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Date:   Mon, 11 Jan 2021 06:44:09 -0500 (EST)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     David Laight <David.Laight@ACULAB.COM>
cc:     "'Al Viro'" <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Vishal Verma <vishal.l.verma@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Eric Sandeen <esandeen@redhat.com>,
        Dave Chinner <dchinner@redhat.com>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Wang Jianchao <jianchao.wan9@gmail.com>,
        "Kani, Toshi" <toshi.kani@hpe.com>,
        "Norton, Scott J" <scott.norton@hpe.com>,
        "Tadakamadla, Rajesh" <rajesh.tadakamadla@hpe.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "linux-nvdimm@lists.01.org" <linux-nvdimm@lists.01.org>
Subject: RE: [RFC v2] nvfs: a filesystem for persistent memory
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On Mon, 11 Jan 2021, David Laight wrote:

> 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

I removed the second "while" statement and fixed the arguments to 
copy_to_iter - other than that, Al's function works.

Mikuklas