seek.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Copyright (C) 2017 Red Hat, Inc.
 * Copyright (c) 2018 Christoph Hellwig.
 */
#include <linux/module.h>
#include <linux/compiler.h>
#include <linux/fs.h>
#include <linux/iomap.h>
#include <linux/pagemap.h>
#include <linux/pagevec.h>

/*
 * Seek for SEEK_DATA / SEEK_HOLE within @page, starting at @lastoff.
 * Returns true if found and updates @lastoff to the offset in file.
 */
static bool
page_seek_hole_data(struct inode *inode, struct page *page, loff_t *lastoff,
		int whence)
{
	const struct address_space_operations *ops = inode->i_mapping->a_ops;
	unsigned int bsize = i_blocksize(inode), off;
	bool seek_data = whence == SEEK_DATA;
	loff_t poff = page_offset(page);

	if (WARN_ON_ONCE(*lastoff >= poff + PAGE_SIZE))
		return false;

	if (*lastoff < poff) {
		/*
		 * Last offset smaller than the start of the page means we found
		 * a hole:
		 */
		if (whence == SEEK_HOLE)
			return true;
		*lastoff = poff;
	}

	/*
	 * Just check the page unless we can and should check block ranges:
	 */
	if (bsize == PAGE_SIZE || !ops->is_partially_uptodate)
		return PageUptodate(page) == seek_data;

	lock_page(page);
	if (unlikely(page->mapping != inode->i_mapping))
		goto out_unlock_not_found;

	for (off = 0; off < PAGE_SIZE; off += bsize) {
		if (offset_in_page(*lastoff) >= off + bsize)
			continue;
		if (ops->is_partially_uptodate(page, off, bsize) == seek_data) {
			unlock_page(page);
			return true;
		}
		*lastoff = poff + off + bsize;
	}

out_unlock_not_found:
	unlock_page(page);
	return false;
}

/*
 * Seek for SEEK_DATA / SEEK_HOLE in the page cache.
 *
 * Within unwritten extents, the page cache determines which parts are holes
 * and which are data: uptodate buffer heads count as data; everything else
 * counts as a hole.
 *
 * Returns the resulting offset on successs, and -ENOENT otherwise.
 */
static loff_t
page_cache_seek_hole_data(struct inode *inode, loff_t offset, loff_t length,
		int whence)
{
	pgoff_t index = offset >> PAGE_SHIFT;
	pgoff_t end = DIV_ROUND_UP(offset + length, PAGE_SIZE);
	loff_t lastoff = offset;
	struct pagevec pvec;

	if (length <= 0)
		return -ENOENT;

	pagevec_init(&pvec);

	do {
		unsigned nr_pages, i;

		nr_pages = pagevec_lookup_range(&pvec, inode->i_mapping, &index,
						end - 1);
		if (nr_pages == 0)
			break;

		for (i = 0; i < nr_pages; i++) {
			struct page *page = pvec.pages[i];

			if (page_seek_hole_data(inode, page, &lastoff, whence))
				goto check_range;
			lastoff = page_offset(page) + PAGE_SIZE;
		}
		pagevec_release(&pvec);
	} while (index < end);

	/* When no page at lastoff and we are not done, we found a hole. */
	if (whence != SEEK_HOLE)
		goto not_found;

check_range:
	if (lastoff < offset + length)
		goto out;
not_found:
	lastoff = -ENOENT;
out:
	pagevec_release(&pvec);
	return lastoff;
}


static loff_t
iomap_seek_hole_actor(struct inode *inode, loff_t offset, loff_t length,
		      void *data, struct iomap *iomap, struct iomap *srcmap)
{
	switch (iomap->type) {
	case IOMAP_UNWRITTEN:
		offset = page_cache_seek_hole_data(inode, offset, length,
						   SEEK_HOLE);
		if (offset < 0)
			return length;
		fallthrough;
	case IOMAP_HOLE:
		*(loff_t *)data = offset;
		return 0;
	default:
		return length;
	}
}

loff_t
iomap_seek_hole(struct inode *inode, loff_t offset, const struct iomap_ops *ops)
{
	loff_t size = i_size_read(inode);
	loff_t length = size - offset;
	loff_t ret;

	/* Nothing to be found before or beyond the end of the file. */
	if (offset < 0 || offset >= size)
		return -ENXIO;

	while (length > 0) {
		ret = iomap_apply(inode, offset, length, IOMAP_REPORT, ops,
				  &offset, iomap_seek_hole_actor);
		if (ret < 0)
			return ret;
		if (ret == 0)
			break;

		offset += ret;
		length -= ret;
	}

	return offset;
}
EXPORT_SYMBOL_GPL(iomap_seek_hole);

static loff_t
iomap_seek_data_actor(struct inode *inode, loff_t offset, loff_t length,
		      void *data, struct iomap *iomap, struct iomap *srcmap)
{
	switch (iomap->type) {
	case IOMAP_HOLE:
		return length;
	case IOMAP_UNWRITTEN:
		offset = page_cache_seek_hole_data(inode, offset, length,
						   SEEK_DATA);
		if (offset < 0)
			return length;
		fallthrough;
	default:
		*(loff_t *)data = offset;
		return 0;
	}
}

loff_t
iomap_seek_data(struct inode *inode, loff_t offset, const struct iomap_ops *ops)
{
	loff_t size = i_size_read(inode);
	loff_t ret;

	/* Nothing to be found before or beyond the end of the file. */
	if (offset < 0 || offset >= size)
		return -ENXIO;

	while (offset < size) {
		ret = iomap_apply(inode, offset, size - offset, IOMAP_REPORT,
				  ops, &offset, iomap_seek_data_actor);
		if (ret < 0)
			return ret;
		if (ret == 0)
			return offset;
		offset += ret;
	}

	/* We've reached the end of the file without finding data */
	return -ENXIO;
}
EXPORT_SYMBOL_GPL(iomap_seek_data);