Re: [PATCH v10 08/14] btrfs: add BTRFS_IOC_ENCODED_READ

Nikolay Borisov
2021-08-20T12:30:08Z

On 18.08.21 г. 0:06, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
>
> There are 4 main cases:
>
> 1. Inline extents: we copy the data straight out of the extent buffer.
> 2. Hole/preallocated extents: we fill in zeroes.
> 3. Regular, uncompressed extents: we read the sectors we need directly
> from disk.
> 4. Regular, compressed extents: we read the entire compressed extent
> from disk and indicate what subset of the decompressed extent is in
> the file.
>
> This initial implementation simplifies a few things that can be improved
> in the future:
>
> - We hold the inode lock during the operation.
> - Cases 1, 3, and 4 allocate temporary memory to read into before
> copying out to userspace.
> - We don't do read repair, because it turns out that read repair is
> currently broken for compressed data.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> fs/btrfs/ctree.h | 4 +
> fs/btrfs/inode.c | 489 +++++++++++++++++++++++++++++++++++++++++++++++
> fs/btrfs/ioctl.c | 111 +++++++++++
> 3 files changed, 604 insertions(+)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 973616d80080..b68d8ea42f6e 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3219,6 +3219,10 @@ int btrfs_writepage_cow_fixup(struct page *page);
> void btrfs_writepage_endio_finish_ordered(struct btrfs_inode *inode,
> struct page *page, u64 start,
> u64 end, bool uptodate);
> +struct btrfs_ioctl_encoded_io_args;
> +ssize_t btrfs_encoded_read(struct kiocb *iocb, struct iov_iter *iter,
> + struct btrfs_ioctl_encoded_io_args *encoded);
> +
> extern const struct dentry_operations btrfs_dentry_operations;
> extern const struct iomap_ops btrfs_dio_iomap_ops;
> extern const struct iomap_dio_ops btrfs_dio_ops;
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 0b5ff14aa7fd..6d7fae859fb5 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -10496,6 +10496,495 @@ void btrfs_set_range_writeback(struct btrfs_inode *inode, u64 start, u64 end)
> }
> }
>
> +static int btrfs_encoded_io_compression_from_extent(int compress_type)
> +{
> + switch (compress_type) {
> + case BTRFS_COMPRESS_NONE:
> + return BTRFS_ENCODED_IO_COMPRESSION_NONE;
> + case BTRFS_COMPRESS_ZLIB:
> + return BTRFS_ENCODED_IO_COMPRESSION_ZLIB;
> + case BTRFS_COMPRESS_LZO:
> + /*
> + * The LZO format depends on the page size. 64k is the maximum
> + * sectorsize (and thus page size) that we support.
> + */
> + if (PAGE_SIZE < SZ_4K || PAGE_SIZE > SZ_64K)
> + return -EINVAL;
> + return BTRFS_ENCODED_IO_COMPRESSION_LZO_4K + (PAGE_SHIFT - 12);
> + case BTRFS_COMPRESS_ZSTD:
> + return BTRFS_ENCODED_IO_COMPRESSION_ZSTD;
> + default:
> + return -EUCLEAN;
> + }
> +}
> +
> +static ssize_t btrfs_encoded_read_inline(
> + struct kiocb *iocb,
> + struct iov_iter *iter, u64 start,
> + u64 lockend,
> + struct extent_state **cached_state,
> + u64 extent_start, size_t count,
> + struct btrfs_ioctl_encoded_io_args *encoded,
> + bool *unlocked)
> +{
> + struct inode *inode = file_inode(iocb->ki_filp);
> + struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
> + struct btrfs_path *path;
> + struct extent_buffer *leaf;
> + struct btrfs_file_extent_item *item;
> + u64 ram_bytes;
> + unsigned long ptr;
> + void *tmp;
> + ssize_t ret;
> +
> + path = btrfs_alloc_path();
> + if (!path) {
> + ret = -ENOMEM;
> + goto out;
> + }
> + ret = btrfs_lookup_file_extent(NULL, BTRFS_I(inode)->root, path,
> + btrfs_ino(BTRFS_I(inode)), extent_start,
> + 0);
> + if (ret) {
> + if (ret > 0) {
> + /* The extent item disappeared? */
> + ret = -EIO;
> + }
> + goto out;
> + }
> + leaf = path->nodes[0];
> + item = btrfs_item_ptr(leaf, path->slots[0],
> + struct btrfs_file_extent_item);
> +
> + ram_bytes = btrfs_file_extent_ram_bytes(leaf, item);
> + ptr = btrfs_file_extent_inline_start(item);
> +
> + encoded->len = (min_t(u64, extent_start + ram_bytes, inode->i_size) -
> + iocb->ki_pos);
> + ret = btrfs_encoded_io_compression_from_extent(
> + btrfs_file_extent_compression(leaf, item));
> + if (ret < 0)
> + goto out;
> + encoded->compression = ret;
> + if (encoded->compression) {
> + size_t inline_size;
> +
> + inline_size = btrfs_file_extent_inline_item_len(leaf,
> + btrfs_item_nr(path->slots[0]));
> + if (inline_size > count) {
> + ret = -ENOBUFS;
> + goto out;
> + }
> + count = inline_size;
> + encoded->unencoded_len = ram_bytes;
> + encoded->unencoded_offset = iocb->ki_pos - extent_start;
> + } else {
> + encoded->len = encoded->unencoded_len = count =
> + min_t(u64, count, encoded->len);
> + ptr += iocb->ki_pos - extent_start;
> + }
> +
> + tmp = kmalloc(count, GFP_NOFS);
> + if (!tmp) {
> + ret = -ENOMEM;
> + goto out;
> + }
> + read_extent_buffer(leaf, tmp, ptr, count);
> + btrfs_release_path(path);
> + unlock_extent_cached(io_tree, start, lockend, cached_state);
> + inode_unlock_shared(inode);
> + *unlocked = true;
> +
> + ret = copy_to_iter(tmp, count, iter);
> + if (ret != count)
> + ret = -EFAULT;
> + kfree(tmp);
> +out:
> + btrfs_free_path(path);
> + return ret;
> +}
> +
> +struct btrfs_encoded_read_private {
> + struct inode *inode;
> + wait_queue_head_t wait;
> + atomic_t pending;
> + blk_status_t status;
> + bool skip_csum;
> +};
> +
> +static blk_status_t submit_encoded_read_bio(struct inode *inode,
> + struct bio *bio, int mirror_num,
> + unsigned long bio_flags)
> +{
> + struct btrfs_encoded_read_private *priv = bio->bi_private;
> + struct btrfs_io_bio *io_bio = btrfs_io_bio(bio);
> + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> + blk_status_t ret;
> +
> + if (!priv->skip_csum) {
> + ret = btrfs_lookup_bio_sums(inode, bio, NULL);
> + if (ret)
> + return ret;
> + }
> +
> + ret = btrfs_bio_wq_end_io(fs_info, bio, BTRFS_WQ_ENDIO_DATA);
> + if (ret) {
> + btrfs_io_bio_free_csum(io_bio);
> + return ret;
> + }
> +
> + atomic_inc(&priv->pending);
> + ret = btrfs_map_bio(fs_info, bio, mirror_num);
> + if (ret) {
> + atomic_dec(&priv->pending);
> + btrfs_io_bio_free_csum(io_bio);
> + }
> + return ret;
> +}
> +
> +static blk_status_t btrfs_encoded_read_check_bio(struct btrfs_io_bio *io_bio)
> +{
> + const bool uptodate = io_bio->bio.bi_status == BLK_STS_OK;
> + struct btrfs_encoded_read_private *priv = io_bio->bio.bi_private;
> + struct inode *inode = priv->inode;
> + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> + u32 sectorsize = fs_info->sectorsize;
> + struct bio_vec *bvec;
> + struct bvec_iter_all iter_all;
> + u64 start = io_bio->logical;
> + u32 bio_offset = 0;
> +
> + if (priv->skip_csum || !uptodate)
> + return io_bio->bio.bi_status;
> +
> + bio_for_each_segment_all(bvec, &io_bio->bio, iter_all) {
> + unsigned int i, nr_sectors, pgoff;
> +
> + nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info, bvec->bv_len);
> + pgoff = bvec->bv_offset;
> + for (i = 0; i < nr_sectors; i++) {
> + ASSERT(pgoff < PAGE_SIZE);
> + if (check_data_csum(inode, io_bio, bio_offset,
> + bvec->bv_page, pgoff, start))
> + return BLK_STS_IOERR;
> + start += sectorsize;
> + bio_offset += sectorsize;
> + pgoff += sectorsize;
> + }
> + }
> + return BLK_STS_OK;
> +}
> +
> +static void btrfs_encoded_read_endio(struct bio *bio)
> +{
> + struct btrfs_encoded_read_private *priv = bio->bi_private;
> + struct btrfs_io_bio *io_bio = btrfs_io_bio(bio);
> + blk_status_t status;
> +
> + status = btrfs_encoded_read_check_bio(io_bio);
> + if (status) {
> + /*
> + * The memory barrier implied by the atomic_dec_return() here
> + * pairs with the memory barrier implied by the
> + * atomic_dec_return() or io_wait_event() in
> + * btrfs_encoded_read_regular_fill_pages() to ensure that this
> + * write is observed before the load of status in
> + * btrfs_encoded_read_regular_fill_pages().
> + */
> + WRITE_ONCE(priv->status, status);
> + }
> + if (!atomic_dec_return(&priv->pending))
> + wake_up(&priv->wait);
> + btrfs_io_bio_free_csum(io_bio);
> + bio_put(bio);
> +}
> +
> +static int btrfs_encoded_read_regular_fill_pages(struct inode *inode, u64 offset,
> + u64 disk_io_size, struct page **pages)
> +{
> + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> + struct btrfs_encoded_read_private priv = {
> + .inode = inode,
> + .pending = ATOMIC_INIT(1),
> + .skip_csum = BTRFS_I(inode)->flags & BTRFS_INODE_NODATASUM,
> + };
> + unsigned long i = 0;
> + u64 cur = 0;
> + int ret;
> +
> + init_waitqueue_head(&priv.wait);
> + /*
> + * Submit bios for the extent, splitting due to bio or stripe limits as
> + * necessary.
> + */
> + while (cur < disk_io_size) {
> + struct extent_map *em;
> + struct btrfs_io_geometry geom;
> + struct bio *bio = NULL;
> + u64 remaining;
> +
> + em = btrfs_get_chunk_map(fs_info, offset + cur,
> + disk_io_size - cur);
> + if (IS_ERR(em)) {
> + ret = PTR_ERR(em);
> + } else {
> + ret = btrfs_get_io_geometry(fs_info, em, BTRFS_MAP_READ,
> + offset + cur, &geom);
> + free_extent_map(em);
> + }
> + if (ret) {
> + WRITE_ONCE(priv.status, errno_to_blk_status(ret));
> + break;
> + }
> + remaining = min(geom.len, disk_io_size - cur);
> + while (bio || remaining) {
> + size_t bytes = min_t(u64, remaining, PAGE_SIZE);
> +
> + if (!bio) {
> + bio = btrfs_bio_alloc(offset + cur);
> + bio->bi_end_io = btrfs_encoded_read_endio;
> + bio->bi_private = &priv;
> + bio->bi_opf = REQ_OP_READ;
> + }
> +
> + if (!bytes ||
> + bio_add_page(bio, pages[i], bytes, 0) < bytes) {
> + blk_status_t status;
> +
> + status = submit_encoded_read_bio(inode, bio, 0,
> + 0);
> + if (status) {
> + WRITE_ONCE(priv.status, status);
> + bio_put(bio);
> + goto out;
> + }
> + bio = NULL;
> + continue;
> + }
> +
> + i++;
> + cur += bytes;
> + remaining -= bytes;
> + }
> + }
> +
> +out:
> + if (atomic_dec_return(&priv.pending))
> + io_wait_event(priv.wait, !atomic_read(&priv.pending));
> + /* See btrfs_encoded_read_endio() for ordering. */
> + return blk_status_to_errno(READ_ONCE(priv.status));
> +}
> +
> +static ssize_t btrfs_encoded_read_regular(struct kiocb *iocb,
> + struct iov_iter *iter,
> + u64 start, u64 lockend,
> + struct extent_state **cached_state,
> + u64 offset, u64 disk_io_size,
> + size_t count, bool compressed,
> + bool *unlocked)
> +{
> + struct inode *inode = file_inode(iocb->ki_filp);
> + struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
> + struct page **pages;
> + unsigned long nr_pages, i;
> + u64 cur;
> + size_t page_offset;
> + ssize_t ret;
> +
> + nr_pages = DIV_ROUND_UP(disk_io_size, PAGE_SIZE);
> + pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
> + if (!pages)
> + return -ENOMEM;
> + for (i = 0; i < nr_pages; i++) {
> + pages[i] = alloc_page(GFP_NOFS | __GFP_HIGHMEM);
> + if (!pages[i]) {
> + ret = -ENOMEM;
> + goto out;
> + }
> + }
> +
> + ret = btrfs_encoded_read_regular_fill_pages(inode, offset, disk_io_size,
> + pages);
> + if (ret)
> + goto out;
> +
> + unlock_extent_cached(io_tree, start, lockend, cached_state);
> + inode_unlock_shared(inode);
> + *unlocked = true;
> +
> + if (compressed) {
> + i = 0;
> + page_offset = 0;
> + } else {
> + i = (iocb->ki_pos - start) >> PAGE_SHIFT;
> + page_offset = (iocb->ki_pos - start) & (PAGE_SIZE - 1);
> + }
> + cur = 0;
> + while (cur < count) {
> + size_t bytes = min_t(size_t, count - cur,
> + PAGE_SIZE - page_offset);
> +
> + if (copy_page_to_iter(pages[i], page_offset, bytes,
> + iter) != bytes) {
> + ret = -EFAULT;
> + goto out;
> + }
> + i++;
> + cur += bytes;
> + page_offset = 0;
> + }
> + ret = count;
> +out:
> + for (i = 0; i < nr_pages; i++) {
> + if (pages[i])
> + __free_page(pages[i]);
> + }
> + kfree(pages);
> + return ret;
> +}
> +
> +ssize_t btrfs_encoded_read(struct kiocb *iocb, struct iov_iter *iter,
> + struct btrfs_ioctl_encoded_io_args *encoded)
> +{
> + struct inode *inode = file_inode(iocb->ki_filp);
> + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
> + struct extent_io_tree *io_tree = &BTRFS_I(inode)->io_tree;
> + ssize_t ret;
> + size_t count = iov_iter_count(iter);
> + u64 start, lockend, offset, disk_io_size;
> + struct extent_state *cached_state = NULL;
> + struct extent_map *em;
> + bool unlocked = false;
> +
> + file_accessed(iocb->ki_filp);
> +
> + inode_lock_shared(inode);
> +
> + if (iocb->ki_pos >= inode->i_size) {
> + inode_unlock_shared(inode);
> + return 0;

Don't we need to signal beyond EOF reads somehow. As it stands returning
0 means returning zeroed portion of btrfs_ioctl_encoded_io_args to user
space?

> + }
> + start = ALIGN_DOWN(iocb->ki_pos, fs_info->sectorsize);
> + /*
> + * We don't know how long the extent containing iocb->ki_pos is, but if
> + * it's compressed we know that it won't be longer than this.
> + */
> + lockend = start + BTRFS_MAX_UNCOMPRESSED - 1;
> +
> + for (;;) {
> + struct btrfs_ordered_extent *ordered;
> +
> + ret = btrfs_wait_ordered_range(inode, start,
> + lockend - start + 1);
> + if (ret)
> + goto out_unlock_inode;
> + lock_extent_bits(io_tree, start, lockend, &cached_state);
> + ordered = btrfs_lookup_ordered_range(BTRFS_I(inode), start,
> + lockend - start + 1);
> + if (!ordered)
> + break;
> + btrfs_put_ordered_extent(ordered);
> + unlock_extent_cached(io_tree, start, lockend, &cached_state);
> + cond_resched();
> + }
Can't you simply use btrfs_lock_and_flush_ordered_range, the major
difference is btrfs_wait_ordered_range basically instantiates any
pending delalloc, whilst btrfs_lock_and_flush_ordered_range returns with
any, already-instantiated OE run to completion and the range locked ?

> +
> + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, start,
> + lockend - start + 1);
> + if (IS_ERR(em)) {
> + ret = PTR_ERR(em);
> + goto out_unlock_extent;
> + }
> +
> + if (em->block_start == EXTENT_MAP_INLINE) {
> + u64 extent_start = em->start;
> +
> + /*
> + * For inline extents we get everything we need out of the
> + * extent item.
> + */
> + free_extent_map(em);
> + em = NULL;
> + ret = btrfs_encoded_read_inline(iocb, iter, start, lockend,
> + &cached_state, extent_start,
> + count, encoded, &unlocked);
> + goto out;
> + }
> +
> + /*
> + * We only want to return up to EOF even if the extent extends beyond
> + * that.
> + */
> + encoded->len = (min_t(u64, extent_map_end(em), inode->i_size) -
> + iocb->ki_pos);
> + if (em->block_start == EXTENT_MAP_HOLE ||
> + test_bit(EXTENT_FLAG_PREALLOC, &em->flags)) {
> + offset = EXTENT_MAP_HOLE;
> + encoded->len = encoded->unencoded_len = count =
> + min_t(u64, count, encoded->len);
> + } else if (test_bit(EXTENT_FLAG_COMPRESSED, &em->flags)) {
> + offset = em->block_start;
> + /*
> + * Bail if the buffer isn't large enough to return the whole
> + * compressed extent.
> + */
> + if (em->block_len > count) {
> + ret = -ENOBUFS;
> + goto out_em;
> + }
> + disk_io_size = count = em->block_len;
> + encoded->unencoded_len = em->ram_bytes;
> + encoded->unencoded_offset = iocb->ki_pos - em->orig_start;
> + ret = btrfs_encoded_io_compression_from_extent(
> + em->compress_type);
> + if (ret < 0)
> + goto out_em;
> + encoded->compression = ret;
> + } else {
> + offset = em->block_start + (start - em->start);
> + if (encoded->len > count)
> + encoded->len = count;
> + /*
> + * Don't read beyond what we locked. This also limits the page
> + * allocations that we'll do.
> + */
> + disk_io_size = min(lockend + 1,
> + iocb->ki_pos + encoded->len) - start;
> + encoded->len = encoded->unencoded_len = count =
> + start + disk_io_size - iocb->ki_pos;
> + disk_io_size = ALIGN(disk_io_size, fs_info->sectorsize);
> + }
> + free_extent_map(em);
> + em = NULL;
> +
> + if (offset == EXTENT_MAP_HOLE) {
> + unlock_extent_cached(io_tree, start, lockend, &cached_state);
> + inode_unlock_shared(inode);
> + unlocked = true;
> + ret = iov_iter_zero(count, iter);
> + if (ret != count)
> + ret = -EFAULT;
> + } else {
> + ret = btrfs_encoded_read_regular(iocb, iter, start, lockend,
> + &cached_state, offset,
> + disk_io_size, count,
> + encoded->compression,
> + &unlocked);
> + }
> +
> +out:
> + if (ret >= 0)
> + iocb->ki_pos += encoded->len;
> +out_em:
> + free_extent_map(em);
> +out_unlock_extent:
> + if (!unlocked)
> + unlock_extent_cached(io_tree, start, lockend, &cached_state);
> +out_unlock_inode:
> + if (!unlocked)
> + inode_unlock_shared(inode);
> + return ret;
> +}
> +
> #ifdef CONFIG_SWAP
> /*
> * Add an entry indicating a block group or device which is pinned by a

<snip>

> /* Mask out flags that are inappropriate for the given type of inode. */
> @@ -4887,6 +4904,94 @@ static int _btrfs_ioctl_send(struct file *file, void __user *argp, bool compat)
> return ret;
> }
>
> +static int btrfs_ioctl_encoded_read(struct file *file, void __user *argp,
> + bool compat)
> +{
> + struct btrfs_ioctl_encoded_io_args args;

nit: If you make this args = {}; you guarantee it's zeroed out then you
can get rid of the ugly memset further down.

> + size_t copy_end_kernel = offsetofend(struct btrfs_ioctl_encoded_io_args,
> + flags);
> + size_t copy_end;
> + struct iovec iovstack[UIO_FASTIOV];
> + struct iovec *iov = iovstack;
> + struct iov_iter iter;
> + loff_t pos;
> + struct kiocb kiocb;
> + ssize_t ret;
> +
> + if (!capable(CAP_SYS_ADMIN)) {
> + ret = -EPERM;
> + goto out_acct;
> + }
> +
> + if (compat) {
> +#if defined(CONFIG_64BIT) && defined(CONFIG_COMPAT)
> + struct btrfs_ioctl_encoded_io_args_32 args32;
> +
> + copy_end = offsetofend(struct btrfs_ioctl_encoded_io_args_32,
> + flags);
> + if (copy_from_user(&args32, argp, copy_end)) {
> + ret = -EFAULT;
> + goto out_acct;
> + }
> + args.iov = compat_ptr(args32.iov);
> + args.iovcnt = args32.iovcnt;
> + args.offset = args32.offset;
> + args.flags = args32.flags;
> +#else
> + return -ENOTTY;
> +#endif
> + } else {
> + copy_end = copy_end_kernel;
> + if (copy_from_user(&args, argp, copy_end)) {
> + ret = -EFAULT;
> + goto out_acct;
> + }
> + }
> + if (args.flags != 0) {
> + ret = -EINVAL;
> + goto out_acct;
> + }
> + memset((char *)&args + copy_end_kernel, 0,
> + sizeof(args) - copy_end_kernel);
> +
> + ret = import_iovec(READ, args.iov, args.iovcnt, ARRAY_SIZE(iovstack),
> + &iov, &iter);
> + if (ret < 0)
> + goto out_acct;
> +
> + if (iov_iter_count(&iter) == 0) {
> + ret = 0;
> + goto out_iov;
> + }
> + pos = args.offset;
> + ret = rw_verify_area(READ, file, &pos, args.len);
> + if (ret < 0)
> + goto out_iov;
> +
> + init_sync_kiocb(&kiocb, file);
> + ret = kiocb_set_rw_flags(&kiocb, 0);
> + if (ret)
> + goto out_iov;
> + kiocb.ki_pos = pos;
> +
> + ret = btrfs_encoded_read(&kiocb, &iter, &args);
> + if (ret >= 0) {
> + fsnotify_access(file);
> + if (copy_to_user(argp + copy_end,
> + (char *)&args + copy_end_kernel,
> + sizeof(args) - copy_end_kernel))
> + ret = -EFAULT;
> + }
> +
> +out_iov:
> + kfree(iov);
> +out_acct:
> + if (ret > 0)
> + add_rchar(current, ret);
> + inc_syscr(current);
> + return ret;
> +}
> +
> long btrfs_ioctl(struct file *file, unsigned int
> cmd, unsigned long arg)
> {