Re: [RFC PATCH v1.1 2/2] erofs: dax support for non-tailpacking regular file

[Darrick J. Wong]

On Mon, Jul 05, 2021 at 09:21:53PM +0800, Gao Xiang wrote:
> DAX is quite useful for some VM use cases in order to save guest
> memory extremely with minimal lightweight EROFS.
> 
> In order to prepare for such use cases, add preliminary dax support
> for non-tailpacking regular files for now.
> 
> Tested with the DRAM-emulated PMEM and the EROFS image generated by
> "mkfs.erofs -Enoinline_data enwik9.fsdax.img enwik9"
> 
> Cc: nvdimm@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> ---
> change since v1:
>  - update missing hunks due to patch spliting...
>     bdev_dax_supported(...)
>     erofs_file_mmap(...)   
> 
>  fs/erofs/data.c     | 43 +++++++++++++++++++++++++++++++++++++++++--
>  fs/erofs/inode.c    |  5 +++++
>  fs/erofs/internal.h |  2 ++
>  fs/erofs/super.c    | 26 ++++++++++++++++++++++++--
>  4 files changed, 72 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/erofs/data.c b/fs/erofs/data.c
> index 0f82b4cb474c..c188c629be45 100644
> --- a/fs/erofs/data.c
> +++ b/fs/erofs/data.c
> @@ -6,7 +6,7 @@
>  #include "internal.h"
>  #include <linux/prefetch.h>
>  #include <linux/iomap.h>
> -
> +#include <linux/dax.h>
>  #include <trace/events/erofs.h>
>  
>  static void erofs_readendio(struct bio *bio)
> @@ -323,6 +323,7 @@ static int erofs_iomap_begin(struct inode *inode, loff_t offset, loff_t length,
>  		return ret;
>  
>  	iomap->bdev = inode->i_sb->s_bdev;
> +	iomap->dax_dev = EROFS_I_SB(inode)->dax_dev;
>  	iomap->offset = map.m_la;
>  	iomap->length = map.m_llen;
>  
> @@ -382,6 +383,11 @@ static ssize_t erofs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>  	if (!iov_iter_count(to))
>  		return 0;
>  
> +#ifdef CONFIG_FS_DAX
> +	if (IS_DAX(iocb->ki_filp->f_mapping->host))
> +		return dax_iomap_rw(iocb, to, &erofs_iomap_ops);
> +#endif
> +
>  	if (iocb->ki_flags & IOCB_DIRECT) {
>  		int err = erofs_prepare_dio(iocb, to);
>  
> @@ -410,9 +416,42 @@ const struct address_space_operations erofs_raw_access_aops = {
>  	.direct_IO = noop_direct_IO,
>  };
>  
> +#ifdef CONFIG_FS_DAX
> +static vm_fault_t erofs_dax_huge_fault(struct vm_fault *vmf,
> +		enum page_entry_size pe_size)
> +{
> +	return dax_iomap_fault(vmf, pe_size, NULL, NULL, &erofs_iomap_ops);
> +}
> +
> +static vm_fault_t erofs_dax_fault(struct vm_fault *vmf)
> +{
> +	return erofs_dax_huge_fault(vmf, PE_SIZE_PTE);
> +}
> +
> +static const struct vm_operations_struct erofs_dax_vm_ops = {
> +	.fault		= erofs_dax_fault,
> +	.huge_fault	= erofs_dax_huge_fault,
> +};
> +
> +static int erofs_file_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	if (!IS_DAX(file_inode(file)))
> +		return generic_file_readonly_mmap(file, vma);
> +
> +	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
> +		return -EINVAL;
> +
> +	vma->vm_ops = &erofs_dax_vm_ops;
> +	vma->vm_flags |= VM_HUGEPAGE;
> +	return 0;
> +}
> +#else
> +#define erofs_file_mmap	generic_file_readonly_mmap
> +#endif
> +
>  const struct file_operations erofs_file_fops = {
>  	.llseek		= generic_file_llseek,
>  	.read_iter	= erofs_file_read_iter,
> -	.mmap		= generic_file_readonly_mmap,
> +	.mmap		= erofs_file_mmap,
>  	.splice_read	= generic_file_splice_read,
>  };
> diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
> index 00edb7562fea..695b97acb9a6 100644
> --- a/fs/erofs/inode.c
> +++ b/fs/erofs/inode.c
> @@ -174,6 +174,11 @@ static struct page *erofs_read_inode(struct inode *inode,
>  	inode->i_mtime.tv_nsec = inode->i_ctime.tv_nsec;
>  	inode->i_atime.tv_nsec = inode->i_ctime.tv_nsec;
>  
> +	inode->i_flags &= ~S_DAX;
> +	if (test_opt(&sbi->ctx, DAX) && S_ISREG(inode->i_mode) &&
> +	    vi->datalayout == EROFS_INODE_FLAT_PLAIN)
> +		inode->i_flags |= S_DAX;
> +
>  	if (!nblks)
>  		/* measure inode.i_blocks as generic filesystems */
>  		inode->i_blocks = roundup(inode->i_size, EROFS_BLKSIZ) >> 9;
> diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
> index 2669c785d548..8b0542d35148 100644
> --- a/fs/erofs/internal.h
> +++ b/fs/erofs/internal.h
> @@ -83,6 +83,7 @@ struct erofs_sb_info {
>  
>  	struct erofs_sb_lz4_info lz4;
>  #endif	/* CONFIG_EROFS_FS_ZIP */
> +	struct dax_device *dax_dev;
>  	u32 blocks;
>  	u32 meta_blkaddr;
>  #ifdef CONFIG_EROFS_FS_XATTR
> @@ -115,6 +116,7 @@ struct erofs_sb_info {
>  /* Mount flags set via mount options or defaults */
>  #define EROFS_MOUNT_XATTR_USER		0x00000010
>  #define EROFS_MOUNT_POSIX_ACL		0x00000020
> +#define EROFS_MOUNT_DAX			0x00000040
>  
>  #define clear_opt(ctx, option)	((ctx)->mount_opt &= ~EROFS_MOUNT_##option)
>  #define set_opt(ctx, option)	((ctx)->mount_opt |= EROFS_MOUNT_##option)
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 8fc6c04b54f4..b44a964ab24f 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -11,6 +11,7 @@
>  #include <linux/crc32c.h>
>  #include <linux/fs_context.h>
>  #include <linux/fs_parser.h>
> +#include <linux/dax.h>
>  #include "xattr.h"
>  
>  #define CREATE_TRACE_POINTS
> @@ -355,6 +356,7 @@ enum {
>  	Opt_user_xattr,
>  	Opt_acl,
>  	Opt_cache_strategy,
> +	Opt_dax,
>  	Opt_err
>  };
>  
> @@ -370,6 +372,7 @@ static const struct fs_parameter_spec erofs_fs_parameters[] = {
>  	fsparam_flag_no("acl",		Opt_acl),
>  	fsparam_enum("cache_strategy",	Opt_cache_strategy,
>  		     erofs_param_cache_strategy),
> +	fsparam_flag("dax",             Opt_dax),
>  	{}
>  };
>  
> @@ -410,6 +413,14 @@ static int erofs_fc_parse_param(struct fs_context *fc,
>  		ctx->cache_strategy = result.uint_32;
>  #else
>  		errorfc(fc, "compression not supported, cache_strategy ignored");
> +#endif
> +		break;
> +	case Opt_dax:
> +#ifdef CONFIG_FS_DAX
> +		warnfc(fc, "DAX enabled. Warning: EXPERIMENTAL, use at your own risk");
> +		set_opt(ctx, DAX);

You might want to allow 'dax=always' and 'dax=never' to maintain parity
with xfs/ext4's mount options...

--D

> +#else
> +		errorfc(fc, "dax options not supported");
>  #endif
>  		break;
>  	default:
> @@ -496,10 +507,17 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
>  		return -ENOMEM;
>  
>  	sb->s_fs_info = sbi;
> +	sbi->dax_dev = fs_dax_get_by_bdev(sb->s_bdev);
>  	err = erofs_read_superblock(sb);
>  	if (err)
>  		return err;
>  
> +	if (test_opt(ctx, DAX) &&
> +	    !bdev_dax_supported(sb->s_bdev, EROFS_BLKSIZ)) {
> +		errorfc(fc, "DAX unsupported by block device. Turning off DAX.");
> +		clear_opt(ctx, DAX);
> +	}
> +
>  	sb->s_flags |= SB_RDONLY | SB_NOATIME;
>  	sb->s_maxbytes = MAX_LFS_FILESIZE;
>  	sb->s_time_gran = 1;
> @@ -609,6 +627,8 @@ static void erofs_kill_sb(struct super_block *sb)
>  	sbi = EROFS_SB(sb);
>  	if (!sbi)
>  		return;
> +	if (sbi->dax_dev)
> +		fs_put_dax(sbi->dax_dev);
>  	kfree(sbi);
>  	sb->s_fs_info = NULL;
>  }
> @@ -711,8 +731,8 @@ static int erofs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  
>  static int erofs_show_options(struct seq_file *seq, struct dentry *root)
>  {
> -	struct erofs_sb_info *sbi __maybe_unused = EROFS_SB(root->d_sb);
> -	struct erofs_fs_context *ctx __maybe_unused = &sbi->ctx;
> +	struct erofs_sb_info *sbi = EROFS_SB(root->d_sb);
> +	struct erofs_fs_context *ctx = &sbi->ctx;
>  
>  #ifdef CONFIG_EROFS_FS_XATTR
>  	if (test_opt(ctx, XATTR_USER))
> @@ -734,6 +754,8 @@ static int erofs_show_options(struct seq_file *seq, struct dentry *root)
>  	else if (ctx->cache_strategy == EROFS_ZIP_CACHE_READAROUND)
>  		seq_puts(seq, ",cache_strategy=readaround");
>  #endif
> +	if (test_opt(ctx, DAX))
> +		seq_puts(seq, ",dax");
>  	return 0;
>  }
>  
> -- 
> 2.24.4
>