fs/erofs/decompressor.c - linux - Git at Google

 // 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_lz4_decompress_ctx {
 	struct z_erofs_decompress_req *rq;
 	/* # of encoded, decoded pages */
 	unsigned int inpages, outpages;
 	/* decoded block total length (used for in-place decompression) */
 	unsigned int oend;
 };

 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 {
 		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);
 }

 /*
  * Fill all gaps with bounce pages if it's a sparse page list. Also check if
  * all physical pages are consecutive, which can be seen for moderate CR.
  */
 static int z_erofs_lz4_prepare_dstpages(struct z_erofs_lz4_decompress_ctx *ctx,
 					struct page **pagepool)
 {
 	struct z_erofs_decompress_req *rq = ctx->rq;
 	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 < ctx->outpages; ++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 (!rq->fillgaps && 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_lz4_handle_overlap(struct z_erofs_lz4_decompress_ctx *ctx,
 			void *inpage, unsigned int *inputmargin, int *maptype,
 			bool may_inplace)
 {
 	struct z_erofs_decompress_req *rq = ctx->rq;
 	unsigned int omargin, total, i, j;
 	struct page **in;
 	void *src, *tmp;

 	if (rq->inplace_io) {
 		omargin = PAGE_ALIGN(ctx->oend) - ctx->oend;
 		if (rq->partial_decoding || !may_inplace ||
 		    omargin < LZ4_DECOMPRESS_INPLACE_MARGIN(rq->inputsize))
 			goto docopy;

 		for (i = 0; i < ctx->inpages; ++i) {
 			DBG_BUGON(rq->in[i] == NULL);
 			for (j = 0; j < ctx->outpages - ctx->inpages + i; ++j)
 				if (rq->out[j] == rq->in[i])
 					goto docopy;
 		}
 	}

 	if (ctx->inpages <= 1) {
 		*maptype = 0;
 		return inpage;
 	}
 	kunmap_atomic(inpage);
 	might_sleep();
 	src = erofs_vm_map_ram(rq->in, ctx->inpages);
 	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(ctx->inpages);
 	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;
 }

 /*
  * Get the exact inputsize with zero_padding feature.
  *  - For LZ4, it should work if zero_padding feature is on (5.3+);
  *  - For MicroLZMA, it'd be enabled all the time.
  */
 int z_erofs_fixup_insize(struct z_erofs_decompress_req *rq, const char *padbuf,
 			 unsigned int padbufsize)
 {
 	const char *padend;

 	padend = memchr_inv(padbuf, 0, padbufsize);
 	if (!padend)
 		return -EFSCORRUPTED;
 	rq->inputsize -= padend - padbuf;
 	rq->pageofs_in += padend - padbuf;
 	return 0;
 }

 static int z_erofs_lz4_decompress_mem(struct z_erofs_lz4_decompress_ctx *ctx,
 				      u8 *out)
 {
 	struct z_erofs_decompress_req *rq = ctx->rq;
 	bool support_0padding = false, may_inplace = false;
 	unsigned int inputmargin;
 	u8 *headpage, *src;
 	int ret, maptype;

 	DBG_BUGON(*rq->in == NULL);
 	headpage = kmap_atomic(*rq->in);

 	/* LZ4 decompression inplace is only safe if zero_padding is enabled */
 	if (erofs_sb_has_zero_padding(EROFS_SB(rq->sb))) {
 		support_0padding = true;
 		ret = z_erofs_fixup_insize(rq, headpage + rq->pageofs_in,
 				min_t(unsigned int, rq->inputsize,
 				      EROFS_BLKSIZ - rq->pageofs_in));
 		if (ret) {
 			kunmap_atomic(headpage);
 			return ret;
 		}
 		may_inplace = !((rq->pageofs_in + rq->inputsize) &
 				(EROFS_BLKSIZ - 1));
 	}

 	inputmargin = rq->pageofs_in;
 	src = z_erofs_lz4_handle_overlap(ctx, headpage, &inputmargin,
 					 &maptype, may_inplace);
 	if (IS_ERR(src))
 		return PTR_ERR(src);

 	/* 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;
 	} else {
 		ret = 0;
 	}

 	if (maptype == 0) {
 		kunmap_atomic(headpage);
 	} else if (maptype == 1) {
 		vm_unmap_ram(src, ctx->inpages);
 	} else if (maptype == 2) {
 		erofs_put_pcpubuf(src);
 	} else {
 		DBG_BUGON(1);
 		return -EFAULT;
 	}
 	return ret;
 }

 static int z_erofs_lz4_decompress(struct z_erofs_decompress_req *rq,
 				  struct page **pagepool)
 {
 	struct z_erofs_lz4_decompress_ctx ctx;
 	unsigned int dst_maptype;
 	void *dst;
 	int ret;

 	ctx.rq = rq;
 	ctx.oend = rq->pageofs_out + rq->outputsize;
 	ctx.outpages = PAGE_ALIGN(ctx.oend) >> PAGE_SHIFT;
 	ctx.inpages = PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT;

 	/* one optimized fast path only for non bigpcluster cases yet */
 	if (ctx.inpages == 1 && ctx.outpages == 1 && !rq->inplace_io) {
 		DBG_BUGON(!*rq->out);
 		dst = kmap_atomic(*rq->out);
 		dst_maptype = 0;
 		goto dstmap_out;
 	}

 	/* general decoding path which can be used for all cases */
 	ret = z_erofs_lz4_prepare_dstpages(&ctx, pagepool);
 	if (ret < 0) {
 		return ret;
 	} else if (ret > 0) {
 		dst = page_address(*rq->out);
 		dst_maptype = 1;
 	} else {
 		dst = erofs_vm_map_ram(rq->out, ctx.outpages);
 		if (!dst)
 			return -ENOMEM;
 		dst_maptype = 2;
 	}

 dstmap_out:
 	ret = z_erofs_lz4_decompress_mem(&ctx, dst + rq->pageofs_out);
 	if (!dst_maptype)
 		kunmap_atomic(dst);
 	else if (dst_maptype == 2)
 		vm_unmap_ram(dst, ctx.outpages);
 	return ret;
 }

 static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
 				   struct page **pagepool)
 {
 	const unsigned int inpages = PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT;
 	const unsigned int outpages =
 		PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
 	const unsigned int righthalf = min_t(unsigned int, rq->outputsize,
 					     PAGE_SIZE - rq->pageofs_out);
 	const unsigned int lefthalf = rq->outputsize - righthalf;
 	const unsigned int interlaced_offset =
 		rq->alg == Z_EROFS_COMPRESSION_SHIFTED ? 0 : rq->pageofs_out;
 	unsigned char *src, *dst;

 	if (outpages > 2 && rq->alg == Z_EROFS_COMPRESSION_SHIFTED) {
 		DBG_BUGON(1);
 		return -EFSCORRUPTED;
 	}

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

 	src = kmap_local_page(rq->in[inpages - 1]) + rq->pageofs_in;
 	if (rq->out[0]) {
 		dst = kmap_local_page(rq->out[0]);
 		memcpy(dst + rq->pageofs_out, src + interlaced_offset,
 		       righthalf);
 		kunmap_local(dst);
 	}

 	if (outpages > inpages) {
 		DBG_BUGON(!rq->out[outpages - 1]);
 		if (rq->out[outpages - 1] != rq->in[inpages - 1]) {
 			dst = kmap_local_page(rq->out[outpages - 1]);
 			memcpy(dst, interlaced_offset ? src :
 					(src + righthalf), lefthalf);
 			kunmap_local(dst);
 		} else if (!interlaced_offset) {
 			memmove(src, src + righthalf, lefthalf);
 		}
 	}
 	kunmap_local(src);
 	return 0;
 }

 static struct z_erofs_decompressor decompressors[] = {
 	[Z_EROFS_COMPRESSION_SHIFTED] = {
 		.decompress = z_erofs_transform_plain,
 		.name = "shifted"
 	},
 	[Z_EROFS_COMPRESSION_INTERLACED] = {
 		.decompress = z_erofs_transform_plain,
 		.name = "interlaced"
 	},
 	[Z_EROFS_COMPRESSION_LZ4] = {
 		.decompress = z_erofs_lz4_decompress,
 		.name = "lz4"
 	},
 #ifdef CONFIG_EROFS_FS_ZIP_LZMA
 	[Z_EROFS_COMPRESSION_LZMA] = {
 		.decompress = z_erofs_lzma_decompress,
 		.name = "lzma"
 	},
 #endif
 };

 int z_erofs_decompress(struct z_erofs_decompress_req *rq,
 		       struct page **pagepool)
 {
 	return decompressors[rq->alg].decompress(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_lz4_decompress_ctx {
	struct z_erofs_decompress_req *rq;
	/* # of encoded, decoded pages */
	unsigned int inpages, outpages;
	/* decoded block total length (used for in-place decompression) */
	unsigned int oend;
	};

	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 {
	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);
	}

	/*
	* Fill all gaps with bounce pages if it's a sparse page list. Also check if
	* all physical pages are consecutive, which can be seen for moderate CR.
	*/
	static int z_erofs_lz4_prepare_dstpages(struct z_erofs_lz4_decompress_ctx *ctx,
	struct page **pagepool)
	{
	struct z_erofs_decompress_req *rq = ctx->rq;
	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 < ctx->outpages; ++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 (!rq->fillgaps && 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_lz4_handle_overlap(struct z_erofs_lz4_decompress_ctx ctx,
	void inpage, unsigned int inputmargin, int *maptype,
	bool may_inplace)
	{
	struct z_erofs_decompress_req *rq = ctx->rq;
	unsigned int omargin, total, i, j;
	struct page **in;
	void src, tmp;

	if (rq->inplace_io) {
	omargin = PAGE_ALIGN(ctx->oend) - ctx->oend;
	if (rq->partial_decoding \|\| !may_inplace \|\|
	omargin < LZ4_DECOMPRESS_INPLACE_MARGIN(rq->inputsize))
	goto docopy;

	for (i = 0; i < ctx->inpages; ++i) {
	DBG_BUGON(rq->in[i] == NULL);
	for (j = 0; j < ctx->outpages - ctx->inpages + i; ++j)
	if (rq->out[j] == rq->in[i])
	goto docopy;
	}
	}

	if (ctx->inpages <= 1) {
	*maptype = 0;
	return inpage;
	}
	kunmap_atomic(inpage);
	might_sleep();
	src = erofs_vm_map_ram(rq->in, ctx->inpages);
	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(ctx->inpages);
	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;
	}

	/*
	* Get the exact inputsize with zero_padding feature.
	* - For LZ4, it should work if zero_padding feature is on (5.3+);
	* - For MicroLZMA, it'd be enabled all the time.
	*/
	int z_erofs_fixup_insize(struct z_erofs_decompress_req rq, const char padbuf,
	unsigned int padbufsize)
	{
	const char *padend;

	padend = memchr_inv(padbuf, 0, padbufsize);
	if (!padend)
	return -EFSCORRUPTED;
	rq->inputsize -= padend - padbuf;
	rq->pageofs_in += padend - padbuf;
	return 0;
	}

	static int z_erofs_lz4_decompress_mem(struct z_erofs_lz4_decompress_ctx *ctx,
	u8 *out)
	{
	struct z_erofs_decompress_req *rq = ctx->rq;
	bool support_0padding = false, may_inplace = false;
	unsigned int inputmargin;
	u8 headpage, src;
	int ret, maptype;

	DBG_BUGON(*rq->in == NULL);
	headpage = kmap_atomic(*rq->in);

	/* LZ4 decompression inplace is only safe if zero_padding is enabled */
	if (erofs_sb_has_zero_padding(EROFS_SB(rq->sb))) {
	support_0padding = true;
	ret = z_erofs_fixup_insize(rq, headpage + rq->pageofs_in,
	min_t(unsigned int, rq->inputsize,
	EROFS_BLKSIZ - rq->pageofs_in));
	if (ret) {
	kunmap_atomic(headpage);
	return ret;
	}
	may_inplace = !((rq->pageofs_in + rq->inputsize) &
	(EROFS_BLKSIZ - 1));
	}

	inputmargin = rq->pageofs_in;
	src = z_erofs_lz4_handle_overlap(ctx, headpage, &inputmargin,
	&maptype, may_inplace);
	if (IS_ERR(src))
	return PTR_ERR(src);

	/* 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;
	} else {
	ret = 0;
	}

	if (maptype == 0) {
	kunmap_atomic(headpage);
	} else if (maptype == 1) {
	vm_unmap_ram(src, ctx->inpages);
	} else if (maptype == 2) {
	erofs_put_pcpubuf(src);
	} else {
	DBG_BUGON(1);
	return -EFAULT;
	}
	return ret;
	}

	static int z_erofs_lz4_decompress(struct z_erofs_decompress_req *rq,
	struct page **pagepool)
	{
	struct z_erofs_lz4_decompress_ctx ctx;
	unsigned int dst_maptype;
	void *dst;
	int ret;

	ctx.rq = rq;
	ctx.oend = rq->pageofs_out + rq->outputsize;
	ctx.outpages = PAGE_ALIGN(ctx.oend) >> PAGE_SHIFT;
	ctx.inpages = PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT;

	/* one optimized fast path only for non bigpcluster cases yet */
	if (ctx.inpages == 1 && ctx.outpages == 1 && !rq->inplace_io) {
	DBG_BUGON(!*rq->out);
	dst = kmap_atomic(*rq->out);
	dst_maptype = 0;
	goto dstmap_out;
	}

	/* general decoding path which can be used for all cases */
	ret = z_erofs_lz4_prepare_dstpages(&ctx, pagepool);
	if (ret < 0) {
	return ret;
	} else if (ret > 0) {
	dst = page_address(*rq->out);
	dst_maptype = 1;
	} else {
	dst = erofs_vm_map_ram(rq->out, ctx.outpages);
	if (!dst)
	return -ENOMEM;
	dst_maptype = 2;
	}

	dstmap_out:
	ret = z_erofs_lz4_decompress_mem(&ctx, dst + rq->pageofs_out);
	if (!dst_maptype)
	kunmap_atomic(dst);
	else if (dst_maptype == 2)
	vm_unmap_ram(dst, ctx.outpages);
	return ret;
	}

	static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
	struct page **pagepool)
	{
	const unsigned int inpages = PAGE_ALIGN(rq->inputsize) >> PAGE_SHIFT;
	const unsigned int outpages =
	PAGE_ALIGN(rq->pageofs_out + rq->outputsize) >> PAGE_SHIFT;
	const unsigned int righthalf = min_t(unsigned int, rq->outputsize,
	PAGE_SIZE - rq->pageofs_out);
	const unsigned int lefthalf = rq->outputsize - righthalf;
	const unsigned int interlaced_offset =
	rq->alg == Z_EROFS_COMPRESSION_SHIFTED ? 0 : rq->pageofs_out;
	unsigned char src, dst;

	if (outpages > 2 && rq->alg == Z_EROFS_COMPRESSION_SHIFTED) {
	DBG_BUGON(1);
	return -EFSCORRUPTED;
	}

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

	src = kmap_local_page(rq->in[inpages - 1]) + rq->pageofs_in;
	if (rq->out[0]) {
	dst = kmap_local_page(rq->out[0]);
	memcpy(dst + rq->pageofs_out, src + interlaced_offset,
	righthalf);
	kunmap_local(dst);
	}

	if (outpages > inpages) {
	DBG_BUGON(!rq->out[outpages - 1]);
	if (rq->out[outpages - 1] != rq->in[inpages - 1]) {
	dst = kmap_local_page(rq->out[outpages - 1]);
	memcpy(dst, interlaced_offset ? src :
	(src + righthalf), lefthalf);
	kunmap_local(dst);
	} else if (!interlaced_offset) {
	memmove(src, src + righthalf, lefthalf);
	}
	}
	kunmap_local(src);
	return 0;
	}

	static struct z_erofs_decompressor decompressors[] = {
	[Z_EROFS_COMPRESSION_SHIFTED] = {
	.decompress = z_erofs_transform_plain,
	.name = "shifted"
	},
	[Z_EROFS_COMPRESSION_INTERLACED] = {
	.decompress = z_erofs_transform_plain,
	.name = "interlaced"
	},
	[Z_EROFS_COMPRESSION_LZ4] = {
	.decompress = z_erofs_lz4_decompress,
	.name = "lz4"
	},
	#ifdef CONFIG_EROFS_FS_ZIP_LZMA
	[Z_EROFS_COMPRESSION_LZMA] = {
	.decompress = z_erofs_lzma_decompress,
	.name = "lzma"
	},
	#endif
	};

	int z_erofs_decompress(struct z_erofs_decompress_req *rq,
	struct page **pagepool)
	{
	return decompressors[rq->alg].decompress(rq, pagepool);
	}