fs/zonefs/file.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Simple file system for zoned block devices exposing zones as files.
 *
 * Copyright (C) 2022 Western Digital Corporation or its affiliates.
 */
#include <linux/module.h>
#include <linux/pagemap.h>
#include <linux/iomap.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/statfs.h>
#include <linux/writeback.h>
#include <linux/quotaops.h>
#include <linux/seq_file.h>
#include <linux/parser.h>
#include <linux/uio.h>
#include <linux/mman.h>
#include <linux/sched/mm.h>
#include <linux/task_io_accounting_ops.h>

#include "zonefs.h"

#include "trace.h"

static int zonefs_read_iomap_begin(struct inode *inode, loff_t offset,
				   loff_t length, unsigned int flags,
				   struct iomap *iomap, struct iomap *srcmap)
{
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct super_block *sb = inode->i_sb;
	loff_t isize;

	/*
	 * All blocks are always mapped below EOF. If reading past EOF,
	 * act as if there is a hole up to the file maximum size.
	 */
	mutex_lock(&zi->i_truncate_mutex);
	iomap->bdev = inode->i_sb->s_bdev;
	iomap->offset = ALIGN_DOWN(offset, sb->s_blocksize);
	isize = i_size_read(inode);
	if (iomap->offset >= isize) {
		iomap->type = IOMAP_HOLE;
		iomap->addr = IOMAP_NULL_ADDR;
		iomap->length = length;
	} else {
		iomap->type = IOMAP_MAPPED;
		iomap->addr = (z->z_sector << SECTOR_SHIFT) + iomap->offset;
		iomap->length = isize - iomap->offset;
	}
	mutex_unlock(&zi->i_truncate_mutex);

	trace_zonefs_iomap_begin(inode, iomap);

	return 0;
}

static const struct iomap_ops zonefs_read_iomap_ops = {
	.iomap_begin	= zonefs_read_iomap_begin,
};

static int zonefs_write_iomap_begin(struct inode *inode, loff_t offset,
				    loff_t length, unsigned int flags,
				    struct iomap *iomap, struct iomap *srcmap)
{
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct super_block *sb = inode->i_sb;
	loff_t isize;

	/* All write I/Os should always be within the file maximum size */
	if (WARN_ON_ONCE(offset + length > z->z_capacity))
		return -EIO;

	/*
	 * Sequential zones can only accept direct writes. This is already
	 * checked when writes are issued, so warn if we see a page writeback
	 * operation.
	 */
	if (WARN_ON_ONCE(zonefs_zone_is_seq(z) && !(flags & IOMAP_DIRECT)))
		return -EIO;

	/*
	 * For conventional zones, all blocks are always mapped. For sequential
	 * zones, all blocks after always mapped below the inode size (zone
	 * write pointer) and unwriten beyond.
	 */
	mutex_lock(&zi->i_truncate_mutex);
	iomap->bdev = inode->i_sb->s_bdev;
	iomap->offset = ALIGN_DOWN(offset, sb->s_blocksize);
	iomap->addr = (z->z_sector << SECTOR_SHIFT) + iomap->offset;
	isize = i_size_read(inode);
	if (iomap->offset >= isize) {
		iomap->type = IOMAP_UNWRITTEN;
		iomap->length = z->z_capacity - iomap->offset;
	} else {
		iomap->type = IOMAP_MAPPED;
		iomap->length = isize - iomap->offset;
	}
	mutex_unlock(&zi->i_truncate_mutex);

	trace_zonefs_iomap_begin(inode, iomap);

	return 0;
}

static const struct iomap_ops zonefs_write_iomap_ops = {
	.iomap_begin	= zonefs_write_iomap_begin,
};

static int zonefs_read_folio(struct file *unused, struct folio *folio)
{
	return iomap_read_folio(folio, &zonefs_read_iomap_ops);
}

static void zonefs_readahead(struct readahead_control *rac)
{
	iomap_readahead(rac, &zonefs_read_iomap_ops);
}

/*
 * Map blocks for page writeback. This is used only on conventional zone files,
 * which implies that the page range can only be within the fixed inode size.
 */
static int zonefs_write_map_blocks(struct iomap_writepage_ctx *wpc,
				   struct inode *inode, loff_t offset)
{
	struct zonefs_zone *z = zonefs_inode_zone(inode);

	if (WARN_ON_ONCE(zonefs_zone_is_seq(z)))
		return -EIO;
	if (WARN_ON_ONCE(offset >= i_size_read(inode)))
		return -EIO;

	/* If the mapping is already OK, nothing needs to be done */
	if (offset >= wpc->iomap.offset &&
	    offset < wpc->iomap.offset + wpc->iomap.length)
		return 0;

	return zonefs_write_iomap_begin(inode, offset,
					z->z_capacity - offset,
					IOMAP_WRITE, &wpc->iomap, NULL);
}

static const struct iomap_writeback_ops zonefs_writeback_ops = {
	.map_blocks		= zonefs_write_map_blocks,
};

static int zonefs_writepages(struct address_space *mapping,
			     struct writeback_control *wbc)
{
	struct iomap_writepage_ctx wpc = { };

	return iomap_writepages(mapping, wbc, &wpc, &zonefs_writeback_ops);
}

static int zonefs_swap_activate(struct swap_info_struct *sis,
				struct file *swap_file, sector_t *span)
{
	struct inode *inode = file_inode(swap_file);

	if (zonefs_inode_is_seq(inode)) {
		zonefs_err(inode->i_sb,
			   "swap file: not a conventional zone file\n");
		return -EINVAL;
	}

	return iomap_swapfile_activate(sis, swap_file, span,
				       &zonefs_read_iomap_ops);
}

const struct address_space_operations zonefs_file_aops = {
	.read_folio		= zonefs_read_folio,
	.readahead		= zonefs_readahead,
	.writepages		= zonefs_writepages,
	.dirty_folio		= filemap_dirty_folio,
	.release_folio		= iomap_release_folio,
	.invalidate_folio	= iomap_invalidate_folio,
	.migrate_folio		= filemap_migrate_folio,
	.is_partially_uptodate	= iomap_is_partially_uptodate,
	.error_remove_page	= generic_error_remove_page,
	.direct_IO		= noop_direct_IO,
	.swap_activate		= zonefs_swap_activate,
};

int zonefs_file_truncate(struct inode *inode, loff_t isize)
{
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	loff_t old_isize;
	enum req_op op;
	int ret = 0;

	/*
	 * Only sequential zone files can be truncated and truncation is allowed
	 * only down to a 0 size, which is equivalent to a zone reset, and to
	 * the maximum file size, which is equivalent to a zone finish.
	 */
	if (!zonefs_zone_is_seq(z))
		return -EPERM;

	if (!isize)
		op = REQ_OP_ZONE_RESET;
	else if (isize == z->z_capacity)
		op = REQ_OP_ZONE_FINISH;
	else
		return -EPERM;

	inode_dio_wait(inode);

	/* Serialize against page faults */
	filemap_invalidate_lock(inode->i_mapping);

	/* Serialize against zonefs_iomap_begin() */
	mutex_lock(&zi->i_truncate_mutex);

	old_isize = i_size_read(inode);
	if (isize == old_isize)
		goto unlock;

	ret = zonefs_inode_zone_mgmt(inode, op);
	if (ret)
		goto unlock;

	/*
	 * If the mount option ZONEFS_MNTOPT_EXPLICIT_OPEN is set,
	 * take care of open zones.
	 */
	if (z->z_flags & ZONEFS_ZONE_OPEN) {
		/*
		 * Truncating a zone to EMPTY or FULL is the equivalent of
		 * closing the zone. For a truncation to 0, we need to
		 * re-open the zone to ensure new writes can be processed.
		 * For a truncation to the maximum file size, the zone is
		 * closed and writes cannot be accepted anymore, so clear
		 * the open flag.
		 */
		if (!isize)
			ret = zonefs_inode_zone_mgmt(inode, REQ_OP_ZONE_OPEN);
		else
			z->z_flags &= ~ZONEFS_ZONE_OPEN;
	}

	zonefs_update_stats(inode, isize);
	truncate_setsize(inode, isize);
	z->z_wpoffset = isize;
	zonefs_inode_account_active(inode);

unlock:
	mutex_unlock(&zi->i_truncate_mutex);
	filemap_invalidate_unlock(inode->i_mapping);

	return ret;
}

static int zonefs_file_fsync(struct file *file, loff_t start, loff_t end,
			     int datasync)
{
	struct inode *inode = file_inode(file);
	int ret = 0;

	if (unlikely(IS_IMMUTABLE(inode)))
		return -EPERM;

	/*
	 * Since only direct writes are allowed in sequential files, page cache
	 * flush is needed only for conventional zone files.
	 */
	if (zonefs_inode_is_cnv(inode))
		ret = file_write_and_wait_range(file, start, end);
	if (!ret)
		ret = blkdev_issue_flush(inode->i_sb->s_bdev);

	if (ret)
		zonefs_io_error(inode, true);

	return ret;
}

static vm_fault_t zonefs_filemap_page_mkwrite(struct vm_fault *vmf)
{
	struct inode *inode = file_inode(vmf->vma->vm_file);
	vm_fault_t ret;

	if (unlikely(IS_IMMUTABLE(inode)))
		return VM_FAULT_SIGBUS;

	/*
	 * Sanity check: only conventional zone files can have shared
	 * writeable mappings.
	 */
	if (zonefs_inode_is_seq(inode))
		return VM_FAULT_NOPAGE;

	sb_start_pagefault(inode->i_sb);
	file_update_time(vmf->vma->vm_file);

	/* Serialize against truncates */
	filemap_invalidate_lock_shared(inode->i_mapping);
	ret = iomap_page_mkwrite(vmf, &zonefs_write_iomap_ops);
	filemap_invalidate_unlock_shared(inode->i_mapping);

	sb_end_pagefault(inode->i_sb);
	return ret;
}

static const struct vm_operations_struct zonefs_file_vm_ops = {
	.fault		= filemap_fault,
	.map_pages	= filemap_map_pages,
	.page_mkwrite	= zonefs_filemap_page_mkwrite,
};

static int zonefs_file_mmap(struct file *file, struct vm_area_struct *vma)
{
	/*
	 * Conventional zones accept random writes, so their files can support
	 * shared writable mappings. For sequential zone files, only read
	 * mappings are possible since there are no guarantees for write
	 * ordering between msync() and page cache writeback.
	 */
	if (zonefs_inode_is_seq(file_inode(file)) &&
	    (vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
		return -EINVAL;

	file_accessed(file);
	vma->vm_ops = &zonefs_file_vm_ops;

	return 0;
}

static loff_t zonefs_file_llseek(struct file *file, loff_t offset, int whence)
{
	loff_t isize = i_size_read(file_inode(file));

	/*
	 * Seeks are limited to below the zone size for conventional zones
	 * and below the zone write pointer for sequential zones. In both
	 * cases, this limit is the inode size.
	 */
	return generic_file_llseek_size(file, offset, whence, isize, isize);
}

static int zonefs_file_write_dio_end_io(struct kiocb *iocb, ssize_t size,
					int error, unsigned int flags)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	struct zonefs_inode_info *zi = ZONEFS_I(inode);

	if (error) {
		zonefs_io_error(inode, true);
		return error;
	}

	if (size && zonefs_inode_is_seq(inode)) {
		/*
		 * Note that we may be seeing completions out of order,
		 * but that is not a problem since a write completed
		 * successfully necessarily means that all preceding writes
		 * were also successful. So we can safely increase the inode
		 * size to the write end location.
		 */
		mutex_lock(&zi->i_truncate_mutex);
		if (i_size_read(inode) < iocb->ki_pos + size) {
			zonefs_update_stats(inode, iocb->ki_pos + size);
			zonefs_i_size_write(inode, iocb->ki_pos + size);
		}
		mutex_unlock(&zi->i_truncate_mutex);
	}

	return 0;
}

static const struct iomap_dio_ops zonefs_write_dio_ops = {
	.end_io			= zonefs_file_write_dio_end_io,
};

static ssize_t zonefs_file_dio_append(struct kiocb *iocb, struct iov_iter *from)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct block_device *bdev = inode->i_sb->s_bdev;
	unsigned int max = bdev_max_zone_append_sectors(bdev);
	struct bio *bio;
	ssize_t size = 0;
	int nr_pages;
	ssize_t ret;

	max = ALIGN_DOWN(max << SECTOR_SHIFT, inode->i_sb->s_blocksize);
	iov_iter_truncate(from, max);

	nr_pages = iov_iter_npages(from, BIO_MAX_VECS);
	if (!nr_pages)
		return 0;

	bio = bio_alloc(bdev, nr_pages,
			REQ_OP_ZONE_APPEND | REQ_SYNC | REQ_IDLE, GFP_NOFS);
	bio->bi_iter.bi_sector = z->z_sector;
	bio->bi_ioprio = iocb->ki_ioprio;
	if (iocb_is_dsync(iocb))
		bio->bi_opf |= REQ_FUA;

	ret = bio_iov_iter_get_pages(bio, from);
	if (unlikely(ret))
		goto out_release;

	size = bio->bi_iter.bi_size;
	task_io_account_write(size);

	if (iocb->ki_flags & IOCB_HIPRI)
		bio_set_polled(bio, iocb);

	ret = submit_bio_wait(bio);

	/*
	 * If the file zone was written underneath the file system, the zone
	 * write pointer may not be where we expect it to be, but the zone
	 * append write can still succeed. So check manually that we wrote where
	 * we intended to, that is, at zi->i_wpoffset.
	 */
	if (!ret) {
		sector_t wpsector =
			z->z_sector + (z->z_wpoffset >> SECTOR_SHIFT);

		if (bio->bi_iter.bi_sector != wpsector) {
			zonefs_warn(inode->i_sb,
				"Corrupted write pointer %llu for zone at %llu\n",
				bio->bi_iter.bi_sector, z->z_sector);
			ret = -EIO;
		}
	}

	zonefs_file_write_dio_end_io(iocb, size, ret, 0);
	trace_zonefs_file_dio_append(inode, size, ret);

out_release:
	bio_release_pages(bio, false);
	bio_put(bio);

	if (ret >= 0) {
		iocb->ki_pos += size;
		return size;
	}

	return ret;
}

/*
 * Do not exceed the LFS limits nor the file zone size. If pos is under the
 * limit it becomes a short access. If it exceeds the limit, return -EFBIG.
 */
static loff_t zonefs_write_check_limits(struct file *file, loff_t pos,
					loff_t count)
{
	struct inode *inode = file_inode(file);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	loff_t limit = rlimit(RLIMIT_FSIZE);
	loff_t max_size = z->z_capacity;

	if (limit != RLIM_INFINITY) {
		if (pos >= limit) {
			send_sig(SIGXFSZ, current, 0);
			return -EFBIG;
		}
		count = min(count, limit - pos);
	}

	if (!(file->f_flags & O_LARGEFILE))
		max_size = min_t(loff_t, MAX_NON_LFS, max_size);

	if (unlikely(pos >= max_size))
		return -EFBIG;

	return min(count, max_size - pos);
}

static ssize_t zonefs_write_checks(struct kiocb *iocb, struct iov_iter *from)
{
	struct file *file = iocb->ki_filp;
	struct inode *inode = file_inode(file);
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	loff_t count;

	if (IS_SWAPFILE(inode))
		return -ETXTBSY;

	if (!iov_iter_count(from))
		return 0;

	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
		return -EINVAL;

	if (iocb->ki_flags & IOCB_APPEND) {
		if (zonefs_zone_is_cnv(z))
			return -EINVAL;
		mutex_lock(&zi->i_truncate_mutex);
		iocb->ki_pos = z->z_wpoffset;
		mutex_unlock(&zi->i_truncate_mutex);
	}

	count = zonefs_write_check_limits(file, iocb->ki_pos,
					  iov_iter_count(from));
	if (count < 0)
		return count;

	iov_iter_truncate(from, count);
	return iov_iter_count(from);
}

/*
 * Handle direct writes. For sequential zone files, this is the only possible
 * write path. For these files, check that the user is issuing writes
 * sequentially from the end of the file. This code assumes that the block layer
 * delivers write requests to the device in sequential order. This is always the
 * case if a block IO scheduler implementing the ELEVATOR_F_ZBD_SEQ_WRITE
 * elevator feature is being used (e.g. mq-deadline). The block layer always
 * automatically select such an elevator for zoned block devices during the
 * device initialization.
 */
static ssize_t zonefs_file_dio_write(struct kiocb *iocb, struct iov_iter *from)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct super_block *sb = inode->i_sb;
	bool sync = is_sync_kiocb(iocb);
	bool append = false;
	ssize_t ret, count;

	/*
	 * For async direct IOs to sequential zone files, refuse IOCB_NOWAIT
	 * as this can cause write reordering (e.g. the first aio gets EAGAIN
	 * on the inode lock but the second goes through but is now unaligned).
	 */
	if (zonefs_zone_is_seq(z) && !sync && (iocb->ki_flags & IOCB_NOWAIT))
		return -EOPNOTSUPP;

	if (iocb->ki_flags & IOCB_NOWAIT) {
		if (!inode_trylock(inode))
			return -EAGAIN;
	} else {
		inode_lock(inode);
	}

	count = zonefs_write_checks(iocb, from);
	if (count <= 0) {
		ret = count;
		goto inode_unlock;
	}

	if ((iocb->ki_pos | count) & (sb->s_blocksize - 1)) {
		ret = -EINVAL;
		goto inode_unlock;
	}

	/* Enforce sequential writes (append only) in sequential zones */
	if (zonefs_zone_is_seq(z)) {
		mutex_lock(&zi->i_truncate_mutex);
		if (iocb->ki_pos != z->z_wpoffset) {
			mutex_unlock(&zi->i_truncate_mutex);
			ret = -EINVAL;
			goto inode_unlock;
		}
		mutex_unlock(&zi->i_truncate_mutex);
		append = sync;
	}

	if (append)
		ret = zonefs_file_dio_append(iocb, from);
	else
		ret = iomap_dio_rw(iocb, from, &zonefs_write_iomap_ops,
				   &zonefs_write_dio_ops, 0, NULL, 0);
	if (zonefs_zone_is_seq(z) &&
	    (ret > 0 || ret == -EIOCBQUEUED)) {
		if (ret > 0)
			count = ret;

		/*
		 * Update the zone write pointer offset assuming the write
		 * operation succeeded. If it did not, the error recovery path
		 * will correct it. Also do active seq file accounting.
		 */
		mutex_lock(&zi->i_truncate_mutex);
		z->z_wpoffset += count;
		zonefs_inode_account_active(inode);
		mutex_unlock(&zi->i_truncate_mutex);
	}

inode_unlock:
	inode_unlock(inode);

	return ret;
}

static ssize_t zonefs_file_buffered_write(struct kiocb *iocb,
					  struct iov_iter *from)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	ssize_t ret;

	/*
	 * Direct IO writes are mandatory for sequential zone files so that the
	 * write IO issuing order is preserved.
	 */
	if (zonefs_inode_is_seq(inode))
		return -EIO;

	if (iocb->ki_flags & IOCB_NOWAIT) {
		if (!inode_trylock(inode))
			return -EAGAIN;
	} else {
		inode_lock(inode);
	}

	ret = zonefs_write_checks(iocb, from);
	if (ret <= 0)
		goto inode_unlock;

	ret = iomap_file_buffered_write(iocb, from, &zonefs_write_iomap_ops);
	if (ret > 0)
		iocb->ki_pos += ret;
	else if (ret == -EIO)
		zonefs_io_error(inode, true);

inode_unlock:
	inode_unlock(inode);
	if (ret > 0)
		ret = generic_write_sync(iocb, ret);

	return ret;
}

static ssize_t zonefs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	struct zonefs_zone *z = zonefs_inode_zone(inode);

	if (unlikely(IS_IMMUTABLE(inode)))
		return -EPERM;

	if (sb_rdonly(inode->i_sb))
		return -EROFS;

	/* Write operations beyond the zone capacity are not allowed */
	if (iocb->ki_pos >= z->z_capacity)
		return -EFBIG;

	if (iocb->ki_flags & IOCB_DIRECT) {
		ssize_t ret = zonefs_file_dio_write(iocb, from);

		if (ret != -ENOTBLK)
			return ret;
	}

	return zonefs_file_buffered_write(iocb, from);
}

static int zonefs_file_read_dio_end_io(struct kiocb *iocb, ssize_t size,
				       int error, unsigned int flags)
{
	if (error) {
		zonefs_io_error(file_inode(iocb->ki_filp), false);
		return error;
	}

	return 0;
}

static const struct iomap_dio_ops zonefs_read_dio_ops = {
	.end_io			= zonefs_file_read_dio_end_io,
};

static ssize_t zonefs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
{
	struct inode *inode = file_inode(iocb->ki_filp);
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct super_block *sb = inode->i_sb;
	loff_t isize;
	ssize_t ret;

	/* Offline zones cannot be read */
	if (unlikely(IS_IMMUTABLE(inode) && !(inode->i_mode & 0777)))
		return -EPERM;

	if (iocb->ki_pos >= z->z_capacity)
		return 0;

	if (iocb->ki_flags & IOCB_NOWAIT) {
		if (!inode_trylock_shared(inode))
			return -EAGAIN;
	} else {
		inode_lock_shared(inode);
	}

	/* Limit read operations to written data */
	mutex_lock(&zi->i_truncate_mutex);
	isize = i_size_read(inode);
	if (iocb->ki_pos >= isize) {
		mutex_unlock(&zi->i_truncate_mutex);
		ret = 0;
		goto inode_unlock;
	}
	iov_iter_truncate(to, isize - iocb->ki_pos);
	mutex_unlock(&zi->i_truncate_mutex);

	if (iocb->ki_flags & IOCB_DIRECT) {
		size_t count = iov_iter_count(to);

		if ((iocb->ki_pos | count) & (sb->s_blocksize - 1)) {
			ret = -EINVAL;
			goto inode_unlock;
		}
		file_accessed(iocb->ki_filp);
		ret = iomap_dio_rw(iocb, to, &zonefs_read_iomap_ops,
				   &zonefs_read_dio_ops, 0, NULL, 0);
	} else {
		ret = generic_file_read_iter(iocb, to);
		if (ret == -EIO)
			zonefs_io_error(inode, false);
	}

inode_unlock:
	inode_unlock_shared(inode);

	return ret;
}

/*
 * Write open accounting is done only for sequential files.
 */
static inline bool zonefs_seq_file_need_wro(struct inode *inode,
					    struct file *file)
{
	if (zonefs_inode_is_cnv(inode))
		return false;

	if (!(file->f_mode & FMODE_WRITE))
		return false;

	return true;
}

static int zonefs_seq_file_write_open(struct inode *inode)
{
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	int ret = 0;

	mutex_lock(&zi->i_truncate_mutex);

	if (!zi->i_wr_refcnt) {
		struct zonefs_sb_info *sbi = ZONEFS_SB(inode->i_sb);
		unsigned int wro = atomic_inc_return(&sbi->s_wro_seq_files);

		if (sbi->s_mount_opts & ZONEFS_MNTOPT_EXPLICIT_OPEN) {

			if (sbi->s_max_wro_seq_files
			    && wro > sbi->s_max_wro_seq_files) {
				atomic_dec(&sbi->s_wro_seq_files);
				ret = -EBUSY;
				goto unlock;
			}

			if (i_size_read(inode) < z->z_capacity) {
				ret = zonefs_inode_zone_mgmt(inode,
							     REQ_OP_ZONE_OPEN);
				if (ret) {
					atomic_dec(&sbi->s_wro_seq_files);
					goto unlock;
				}
				z->z_flags |= ZONEFS_ZONE_OPEN;
				zonefs_inode_account_active(inode);
			}
		}
	}

	zi->i_wr_refcnt++;

unlock:
	mutex_unlock(&zi->i_truncate_mutex);

	return ret;
}

static int zonefs_file_open(struct inode *inode, struct file *file)
{
	int ret;

	ret = generic_file_open(inode, file);
	if (ret)
		return ret;

	if (zonefs_seq_file_need_wro(inode, file))
		return zonefs_seq_file_write_open(inode);

	return 0;
}

static void zonefs_seq_file_write_close(struct inode *inode)
{
	struct zonefs_inode_info *zi = ZONEFS_I(inode);
	struct zonefs_zone *z = zonefs_inode_zone(inode);
	struct super_block *sb = inode->i_sb;
	struct zonefs_sb_info *sbi = ZONEFS_SB(sb);
	int ret = 0;

	mutex_lock(&zi->i_truncate_mutex);

	zi->i_wr_refcnt--;
	if (zi->i_wr_refcnt)
		goto unlock;

	/*
	 * The file zone may not be open anymore (e.g. the file was truncated to
	 * its maximum size or it was fully written). For this case, we only
	 * need to decrement the write open count.
	 */
	if (z->z_flags & ZONEFS_ZONE_OPEN) {
		ret = zonefs_inode_zone_mgmt(inode, REQ_OP_ZONE_CLOSE);
		if (ret) {
			__zonefs_io_error(inode, false);
			/*
			 * Leaving zones explicitly open may lead to a state
			 * where most zones cannot be written (zone resources
			 * exhausted). So take preventive action by remounting
			 * read-only.
			 */
			if (z->z_flags & ZONEFS_ZONE_OPEN &&
			    !(sb->s_flags & SB_RDONLY)) {
				zonefs_warn(sb,
					"closing zone at %llu failed %d\n",
					z->z_sector, ret);
				zonefs_warn(sb,
					"remounting filesystem read-only\n");
				sb->s_flags |= SB_RDONLY;
			}
			goto unlock;
		}

		z->z_flags &= ~ZONEFS_ZONE_OPEN;
		zonefs_inode_account_active(inode);
	}

	atomic_dec(&sbi->s_wro_seq_files);

unlock:
	mutex_unlock(&zi->i_truncate_mutex);
}

static int zonefs_file_release(struct inode *inode, struct file *file)
{
	/*
	 * If we explicitly open a zone we must close it again as well, but the
	 * zone management operation can fail (either due to an IO error or as
	 * the zone has gone offline or read-only). Make sure we don't fail the
	 * close(2) for user-space.
	 */
	if (zonefs_seq_file_need_wro(inode, file))
		zonefs_seq_file_write_close(inode);

	return 0;
}

const struct file_operations zonefs_file_operations = {
	.open		= zonefs_file_open,
	.release	= zonefs_file_release,
	.fsync		= zonefs_file_fsync,
	.mmap		= zonefs_file_mmap,
	.llseek		= zonefs_file_llseek,
	.read_iter	= zonefs_file_read_iter,
	.write_iter	= zonefs_file_write_iter,
	.splice_read	= generic_file_splice_read,
	.splice_write	= iter_file_splice_write,
	.iopoll		= iocb_bio_iopoll,
};