fs/erofs/decompressor.c - LeafOS-Devices/android_kernel_samsung_gta4xl - Gitiles

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * Copyright (C) 2019 HUAWEI, Inc.
  *             https://www.huawei.com/
  */
 #include "compress.h"
 #include <linux/module.h>
 #include <linux/lz4.h>

 #ifndef LZ4_DISTANCE_MAX	/* history window size */
 #define LZ4_DISTANCE_MAX 65535	/* set to maximum value by default */
 #endif

 #define LZ4_MAX_DISTANCE_PAGES	(DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
 #ifndef LZ4_DECOMPRESS_INPLACE_MARGIN
 #define LZ4_DECOMPRESS_INPLACE_MARGIN(srcsize)  (((srcsize) >> 8) + 32)
 #endif

 struct z_erofs_decompressor {
 	/*
 	 * if destpages have sparsed pages, fill them with bounce pages.
 	 * it also check whether destpages indicate continuous physical memory.
 	 */
 	int (*prepare_destpages)(struct z_erofs_decompress_req *rq,
 				 struct list_head *pagepool);
 	int (*decompress)(struct z_erofs_decompress_req *rq, u8 *dst);
 	char *name;
 };

 int z_erofs_load_lz4_config(struct super_block *sb,
 			    struct erofs_super_block *dsb,
 			    struct z_erofs_lz4_cfgs *lz4, int size)
 {
 	struct erofs_sb_info *sbi = EROFS_SB(sb);
 	u16 distance;

 	if (lz4) {
 		if (size < sizeof(struct z_erofs_lz4_cfgs)) {
 			erofs_err(sb, "invalid lz4 cfgs, size=%u", size);
 			return -EINVAL;
 		}
 		distance = le16_to_cpu(lz4->max_distance);

 		sbi->lz4.max_pclusterblks = le16_to_cpu(lz4->max_pclusterblks);
 		if (!sbi->lz4.max_pclusterblks) {
 			sbi->lz4.max_pclusterblks = 1;	/* reserved case */
 		} else if (sbi->lz4.max_pclusterblks >
 			   Z_EROFS_PCLUSTER_MAX_SIZE / EROFS_BLKSIZ) {
 			erofs_err(sb, "too large lz4 pclusterblks %u",
 				  sbi->lz4.max_pclusterblks);
 			return -EINVAL;
 		} else if (sbi->lz4.max_pclusterblks >= 2) {
 			erofs_info(sb, "EXPERIMENTAL big pcluster feature in use. Use at your own risk!");
 		}
 	} else {
 		distance = le16_to_cpu(dsb->u1.lz4_max_distance);
 		sbi->lz4.max_pclusterblks = 1;
 	}

 	sbi->lz4.max_distance_pages = distance ?
 					DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :
 					LZ4_MAX_DISTANCE_PAGES;
 	return erofs_pcpubuf_growsize(sbi->lz4.max_pclusterblks);
 }

 static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
 					 struct list_head *pagepool)
 {
 	const unsigned int nr =
 		PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
 	struct page *availables[LZ4_MAX_DISTANCE_PAGES] = { NULL };
 	unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
 					   BITS_PER_LONG)] = { 0 };
 	unsigned int lz4_max_distance_pages =
 				EROFS_SB(rq->sb)->lz4.max_distance_pages;
 	void *kaddr = NULL;
 	unsigned int i, j, top;

 	top = 0;
 	for (i = j = 0; i < nr; ++i, ++j) {
 		struct page *const page = rq->out[i];
 		struct page *victim;

 		if (j >= lz4_max_distance_pages)
 			j = 0;

 		/* 'valid' bounced can only be tested after a complete round */
 		if (test_bit(j, bounced)) {
 			DBG_BUGON(i < lz4_max_distance_pages);
 			DBG_BUGON(top >= lz4_max_distance_pages);
 			availables[top++] = rq->out[i - lz4_max_distance_pages];
 		}

 		if (page) {
 			__clear_bit(j, bounced);
 			if (!PageHighMem(page)) {
 				if (!i) {
 					kaddr = page_address(page);
 					continue;
 				}
 				if (kaddr &&
 				    kaddr + PAGE_SIZE == page_address(page)) {
 					kaddr += PAGE_SIZE;
 					continue;
 				}
 			}
 			kaddr = NULL;
 			continue;
 		}
 		kaddr = NULL;
 		__set_bit(j, bounced);

 		if (top) {
 			victim = availables[--top];
 			get_page(victim);
 		} else {
 			victim = erofs_allocpage(pagepool,
 						 GFP_KERNEL | __GFP_NOFAIL);
 			set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
 		}
 		rq->out[i] = victim;
 	}
 	return kaddr ? 1 : 0;
 }

 static void *z_erofs_handle_inplace_io(struct z_erofs_decompress_req *rq,
 			void *inpage, void *out, unsigned int *inputmargin,
 			int *maptype, bool support_0padding)
 {
 	unsigned int nrpages_in, nrpages_out;
 	unsigned int ofull, oend, inputsize, total, i;
 	struct page **in;
 	void *src, *tmp;

 	inputsize = rq->inputsize;
 	nrpages_in = PAGE_ALIGN(inputsize) >> PAGE_SHIFT;
 	oend = rq->pageofs_out + rq->outputsize;
 	ofull = PAGE_ALIGN(oend);
 	nrpages_out = ofull >> PAGE_SHIFT;

 	if (rq->inplace_io) {
 		if (rq->partial_decoding || !support_0padding ||
 		    ofull - oend < LZ4_DECOMPRESS_INPLACE_MARGIN(inputsize))
 			goto docopy;

 		for (i = 0; i < nrpages_in; ++i)
 			if (rq->out[nrpages_out - nrpages_in + i] !=
 			    rq->in[i])
 				goto docopy;
 		kunmap_atomic(inpage);
 		*maptype = 3;
 		return out + ((nrpages_out - nrpages_in) << PAGE_SHIFT);
 	}

 	if (nrpages_in <= 1) {
 		*maptype = 0;
 		return inpage;
 	}
 	kunmap_atomic(inpage);
 	src = erofs_vm_map_ram(rq->in, nrpages_in);
 	if (!src)
 		return ERR_PTR(-ENOMEM);
 	*maptype = 1;
 	return src;

 docopy:
 	/* Or copy compressed data which can be overlapped to per-CPU buffer */
 	in = rq->in;
 	src = erofs_get_pcpubuf(nrpages_in);
 	if (!src) {
 		DBG_BUGON(1);
 		kunmap_atomic(inpage);
 		return ERR_PTR(-EFAULT);
 	}

 	tmp = src;
 	total = rq->inputsize;
 	while (total) {
 		unsigned int page_copycnt =
 			min_t(unsigned int, total, PAGE_SIZE - *inputmargin);

 		if (!inpage)
 			inpage = kmap_atomic(*in);
 		memcpy(tmp, inpage + *inputmargin, page_copycnt);
 		kunmap_atomic(inpage);
 		inpage = NULL;
 		tmp += page_copycnt;
 		total -= page_copycnt;
 		++in;
 		*inputmargin = 0;
 	}
 	*maptype = 2;
 	return src;
 }

 static int z_erofs_lz4_decompress(struct z_erofs_decompress_req *rq, u8 *dst)
 {
 	unsigned int inputmargin;
 	u8 *out, *headpage, *src;
 	bool support_0padding;
 	int ret, maptype;

 	DBG_BUGON(*rq->in == NULL);
 	headpage = kmap_atomic(*rq->in);
 	inputmargin = 0;
 	support_0padding = false;

 	/* decompression inplace is only safe when 0padding is enabled */
 	if (erofs_sb_has_lz4_0padding(EROFS_SB(rq->sb))) {
 		support_0padding = true;

 		while (!headpage[inputmargin & ~PAGE_MASK])
 			if (!(++inputmargin & ~PAGE_MASK))
 				break;

 		if (inputmargin >= rq->inputsize) {
 			kunmap_atomic(headpage);
 			return -EIO;
 		}
 	}

 	rq->inputsize -= inputmargin;
 	src = z_erofs_handle_inplace_io(rq, headpage, dst, &inputmargin,
 					&maptype, support_0padding);
 	if (IS_ERR(src))
 		return PTR_ERR(src);

 	out = dst + rq->pageofs_out;
 	/* legacy format could compress extra data in a pcluster. */
 	if (rq->partial_decoding || !support_0padding)
 		ret = LZ4_decompress_safe_partial(src + inputmargin, out,
 				rq->inputsize, rq->outputsize, rq->outputsize);
 	else
 		ret = LZ4_decompress_safe(src + inputmargin, out,
 					  rq->inputsize, rq->outputsize);

 	if (ret != rq->outputsize) {
 		erofs_err(rq->sb, "failed to decompress %d in[%u, %u] out[%u]",
 			  ret, rq->inputsize, inputmargin, rq->outputsize);

 		print_hex_dump(KERN_DEBUG, "[ in]: ", DUMP_PREFIX_OFFSET,
 			       16, 1, src + inputmargin, rq->inputsize, true);
 		print_hex_dump(KERN_DEBUG, "[out]: ", DUMP_PREFIX_OFFSET,
 			       16, 1, out, rq->outputsize, true);

 		if (ret >= 0)
 			memset(out + ret, 0, rq->outputsize - ret);
 		ret = -EIO;
 	}

 	if (maptype == 0) {
 		kunmap_atomic(src);
 	} else if (maptype == 1) {
 		vm_unmap_ram(src, PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT);
 	} else if (maptype == 2) {
 		erofs_put_pcpubuf(src);
 	} else if (maptype != 3) {
 		DBG_BUGON(1);
 		return -EFAULT;
 	}
 	return ret;
 }

 static struct z_erofs_decompressor decompressors[] = {
 	[Z_EROFS_COMPRESSION_SHIFTED] = {
 		.name = "shifted"
 	},
 	[Z_EROFS_COMPRESSION_LZ4] = {
 		.prepare_destpages = z_erofs_lz4_prepare_destpages,
 		.decompress = z_erofs_lz4_decompress,
 		.name = "lz4"
 	},
 };

 static void copy_from_pcpubuf(struct page **out, const char *dst,
 			      unsigned short pageofs_out,
 			      unsigned int outputsize)
 {
 	const char *end = dst + outputsize;
 	const unsigned int righthalf = PAGE_SIZE - pageofs_out;
 	const char *cur = dst - pageofs_out;

 	while (cur < end) {
 		struct page *const page = *out++;

 		if (page) {
 			char *buf = kmap_atomic(page);

 			if (cur >= dst) {
 				memcpy(buf, cur, min_t(uint, PAGE_SIZE,
 						       end - cur));
 			} else {
 				memcpy(buf + pageofs_out, cur + pageofs_out,
 				       min_t(uint, righthalf, end - cur));
 			}
 			kunmap_atomic(buf);
 		}
 		cur += PAGE_SIZE;
 	}
 }

 static int z_erofs_decompress_generic(struct z_erofs_decompress_req *rq,
 				      struct list_head *pagepool)
 {
 	const unsigned int nrpages_out =
 		PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
 	const struct z_erofs_decompressor *alg = decompressors + rq->alg;
 	unsigned int dst_maptype;
 	void *dst;
 	int ret;

 	/* two optimized fast paths only for non bigpcluster cases yet */
 	if (rq->inputsize <= PAGE_SIZE) {
 		if (nrpages_out == 1 && !rq->inplace_io) {
 			DBG_BUGON(!*rq->out);
 			dst = kmap_atomic(*rq->out);
 			dst_maptype = 0;
 			goto dstmap_out;
 		}

 		/*
 		 * For the case of small output size (especially much less
 		 * than PAGE_SIZE), memcpy the decompressed data rather than
 		 * compressed data is preferred.
 		 */
 		if (rq->outputsize <= PAGE_SIZE * 7 / 8) {
 			dst = erofs_get_pcpubuf(1);
 			if (IS_ERR(dst))
 				return PTR_ERR(dst);

 			rq->inplace_io = false;
 			ret = alg->decompress(rq, dst);
 			if (!ret)
 				copy_from_pcpubuf(rq->out, dst, rq->pageofs_out,
 						  rq->outputsize);

 			erofs_put_pcpubuf(dst);
 			return ret;
 		}
 	}

 	/* general decoding path which can be used for all cases */
 	ret = alg->prepare_destpages(rq, pagepool);
 	if (ret < 0)
 		return ret;
 	if (ret) {
 		dst = page_address(*rq->out);
 		dst_maptype = 1;
 		goto dstmap_out;
 	}

 	dst = erofs_vm_map_ram(rq->out, nrpages_out);
 	if (!dst)
 		return -ENOMEM;
 	dst_maptype = 2;

 dstmap_out:
 	ret = alg->decompress(rq, dst);

 	if (!dst_maptype)
 		kunmap_atomic(dst);
 	else if (dst_maptype == 2)
 		vm_unmap_ram(dst, nrpages_out);
 	return ret;
 }

 static int z_erofs_shifted_transform(const struct z_erofs_decompress_req *rq,
 				     struct list_head *pagepool)
 {
 	const unsigned int nrpages_out =
 		PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
 	const unsigned int righthalf = PAGE_SIZE - rq->pageofs_out;
 	unsigned char *src, *dst;

 	if (nrpages_out > 2) {
 		DBG_BUGON(1);
 		return -EIO;
 	}

 	if (rq->out[0] == *rq->in) {
 		DBG_BUGON(nrpages_out != 1);
 		return 0;
 	}

 	src = kmap_atomic(*rq->in);
 	if (rq->out[0]) {
 		dst = kmap_atomic(rq->out[0]);
 		memcpy(dst + rq->pageofs_out, src, righthalf);
 		kunmap_atomic(dst);
 	}

 	if (nrpages_out == 2) {
 		DBG_BUGON(!rq->out[1]);
 		if (rq->out[1] == *rq->in) {
 			memmove(src, src + righthalf, rq->pageofs_out);
 		} else {
 			dst = kmap_atomic(rq->out[1]);
 			memcpy(dst, src + righthalf, rq->pageofs_out);
 			kunmap_atomic(dst);
 		}
 	}
 	kunmap_atomic(src);
 	return 0;
 }

 int z_erofs_decompress(struct z_erofs_decompress_req *rq,
 		       struct list_head *pagepool)
 {
 	if (rq->alg == Z_EROFS_COMPRESSION_SHIFTED)
 		return z_erofs_shifted_transform(rq, pagepool);
 	return z_erofs_decompress_generic(rq, pagepool);
 }
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* Copyright (C) 2019 HUAWEI, Inc.
	* https://www.huawei.com/
	*/
	#include "compress.h"
	#include <linux/module.h>
	#include <linux/lz4.h>

	#ifndef LZ4_DISTANCE_MAX /* history window size */
	#define LZ4_DISTANCE_MAX 65535 /* set to maximum value by default */
	#endif

	#define LZ4_MAX_DISTANCE_PAGES (DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
	#ifndef LZ4_DECOMPRESS_INPLACE_MARGIN
	#define LZ4_DECOMPRESS_INPLACE_MARGIN(srcsize) (((srcsize) >> 8) + 32)
	#endif

	struct z_erofs_decompressor {
	/*
	* if destpages have sparsed pages, fill them with bounce pages.
	* it also check whether destpages indicate continuous physical memory.
	*/
	int (prepare_destpages)(struct z_erofs_decompress_req rq,
	struct list_head *pagepool);
	int (decompress)(struct z_erofs_decompress_req rq, u8 *dst);
	char *name;
	};

	int z_erofs_load_lz4_config(struct super_block *sb,
	struct erofs_super_block *dsb,
	struct z_erofs_lz4_cfgs *lz4, int size)
	{
	struct erofs_sb_info *sbi = EROFS_SB(sb);
	u16 distance;

	if (lz4) {
	if (size < sizeof(struct z_erofs_lz4_cfgs)) {
	erofs_err(sb, "invalid lz4 cfgs, size=%u", size);
	return -EINVAL;
	}
	distance = le16_to_cpu(lz4->max_distance);

	sbi->lz4.max_pclusterblks = le16_to_cpu(lz4->max_pclusterblks);
	if (!sbi->lz4.max_pclusterblks) {
	sbi->lz4.max_pclusterblks = 1; /* reserved case */
	} else if (sbi->lz4.max_pclusterblks >
	Z_EROFS_PCLUSTER_MAX_SIZE / EROFS_BLKSIZ) {
	erofs_err(sb, "too large lz4 pclusterblks %u",
	sbi->lz4.max_pclusterblks);
	return -EINVAL;
	} else if (sbi->lz4.max_pclusterblks >= 2) {
	erofs_info(sb, "EXPERIMENTAL big pcluster feature in use. Use at your own risk!");
	}
	} else {
	distance = le16_to_cpu(dsb->u1.lz4_max_distance);
	sbi->lz4.max_pclusterblks = 1;
	}

	sbi->lz4.max_distance_pages = distance ?
	DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :
	LZ4_MAX_DISTANCE_PAGES;
	return erofs_pcpubuf_growsize(sbi->lz4.max_pclusterblks);
	}

	static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
	struct list_head *pagepool)
	{
	const unsigned int nr =
	PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
	struct page *availables[LZ4_MAX_DISTANCE_PAGES] = { NULL };
	unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
	BITS_PER_LONG)] = { 0 };
	unsigned int lz4_max_distance_pages =
	EROFS_SB(rq->sb)->lz4.max_distance_pages;
	void *kaddr = NULL;
	unsigned int i, j, top;

	top = 0;
	for (i = j = 0; i < nr; ++i, ++j) {
	struct page *const page = rq->out[i];
	struct page *victim;

	if (j >= lz4_max_distance_pages)
	j = 0;

	/* 'valid' bounced can only be tested after a complete round */
	if (test_bit(j, bounced)) {
	DBG_BUGON(i < lz4_max_distance_pages);
	DBG_BUGON(top >= lz4_max_distance_pages);
	availables[top++] = rq->out[i - lz4_max_distance_pages];
	}

	if (page) {
	__clear_bit(j, bounced);
	if (!PageHighMem(page)) {
	if (!i) {
	kaddr = page_address(page);
	continue;
	}
	if (kaddr &&
	kaddr + PAGE_SIZE == page_address(page)) {
	kaddr += PAGE_SIZE;
	continue;
	}
	}
	kaddr = NULL;
	continue;
	}
	kaddr = NULL;
	__set_bit(j, bounced);

	if (top) {
	victim = availables[--top];
	get_page(victim);
	} else {
	victim = erofs_allocpage(pagepool,
	GFP_KERNEL \| __GFP_NOFAIL);
	set_page_private(victim, Z_EROFS_SHORTLIVED_PAGE);
	}
	rq->out[i] = victim;
	}
	return kaddr ? 1 : 0;
	}

	static void z_erofs_handle_inplace_io(struct z_erofs_decompress_req rq,
	void inpage, void out, unsigned int *inputmargin,
	int *maptype, bool support_0padding)
	{
	unsigned int nrpages_in, nrpages_out;
	unsigned int ofull, oend, inputsize, total, i;
	struct page **in;
	void src, tmp;

	inputsize = rq->inputsize;
	nrpages_in = PAGE_ALIGN(inputsize) >> PAGE_SHIFT;
	oend = rq->pageofs_out + rq->outputsize;
	ofull = PAGE_ALIGN(oend);
	nrpages_out = ofull >> PAGE_SHIFT;

	if (rq->inplace_io) {
	if (rq->partial_decoding \|\| !support_0padding \|\|
	ofull - oend < LZ4_DECOMPRESS_INPLACE_MARGIN(inputsize))
	goto docopy;

	for (i = 0; i < nrpages_in; ++i)
	if (rq->out[nrpages_out - nrpages_in + i] !=
	rq->in[i])
	goto docopy;
	kunmap_atomic(inpage);
	*maptype = 3;
	return out + ((nrpages_out - nrpages_in) << PAGE_SHIFT);
	}

	if (nrpages_in <= 1) {
	*maptype = 0;
	return inpage;
	}
	kunmap_atomic(inpage);
	src = erofs_vm_map_ram(rq->in, nrpages_in);
	if (!src)
	return ERR_PTR(-ENOMEM);
	*maptype = 1;
	return src;

	docopy:
	/* Or copy compressed data which can be overlapped to per-CPU buffer */
	in = rq->in;
	src = erofs_get_pcpubuf(nrpages_in);
	if (!src) {
	DBG_BUGON(1);
	kunmap_atomic(inpage);
	return ERR_PTR(-EFAULT);
	}

	tmp = src;
	total = rq->inputsize;
	while (total) {
	unsigned int page_copycnt =
	min_t(unsigned int, total, PAGE_SIZE - *inputmargin);

	if (!inpage)
	inpage = kmap_atomic(*in);
	memcpy(tmp, inpage + *inputmargin, page_copycnt);
	kunmap_atomic(inpage);
	inpage = NULL;
	tmp += page_copycnt;
	total -= page_copycnt;
	++in;
	*inputmargin = 0;
	}
	*maptype = 2;
	return src;
	}

	static int z_erofs_lz4_decompress(struct z_erofs_decompress_req rq, u8 dst)
	{
	unsigned int inputmargin;
	u8 out, headpage, *src;
	bool support_0padding;
	int ret, maptype;

	DBG_BUGON(*rq->in == NULL);
	headpage = kmap_atomic(*rq->in);
	inputmargin = 0;
	support_0padding = false;

	/* decompression inplace is only safe when 0padding is enabled */
	if (erofs_sb_has_lz4_0padding(EROFS_SB(rq->sb))) {
	support_0padding = true;

	while (!headpage[inputmargin & ~PAGE_MASK])
	if (!(++inputmargin & ~PAGE_MASK))
	break;

	if (inputmargin >= rq->inputsize) {
	kunmap_atomic(headpage);
	return -EIO;
	}
	}

	rq->inputsize -= inputmargin;
	src = z_erofs_handle_inplace_io(rq, headpage, dst, &inputmargin,
	&maptype, support_0padding);
	if (IS_ERR(src))
	return PTR_ERR(src);

	out = dst + rq->pageofs_out;
	/* legacy format could compress extra data in a pcluster. */
	if (rq->partial_decoding \|\| !support_0padding)
	ret = LZ4_decompress_safe_partial(src + inputmargin, out,
	rq->inputsize, rq->outputsize, rq->outputsize);
	else
	ret = LZ4_decompress_safe(src + inputmargin, out,
	rq->inputsize, rq->outputsize);

	if (ret != rq->outputsize) {
	erofs_err(rq->sb, "failed to decompress %d in[%u, %u] out[%u]",
	ret, rq->inputsize, inputmargin, rq->outputsize);

	print_hex_dump(KERN_DEBUG, "[ in]: ", DUMP_PREFIX_OFFSET,
	16, 1, src + inputmargin, rq->inputsize, true);
	print_hex_dump(KERN_DEBUG, "[out]: ", DUMP_PREFIX_OFFSET,
	16, 1, out, rq->outputsize, true);

	if (ret >= 0)
	memset(out + ret, 0, rq->outputsize - ret);
	ret = -EIO;
	}

	if (maptype == 0) {
	kunmap_atomic(src);
	} else if (maptype == 1) {
	vm_unmap_ram(src, PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT);
	} else if (maptype == 2) {
	erofs_put_pcpubuf(src);
	} else if (maptype != 3) {
	DBG_BUGON(1);
	return -EFAULT;
	}
	return ret;
	}

	static struct z_erofs_decompressor decompressors[] = {
	[Z_EROFS_COMPRESSION_SHIFTED] = {
	.name = "shifted"
	},
	[Z_EROFS_COMPRESSION_LZ4] = {
	.prepare_destpages = z_erofs_lz4_prepare_destpages,
	.decompress = z_erofs_lz4_decompress,
	.name = "lz4"
	},
	};

	static void copy_from_pcpubuf(struct page *out, const char dst,
	unsigned short pageofs_out,
	unsigned int outputsize)
	{
	const char *end = dst + outputsize;
	const unsigned int righthalf = PAGE_SIZE - pageofs_out;
	const char *cur = dst - pageofs_out;

	while (cur < end) {
	struct page const page = out++;

	if (page) {
	char *buf = kmap_atomic(page);

	if (cur >= dst) {
	memcpy(buf, cur, min_t(uint, PAGE_SIZE,
	end - cur));
	} else {
	memcpy(buf + pageofs_out, cur + pageofs_out,
	min_t(uint, righthalf, end - cur));
	}
	kunmap_atomic(buf);
	}
	cur += PAGE_SIZE;
	}
	}

	static int z_erofs_decompress_generic(struct z_erofs_decompress_req *rq,
	struct list_head *pagepool)
	{
	const unsigned int nrpages_out =
	PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
	const struct z_erofs_decompressor *alg = decompressors + rq->alg;
	unsigned int dst_maptype;
	void *dst;
	int ret;

	/* two optimized fast paths only for non bigpcluster cases yet */
	if (rq->inputsize <= PAGE_SIZE) {
	if (nrpages_out == 1 && !rq->inplace_io) {
	DBG_BUGON(!*rq->out);
	dst = kmap_atomic(*rq->out);
	dst_maptype = 0;
	goto dstmap_out;
	}

	/*
	* For the case of small output size (especially much less
	* than PAGE_SIZE), memcpy the decompressed data rather than
	* compressed data is preferred.
	*/
	if (rq->outputsize <= PAGE_SIZE * 7 / 8) {
	dst = erofs_get_pcpubuf(1);
	if (IS_ERR(dst))
	return PTR_ERR(dst);

	rq->inplace_io = false;
	ret = alg->decompress(rq, dst);
	if (!ret)
	copy_from_pcpubuf(rq->out, dst, rq->pageofs_out,
	rq->outputsize);

	erofs_put_pcpubuf(dst);
	return ret;
	}
	}

	/* general decoding path which can be used for all cases */
	ret = alg->prepare_destpages(rq, pagepool);
	if (ret < 0)
	return ret;
	if (ret) {
	dst = page_address(*rq->out);
	dst_maptype = 1;
	goto dstmap_out;
	}

	dst = erofs_vm_map_ram(rq->out, nrpages_out);
	if (!dst)
	return -ENOMEM;
	dst_maptype = 2;

	dstmap_out:
	ret = alg->decompress(rq, dst);

	if (!dst_maptype)
	kunmap_atomic(dst);
	else if (dst_maptype == 2)
	vm_unmap_ram(dst, nrpages_out);
	return ret;
	}

	static int z_erofs_shifted_transform(const struct z_erofs_decompress_req *rq,
	struct list_head *pagepool)
	{
	const unsigned int nrpages_out =
	PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
	const unsigned int righthalf = PAGE_SIZE - rq->pageofs_out;
	unsigned char src, dst;

	if (nrpages_out > 2) {
	DBG_BUGON(1);
	return -EIO;
	}

	if (rq->out[0] == *rq->in) {
	DBG_BUGON(nrpages_out != 1);
	return 0;
	}

	src = kmap_atomic(*rq->in);
	if (rq->out[0]) {
	dst = kmap_atomic(rq->out[0]);
	memcpy(dst + rq->pageofs_out, src, righthalf);
	kunmap_atomic(dst);
	}

	if (nrpages_out == 2) {
	DBG_BUGON(!rq->out[1]);
	if (rq->out[1] == *rq->in) {
	memmove(src, src + righthalf, rq->pageofs_out);
	} else {
	dst = kmap_atomic(rq->out[1]);
	memcpy(dst, src + righthalf, rq->pageofs_out);
	kunmap_atomic(dst);
	}
	}
	kunmap_atomic(src);
	return 0;
	}

	int z_erofs_decompress(struct z_erofs_decompress_req *rq,
	struct list_head *pagepool)
	{
	if (rq->alg == Z_EROFS_COMPRESSION_SHIFTED)
	return z_erofs_shifted_transform(rq, pagepool);
	return z_erofs_decompress_generic(rq, pagepool);
	}