, so please update manpage of
fallocate(2).

> ---
>  fs/hugetlbfs/inode.c    | 169 +++++++++++++++++++++++++++++++++++++++++++++++-
>  include/linux/hugetlb.h |   3 +
>  mm/hugetlb.c            |   2 +-
>  3 files changed, 172 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> index dfa88a5..4b1535f 100644
> --- a/fs/hugetlbfs/inode.c
> +++ b/fs/hugetlbfs/inode.c
> @@ -12,6 +12,7 @@
>  #include <linux/thread_info.h>
>  #include <asm/current.h>
>  #include <linux/sched.h>		/* remove ASAP */
> +#include <linux/falloc.h>
>  #include <linux/fs.h>
>  #include <linux/mount.h>
>  #include <linux/file.h>
> @@ -493,6 +494,171 @@ static int hugetlb_vmtruncate(struct inode *inode, loff_t offset)
>  	return 0;
>  }
>  
> +static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> +{
> +	struct hstate *h = hstate_inode(inode);
> +	unsigned long hpage_size = huge_page_size(h);
> +	loff_t hole_start, hole_end;
> +
> +	/*
> +	 * For hole punch round up the beginning offset of the hole and
> +	 * round down the end.
> +	 */
> +	hole_start = (offset + hpage_size - 1) & huge_page_mask(h);
> +	hole_end = (offset + len) & huge_page_mask(h);

We have round_up/round_up macro, so please use them here.
Then, it's self-descriptive, so you don't have to write comment.

> +
> +	if ((u64)hole_end > (u64)hole_start) {

Why is this casting to u64 necessary?

> +		struct address_space *mapping = inode->i_mapping;
> +
> +		mutex_lock(&inode->i_mutex);
> +		i_mmap_lock_write(mapping);
> +		if (!RB_EMPTY_ROOT(&mapping->i_mmap))
> +			hugetlb_vmdelete_list(&mapping->i_mmap,
> +						hole_start >> PAGE_SHIFT,
> +						hole_end  >> PAGE_SHIFT);
> +		i_mmap_unlock_write(mapping);
> +		remove_inode_hugepages(inode, hole_start, hole_end);
> +		mutex_unlock(&inode->i_mutex);
> +	}
> +
> +	return 0;
> +}
> +
> +static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
> +				loff_t len)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct address_space *mapping = inode->i_mapping;
> +	struct hstate *h = hstate_inode(inode);
> +	struct vm_area_struct pseudo_vma;
> +	unsigned long hpage_size = huge_page_size(h);
> +	unsigned long hpage_shift = huge_page_shift(h);
> +	pgoff_t start, index, end;
> +	int error;
> +	u32 hash;
> +
> +	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
> +		return -EOPNOTSUPP;
> +
> +	if (mode & FALLOC_FL_PUNCH_HOLE)
> +		return hugetlbfs_punch_hole(inode, offset, len);
> +
> +	/*
> +	 * Default preallocate case.
> +	 * For this range, start is rounded down and end is rounded up.
> +	 */
> +	start = offset >> hpage_shift;
> +	end = (offset + len + hpage_size - 1) >> hpage_shift;
> +
> +	mutex_lock(&inode->i_mutex);
> +
> +	/* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */
> +	error = inode_newsize_ok(inode, offset + len);
> +	if (error)
> +		goto out;
> +
> +	/*
> +	 * Initialize a pseudo vma that just contains the policy used
> +	 * when allocating the huge pages.  The actual policy field
> +	 * (vm_policy) is determined based on the index in the loop below.
> +	 */
> +	memset(&pseudo_vma, 0, sizeof(struct vm_area_struct));
> +	pseudo_vma.vm_start = 0;
> +	pseudo_vma.vm_flags |= (VM_HUGETLB | VM_MAYSHARE | VM_SHARED);

Maybe '|' isn't necessary.

> +	pseudo_vma.vm_file = file;
> +
> +	for (index = start; index < end; index++) {
> +		/*
> +		 * This is supposed to be the vaddr where the page is being
> +		 * faulted in, but we have no vaddr here.
> +		 */
> +		struct page *page;
> +		unsigned long addr;
> +		int avoid_reserve = 0;

avoid_reserve is referred only once and never changed, so no need to use
the variable?

> +
> +		cond_resched();
> +
> +		/*
> +		 * fallocate(2) manpage permits EINTR; we may have been
> +		 * interrupted because we are using up too much memory.
> +		 */
> +		if (signal_pending(current)) {
> +			error = -EINTR;
> +			break;
> +		}
> +
> +		/* Get policy based on index */
> +		pseudo_vma.vm_policy =
> +			mpol_shared_policy_lookup(&HUGETLBFS_I(inode)->policy,
> +							index);
> +
> +		/* addr is the offset within the file (zero based) */
> +		addr = index * hpage_size;
> +
> +		/* mutex taken here, fault path and hole punch */
> +		hash = hugetlb_fault_mutex_shared_hash(mapping, index);
> +		hugetlb_fault_mutex_lock(hash);
> +
> +		/* see if page already exists to avoid alloc/free */
> +		page = find_get_page(mapping, index);
> +		if (page) {
> +			put_page(page);
> +			hugetlb_fault_mutex_unlock(hash);

Don't you need mpol_cond_put() here?

> +			continue;
> +		}
> +
> +		page = alloc_huge_page(&pseudo_vma, addr, avoid_reserve);
> +		mpol_cond_put(pseudo_vma.vm_policy);
> +		if (IS_ERR(page)) {
> +			hugetlb_fault_mutex_unlock(hash);
> +			error = PTR_ERR(page);
> +			goto out;
> +		}
> +		clear_huge_page(page, addr, pages_per_huge_page(h));
> +		__SetPageUptodate(page);

Note that recently I added page_huge_active() to mark activeness of hugepages,
so when you rebased to v4.1-rc1+, please insert set_page_huge_active(page) here.

> +		error = huge_add_to_page_cache(page, mapping, index);
> +		if (error) {
> +			/*
> +			 * An entry already exists in the cache.  This implies
> +			 * a region also existed in the reserve map at the time
> +			 * the page was allocated above.  Therefore, no use
> +			 * count was added to the subpool for the page.  Before
> +			 * freeing the page, clear the subpool reference so
> +			 * that the count is not decremented.
> +			 */
> +			set_page_private(page, 0);/* clear spool reference */

This looks unclear to me. Which "count" do you refer to in the comment
"no use count was added to the subpool" or "the count is not decremented"?
I guess spool->used_hpages or spool->rsv_hpages, but alloc_huge_page() above
should call hugepage_subpool_get_pages(), so it's accounted, right?
Could you write comments more specifically?

Thanks,
Naoya Horiguchi

> +			put_page(page);
> +
> +			hugetlb_fault_mutex_unlock(hash);
> +			/* Keep going if we see an -EEXIST */
> +			if (error == -EEXIST) {
> +				error = 0;	/* do not return to user */
> +				continue;
> +			} else
> +				goto out;
> +		}
> +
> +		hugetlb_fault_mutex_unlock(hash);
> +
> +		/*
> +		 * page_put due to reference from alloc_huge_page()
> +		 * unlock_page because locked by add_to_page_cache()
> +		 */
> +		put_page(page);
> +		unlock_page(page);
> +	}
> +
> +	if (!(mode & FALLOC_FL_KEEP_SIZE) && offset + len > inode->i_size)
> +		i_size_write(inode, offset + len);
> +	inode->i_ctime = CURRENT_TIME;
> +	spin_lock(&inode->i_lock);
> +	inode->i_private = NULL;
> +	spin_unlock(&inode->i_lock);
> +out:
> +	mutex_unlock(&inode->i_mutex);
> +	return error;
> +}
> +
>  static int hugetlbfs_setattr(struct dentry *dentry, struct iattr *attr)
>  {
>  	struct inode *inode = dentry->d_inode;
> @@ -804,7 +970,8 @@ const struct file_operations hugetlbfs_file_operations = {
>  	.mmap			= hugetlbfs_file_mmap,
>  	.fsync			= noop_fsync,
>  	.get_unmapped_area	= hugetlb_get_unmapped_area,
> -	.llseek		= default_llseek,
> +	.llseek			= default_llseek,
> +	.fallocate		= hugetlbfs_fallocate,
>  };
>  
>  static const struct inode_operations hugetlbfs_dir_inode_operations = {
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 934f339..fa36b7a 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -327,6 +327,8 @@ struct huge_bootmem_page {
>  #endif
>  };
>  
> +struct page *alloc_huge_page(struct vm_area_struct *vma,
> +				unsigned long addr, int avoid_reserve);
>  struct page *alloc_huge_page_node(struct hstate *h, int nid);
>  struct page *alloc_huge_page_noerr(struct vm_area_struct *vma,
>  				unsigned long addr, int avoid_reserve);
> @@ -481,6 +483,7 @@ static inline bool hugepages_supported(void)
>  
>  #else	/* CONFIG_HUGETLB_PAGE */
>  struct hstate {};
> +#define alloc_huge_page(v, a, r) NULL
>  #define alloc_huge_page_node(h, nid) NULL
>  #define alloc_huge_page_noerr(v, a, r) NULL
>  #define alloc_bootmem_huge_page(h) NULL
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 94c6154..1e95038 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1444,7 +1444,7 @@ static long vma_commit_reservation(struct hstate *h,
>  /* Forward declaration */
>  static int hugetlb_acct_memory(struct hstate *h, long delta);
>  
> -static struct page *alloc_huge_page(struct vm_area_struct *vma,
> +struct page *alloc_huge_page(struct vm_area_struct *vma,
>  				    unsigned long addr, int avoid_reserve)
>  {
>  	struct hugepage_subpool *spool = subpool_vma(vma);
> -- 
> 2.1.0
> --
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