RE: [RFC v2] nvfs: a filesystem for persistent memory

[David Laight <David.Laight@ACULAB.COM>]

From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: 10 January 2021 16:20
> 
> On Thu, Jan 07, 2021 at 08:15:41AM -0500, Mikulas Patocka wrote:
> > Hi
> >
> > I announce a new version of NVFS - a filesystem for persistent memory.
> > 	http://people.redhat.com/~mpatocka/nvfs/
> Utilities, AFAICS
> 
> > 	git://leontynka.twibright.com/nvfs.git
> Seems to hang on git pull at the moment...  Do you have it anywhere else?
> 
> > I found out that on NVFS, reading a file with the read method has 10%
> > better performance than the read_iter method. The benchmark just reads the
> > same 4k page over and over again - and the cost of creating and parsing
> > the kiocb and iov_iter structures is just that high.
> 
> Apples and oranges...  What happens if you take
> 
> ssize_t read_iter_locked(struct file *file, struct iov_iter *to, loff_t *ppos)
> {
> 	struct inode *inode = file_inode(file);
> 	struct nvfs_memory_inode *nmi = i_to_nmi(inode);
> 	struct nvfs_superblock *nvs = inode->i_sb->s_fs_info;
> 	ssize_t total = 0;
> 	loff_t pos = *ppos;
> 	int r;
> 	int shift = nvs->log2_page_size;
> 	size_t i_size;
> 
> 	i_size = inode->i_size;
> 	if (pos >= i_size)
> 		return 0;
> 	iov_iter_truncate(to, i_size - pos);
> 
> 	while (iov_iter_count(to)) {
> 		void *blk, *ptr;
> 		size_t page_mask = (1UL << shift) - 1;
> 		unsigned page_offset = pos & page_mask;
> 		unsigned prealloc = (iov_iter_count(to) + page_mask) >> shift;
> 		unsigned size;
> 
> 		blk = nvfs_bmap(nmi, pos >> shift, &prealloc, NULL, NULL, NULL);
> 		if (unlikely(IS_ERR(blk))) {
> 			r = PTR_ERR(blk);
> 			goto ret_r;
> 		}
> 		size = ((size_t)prealloc << shift) - page_offset;
> 		ptr = blk + page_offset;
> 		if (unlikely(!blk)) {
> 			size = min(size, (unsigned)PAGE_SIZE);
> 			ptr = empty_zero_page;
> 		}
> 		size = copy_to_iter(to, ptr, size);
> 		if (unlikely(!size)) {
> 			r = -EFAULT;
> 			goto ret_r;
> 		}
> 
> 		pos += size;
> 		total += size;
> 	} while (iov_iter_count(to));

That isn't the best formed loop!

	David

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