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Date: Thu, 15 Jan 2015 13:41:06 +0100
From: Jan Kara <jack@...e.cz>
To: ross.zwisler@...ux.intel.com
Cc: akpm@...ux-foundation.org, andreas.dilger@...el.com,
axboe@...nel.dk, boaz@...xistor.com, david@...morbit.com,
hch@....de, jack@...e.cz, kirill.shutemov@...ux.intel.com,
mathieu.desnoyers@...icios.com, matthew.r.wilcox@...el.com,
rdunlap@...radead.org, tytso@....edu, mm-commits@...r.kernel.org,
linux-ext4@...r.kernel.org
Subject: Re: + ext4-add-dax-functionality.patch added to -mm tree
On Mon 12-01-15 15:11:17, Andrew Morton wrote:
> From: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Subject: ext4: add DAX functionality
>
> This is a port of the DAX functionality found in the current version of
> ext2.
>
> [matthew.r.wilcox@...el.com: heavily tweaked]
> Signed-off-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
> Reviewed-by: Andreas Dilger <andreas.dilger@...el.com>
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@...el.com>
> Cc: Boaz Harrosh <boaz@...xistor.com>
> Cc: Christoph Hellwig <hch@....de>
> Cc: Dave Chinner <david@...morbit.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Randy Dunlap <rdunlap@...radead.org>
> Cc: Theodore Ts'o <tytso@....edu>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>
> Documentation/filesystems/dax.txt | 1
> Documentation/filesystems/ext4.txt | 4 +
> fs/ext4/ext4.h | 6 +
> fs/ext4/file.c | 50 ++++++++++++++-
> fs/ext4/indirect.c | 18 +++--
> fs/ext4/inode.c | 89 ++++++++++++++++++---------
> fs/ext4/namei.c | 10 ++-
> fs/ext4/super.c | 39 +++++++++++
> 8 files changed, 180 insertions(+), 37 deletions(-)
>
> diff -puN Documentation/filesystems/dax.txt~ext4-add-dax-functionality Documentation/filesystems/dax.txt
> --- a/Documentation/filesystems/dax.txt~ext4-add-dax-functionality
> +++ a/Documentation/filesystems/dax.txt
> @@ -73,6 +73,7 @@ or a write()) work correctly.
>
> These filesystems may be used for inspiration:
> - ext2: the second extended filesystem, see Documentation/filesystems/ext2.txt
> +- ext4: the fourth extended filesystem, see Documentation/filesystems/ext4.txt
>
>
> Shortcomings
> diff -puN Documentation/filesystems/ext4.txt~ext4-add-dax-functionality Documentation/filesystems/ext4.txt
> --- a/Documentation/filesystems/ext4.txt~ext4-add-dax-functionality
> +++ a/Documentation/filesystems/ext4.txt
> @@ -386,6 +386,10 @@ max_dir_size_kb=n This limits the size o
> i_version Enable 64-bit inode version support. This option is
> off by default.
>
> +dax Use direct access (no page cache). See
> + Documentation/filesystems/dax.txt. Note that
> + this option is incompatible with data=journal.
> +
> Data Mode
> =========
> There are 3 different data modes:
> diff -puN fs/ext4/ext4.h~ext4-add-dax-functionality fs/ext4/ext4.h
> --- a/fs/ext4/ext4.h~ext4-add-dax-functionality
> +++ a/fs/ext4/ext4.h
> @@ -965,6 +965,11 @@ struct ext4_inode_info {
> #define EXT4_MOUNT_ERRORS_MASK 0x00070
> #define EXT4_MOUNT_MINIX_DF 0x00080 /* Mimics the Minix statfs */
> #define EXT4_MOUNT_NOLOAD 0x00100 /* Don't use existing journal*/
> +#ifdef CONFIG_FS_DAX
> +#define EXT4_MOUNT_DAX 0x00200 /* Direct Access */
> +#else
> +#define EXT4_MOUNT_DAX 0
> +#endif
Again, why do you make definition of EXT4_MOUNT_DAX dependent on
CONFIG_FS_DAX?
> diff -puN fs/ext4/file.c~ext4-add-dax-functionality fs/ext4/file.c
> --- a/fs/ext4/file.c~ext4-add-dax-functionality
> +++ a/fs/ext4/file.c
> @@ -95,7 +95,7 @@ ext4_file_write_iter(struct kiocb *iocb,
> struct inode *inode = file_inode(iocb->ki_filp);
> struct mutex *aio_mutex = NULL;
> struct blk_plug plug;
> - int o_direct = file->f_flags & O_DIRECT;
> + int o_direct = io_is_direct(file);
> int overwrite = 0;
> size_t length = iov_iter_count(from);
> ssize_t ret;
> @@ -191,6 +191,27 @@ errout:
> return ret;
> }
>
> +#ifdef CONFIG_FS_DAX
> +static int ext4_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
> +{
> + return dax_fault(vma, vmf, ext4_get_block);
> + /* Is this the right get_block? */
You can remove the comment. It is the right get_block function.
...
> diff -puN fs/ext4/inode.c~ext4-add-dax-functionality fs/ext4/inode.c
> --- a/fs/ext4/inode.c~ext4-add-dax-functionality
> +++ a/fs/ext4/inode.c
> @@ -657,6 +657,18 @@ has_zeroout:
> return retval;
> }
>
> +static void ext4_end_io_unwritten(struct buffer_head *bh, int uptodate)
> +{
> + struct inode *inode = bh->b_assoc_map->host;
> + /* XXX: breaks on 32-bit > 16GB. Is that even supported? */
That should be 16 TB if I'm doing the math right - 32-bit block number *
block size (4k) = 16 TB. And that's the max limit of ext4 (as logical file
offset in blocks has to fit in 32-bits for ext4). So I think you can just
remove the comment. But also see comment below.
> + loff_t offset = (loff_t)(uintptr_t)bh->b_private << inode->i_blkbits;
> + int err;
> + if (!uptodate)
> + return;
> + WARN_ON(!buffer_unwritten(bh));
> + err = ext4_convert_unwritten_extents(NULL, inode, offset, bh->b_size);
> +}
> +
> /* Maximum number of blocks we map for direct IO at once. */
> #define DIO_MAX_BLOCKS 4096
>
> @@ -694,6 +706,11 @@ static int _ext4_get_block(struct inode
>
> map_bh(bh, inode->i_sb, map.m_pblk);
> bh->b_state = (bh->b_state & ~EXT4_MAP_FLAGS) | map.m_flags;
> + if (IS_DAX(inode) && buffer_unwritten(bh) && !io_end) {
> + bh->b_assoc_map = inode->i_mapping;
> + bh->b_private = (void *)(unsigned long)iblock;
> + bh->b_end_io = ext4_end_io_unwritten;
> + }
So why is this needed? It would deserve a comment. It confuses me in
particular because:
1) This is a often a phony bh used just as a container for passed data and
b_end_io is just ignored.
2) Even if it was real bh attached to a page, for DAX we don't do any
writeback and thus ->b_end_io will never get called?
3) And if it does get called, you certainly cannot call
ext4_convert_unwritten_extents() from softirq context where ->b_end_io
gets called.
> if (io_end && io_end->flag & EXT4_IO_END_UNWRITTEN)
> set_buffer_defer_completion(bh);
> bh->b_size = inode->i_sb->s_blocksize * map.m_len;
Honza
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