fs/ext4/readpage.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 /*
  * linux/fs/ext4/readpage.c
  *
  * Copyright (C) 2002, Linus Torvalds.
  * Copyright (C) 2015, Google, Inc.
  *
  * This was originally taken from fs/mpage.c
  *
  * The ext4_mpage_readpages() function here is intended to
  * replace mpage_readahead() in the general case, not just for
  * encrypted files.  It has some limitations (see below), where it
  * will fall back to read_block_full_page(), but these limitations
  * should only be hit when page_size != block_size.
  *
  * This will allow us to attach a callback function to support ext4
  * encryption.
  *
  * If anything unusual happens, such as:
  *
  * - encountering a page which has buffers
  * - encountering a page which has a non-hole after a hole
  * - encountering a page with non-contiguous blocks
  *
  * then this code just gives up and calls the buffer_head-based read function.
  * It does handle a page which has holes at the end - that is a common case:
  * the end-of-file on blocksize < PAGE_SIZE setups.
  *
  */

 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/kdev_t.h>
 #include <linux/gfp.h>
 #include <linux/bio.h>
 #include <linux/fs.h>
 #include <linux/buffer_head.h>
 #include <linux/blkdev.h>
 #include <linux/highmem.h>
 #include <linux/prefetch.h>
 #include <linux/mpage.h>
 #include <linux/writeback.h>
 #include <linux/backing-dev.h>
 #include <linux/pagevec.h>

 #include "ext4.h"

 #define NUM_PREALLOC_POST_READ_CTXS	128

 static struct kmem_cache *bio_post_read_ctx_cache;
 static mempool_t *bio_post_read_ctx_pool;

 /* postprocessing steps for read bios */
 enum bio_post_read_step {
 	STEP_INITIAL = 0,
 	STEP_DECRYPT,
 	STEP_VERITY,
 	STEP_MAX,
 };

 struct bio_post_read_ctx {
 	struct bio *bio;
 	struct work_struct work;
 	unsigned int cur_step;
 	unsigned int enabled_steps;
 };

 static void __read_end_io(struct bio *bio)
 {
 	struct folio_iter fi;

 	bio_for_each_folio_all(fi, bio)
 		folio_end_read(fi.folio, bio->bi_status == 0);
 	if (bio->bi_private)
 		mempool_free(bio->bi_private, bio_post_read_ctx_pool);
 	bio_put(bio);
 }

 static void bio_post_read_processing(struct bio_post_read_ctx *ctx);

 static void decrypt_work(struct work_struct *work)
 {
 	struct bio_post_read_ctx *ctx =
 		container_of(work, struct bio_post_read_ctx, work);
 	struct bio *bio = ctx->bio;

 	if (fscrypt_decrypt_bio(bio))
 		bio_post_read_processing(ctx);
 	else
 		__read_end_io(bio);
 }

 static void verity_work(struct work_struct *work)
 {
 	struct bio_post_read_ctx *ctx =
 		container_of(work, struct bio_post_read_ctx, work);
 	struct bio *bio = ctx->bio;

 	/*
 	 * fsverity_verify_bio() may call readahead() again, and although verity
 	 * will be disabled for that, decryption may still be needed, causing
 	 * another bio_post_read_ctx to be allocated.  So to guarantee that
 	 * mempool_alloc() never deadlocks we must free the current ctx first.
 	 * This is safe because verity is the last post-read step.
 	 */
 	BUILD_BUG_ON(STEP_VERITY + 1 != STEP_MAX);
 	mempool_free(ctx, bio_post_read_ctx_pool);
 	bio->bi_private = NULL;

 	fsverity_verify_bio(bio);

 	__read_end_io(bio);
 }

 static void bio_post_read_processing(struct bio_post_read_ctx *ctx)
 {
 	/*
 	 * We use different work queues for decryption and for verity because
 	 * verity may require reading metadata pages that need decryption, and
 	 * we shouldn't recurse to the same workqueue.
 	 */
 	switch (++ctx->cur_step) {
 	case STEP_DECRYPT:
 		if (ctx->enabled_steps & (1 << STEP_DECRYPT)) {
 			INIT_WORK(&ctx->work, decrypt_work);
 			fscrypt_enqueue_decrypt_work(&ctx->work);
 			return;
 		}
 		ctx->cur_step++;
 		fallthrough;
 	case STEP_VERITY:
 		if (ctx->enabled_steps & (1 << STEP_VERITY)) {
 			INIT_WORK(&ctx->work, verity_work);
 			fsverity_enqueue_verify_work(&ctx->work);
 			return;
 		}
 		ctx->cur_step++;
 		fallthrough;
 	default:
 		__read_end_io(ctx->bio);
 	}
 }

 static bool bio_post_read_required(struct bio *bio)
 {
 	return bio->bi_private && !bio->bi_status;
 }

 /*
  * I/O completion handler for multipage BIOs.
  *
  * The mpage code never puts partial pages into a BIO (except for end-of-file).
  * If a page does not map to a contiguous run of blocks then it simply falls
  * back to block_read_full_folio().
  *
  * Why is this?  If a page's completion depends on a number of different BIOs
  * which can complete in any order (or at the same time) then determining the
  * status of that page is hard.  See end_buffer_async_read() for the details.
  * There is no point in duplicating all that complexity.
  */
 static void mpage_end_io(struct bio *bio)
 {
 	if (bio_post_read_required(bio)) {
 		struct bio_post_read_ctx *ctx = bio->bi_private;

 		ctx->cur_step = STEP_INITIAL;
 		bio_post_read_processing(ctx);
 		return;
 	}
 	__read_end_io(bio);
 }

 static inline bool ext4_need_verity(const struct inode *inode, pgoff_t idx)
 {
 	return fsverity_active(inode) &&
 	       idx < DIV_ROUND_UP(inode->i_size, PAGE_SIZE);
 }

 static void ext4_set_bio_post_read_ctx(struct bio *bio,
 				       const struct inode *inode,
 				       pgoff_t first_idx)
 {
 	unsigned int post_read_steps = 0;

 	if (fscrypt_inode_uses_fs_layer_crypto(inode))
 		post_read_steps |= 1 << STEP_DECRYPT;

 	if (ext4_need_verity(inode, first_idx))
 		post_read_steps |= 1 << STEP_VERITY;

 	if (post_read_steps) {
 		/* Due to the mempool, this never fails. */
 		struct bio_post_read_ctx *ctx =
 			mempool_alloc(bio_post_read_ctx_pool, GFP_NOFS);

 		ctx->bio = bio;
 		ctx->enabled_steps = post_read_steps;
 		bio->bi_private = ctx;
 	}
 }

 static inline loff_t ext4_readpage_limit(struct inode *inode)
 {
 	if (IS_ENABLED(CONFIG_FS_VERITY) && IS_VERITY(inode))
 		return inode->i_sb->s_maxbytes;

 	return i_size_read(inode);
 }

 int ext4_mpage_readpages(struct inode *inode,
 		struct readahead_control *rac, struct folio *folio)
 {
 	struct bio *bio = NULL;
 	sector_t last_block_in_bio = 0;

 	const unsigned blkbits = inode->i_blkbits;
 	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
 	const unsigned blocksize = 1 << blkbits;
 	sector_t next_block;
 	sector_t block_in_file;
 	sector_t last_block;
 	sector_t last_block_in_file;
 	sector_t blocks[MAX_BUF_PER_PAGE];
 	unsigned page_block;
 	struct block_device *bdev = inode->i_sb->s_bdev;
 	int length;
 	unsigned relative_block = 0;
 	struct ext4_map_blocks map;
 	unsigned int nr_pages = rac ? readahead_count(rac) : 1;

 	map.m_pblk = 0;
 	map.m_lblk = 0;
 	map.m_len = 0;
 	map.m_flags = 0;

 	for (; nr_pages; nr_pages--) {
 		int fully_mapped = 1;
 		unsigned first_hole = blocks_per_page;

 		if (rac)
 			folio = readahead_folio(rac);
 		prefetchw(&folio->flags);

 		if (folio_buffers(folio))
 			goto confused;

 		block_in_file = next_block =
 			(sector_t)folio->index << (PAGE_SHIFT - blkbits);
 		last_block = block_in_file + nr_pages * blocks_per_page;
 		last_block_in_file = (ext4_readpage_limit(inode) +
 				      blocksize - 1) >> blkbits;
 		if (last_block > last_block_in_file)
 			last_block = last_block_in_file;
 		page_block = 0;

 		/*
 		 * Map blocks using the previous result first.
 		 */
 		if ((map.m_flags & EXT4_MAP_MAPPED) &&
 		    block_in_file > map.m_lblk &&
 		    block_in_file < (map.m_lblk + map.m_len)) {
 			unsigned map_offset = block_in_file - map.m_lblk;
 			unsigned last = map.m_len - map_offset;

 			for (relative_block = 0; ; relative_block++) {
 				if (relative_block == last) {
 					/* needed? */
 					map.m_flags &= ~EXT4_MAP_MAPPED;
 					break;
 				}
 				if (page_block == blocks_per_page)
 					break;
 				blocks[page_block] = map.m_pblk + map_offset +
 					relative_block;
 				page_block++;
 				block_in_file++;
 			}
 		}

 		/*
 		 * Then do more ext4_map_blocks() calls until we are
 		 * done with this folio.
 		 */
 		while (page_block < blocks_per_page) {
 			if (block_in_file < last_block) {
 				map.m_lblk = block_in_file;
 				map.m_len = last_block - block_in_file;

 				if (ext4_map_blocks(NULL, inode, &map, 0) < 0) {
 				set_error_page:
 					folio_set_error(folio);
 					folio_zero_segment(folio, 0,
 							  folio_size(folio));
 					folio_unlock(folio);
 					goto next_page;
 				}
 			}
 			if ((map.m_flags & EXT4_MAP_MAPPED) == 0) {
 				fully_mapped = 0;
 				if (first_hole == blocks_per_page)
 					first_hole = page_block;
 				page_block++;
 				block_in_file++;
 				continue;
 			}
 			if (first_hole != blocks_per_page)
 				goto confused;		/* hole -> non-hole */

 			/* Contiguous blocks? */
 			if (page_block && blocks[page_block-1] != map.m_pblk-1)
 				goto confused;
 			for (relative_block = 0; ; relative_block++) {
 				if (relative_block == map.m_len) {
 					/* needed? */
 					map.m_flags &= ~EXT4_MAP_MAPPED;
 					break;
 				} else if (page_block == blocks_per_page)
 					break;
 				blocks[page_block] = map.m_pblk+relative_block;
 				page_block++;
 				block_in_file++;
 			}
 		}
 		if (first_hole != blocks_per_page) {
 			folio_zero_segment(folio, first_hole << blkbits,
 					  folio_size(folio));
 			if (first_hole == 0) {
 				if (ext4_need_verity(inode, folio->index) &&
 				    !fsverity_verify_folio(folio))
 					goto set_error_page;
 				folio_end_read(folio, true);
 				continue;
 			}
 		} else if (fully_mapped) {
 			folio_set_mappedtodisk(folio);
 		}

 		/*
 		 * This folio will go to BIO.  Do we need to send this
 		 * BIO off first?
 		 */
 		if (bio && (last_block_in_bio != blocks[0] - 1 ||
 			    !fscrypt_mergeable_bio(bio, inode, next_block))) {
 		submit_and_realloc:
 			submit_bio(bio);
 			bio = NULL;
 		}
 		if (bio == NULL) {
 			/*
 			 * bio_alloc will _always_ be able to allocate a bio if
 			 * __GFP_DIRECT_RECLAIM is set, see bio_alloc_bioset().
 			 */
 			bio = bio_alloc(bdev, bio_max_segs(nr_pages),
 					REQ_OP_READ, GFP_KERNEL);
 			fscrypt_set_bio_crypt_ctx(bio, inode, next_block,
 						  GFP_KERNEL);
 			ext4_set_bio_post_read_ctx(bio, inode, folio->index);
 			bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
 			bio->bi_end_io = mpage_end_io;
 			if (rac)
 				bio->bi_opf |= REQ_RAHEAD;
 		}

 		length = first_hole << blkbits;
 		if (!bio_add_folio(bio, folio, length, 0))
 			goto submit_and_realloc;

 		if (((map.m_flags & EXT4_MAP_BOUNDARY) &&
 		     (relative_block == map.m_len)) ||
 		    (first_hole != blocks_per_page)) {
 			submit_bio(bio);
 			bio = NULL;
 		} else
 			last_block_in_bio = blocks[blocks_per_page - 1];
 		continue;
 	confused:
 		if (bio) {
 			submit_bio(bio);
 			bio = NULL;
 		}
 		if (!folio_test_uptodate(folio))
 			block_read_full_folio(folio, ext4_get_block);
 		else
 			folio_unlock(folio);
 next_page:
 		; /* A label shall be followed by a statement until C23 */
 	}
 	if (bio)
 		submit_bio(bio);
 	return 0;
 }

 int __init ext4_init_post_read_processing(void)
 {
 	bio_post_read_ctx_cache = KMEM_CACHE(bio_post_read_ctx, SLAB_RECLAIM_ACCOUNT);

 	if (!bio_post_read_ctx_cache)
 		goto fail;
 	bio_post_read_ctx_pool =
 		mempool_create_slab_pool(NUM_PREALLOC_POST_READ_CTXS,
 					 bio_post_read_ctx_cache);
 	if (!bio_post_read_ctx_pool)
 		goto fail_free_cache;
 	return 0;

 fail_free_cache:
 	kmem_cache_destroy(bio_post_read_ctx_cache);
 fail:
 	return -ENOMEM;
 }

 void ext4_exit_post_read_processing(void)
 {
 	mempool_destroy(bio_post_read_ctx_pool);
 	kmem_cache_destroy(bio_post_read_ctx_cache);
 }
	// SPDX-License-Identifier: GPL-2.0
	/*
	* linux/fs/ext4/readpage.c
	*
	* Copyright (C) 2002, Linus Torvalds.
	* Copyright (C) 2015, Google, Inc.
	*
	* This was originally taken from fs/mpage.c
	*
	* The ext4_mpage_readpages() function here is intended to
	* replace mpage_readahead() in the general case, not just for
	* encrypted files. It has some limitations (see below), where it
	* will fall back to read_block_full_page(), but these limitations
	* should only be hit when page_size != block_size.
	*
	* This will allow us to attach a callback function to support ext4
	* encryption.
	*
	* If anything unusual happens, such as:
	*
	* - encountering a page which has buffers
	* - encountering a page which has a non-hole after a hole
	* - encountering a page with non-contiguous blocks
	*
	* then this code just gives up and calls the buffer_head-based read function.
	* It does handle a page which has holes at the end - that is a common case:
	* the end-of-file on blocksize < PAGE_SIZE setups.
	*
	*/

	#include <linux/kernel.h>
	#include <linux/export.h>
	#include <linux/mm.h>
	#include <linux/kdev_t.h>
	#include <linux/gfp.h>
	#include <linux/bio.h>
	#include <linux/fs.h>
	#include <linux/buffer_head.h>
	#include <linux/blkdev.h>
	#include <linux/highmem.h>
	#include <linux/prefetch.h>
	#include <linux/mpage.h>
	#include <linux/writeback.h>
	#include <linux/backing-dev.h>
	#include <linux/pagevec.h>

	#include "ext4.h"

	#define NUM_PREALLOC_POST_READ_CTXS 128

	static struct kmem_cache *bio_post_read_ctx_cache;
	static mempool_t *bio_post_read_ctx_pool;

	/* postprocessing steps for read bios */
	enum bio_post_read_step {
	STEP_INITIAL = 0,
	STEP_DECRYPT,
	STEP_VERITY,
	STEP_MAX,
	};

	struct bio_post_read_ctx {
	struct bio *bio;
	struct work_struct work;
	unsigned int cur_step;
	unsigned int enabled_steps;
	};

	static void __read_end_io(struct bio *bio)
	{
	struct folio_iter fi;

	bio_for_each_folio_all(fi, bio)
	folio_end_read(fi.folio, bio->bi_status == 0);
	if (bio->bi_private)
	mempool_free(bio->bi_private, bio_post_read_ctx_pool);
	bio_put(bio);
	}

	static void bio_post_read_processing(struct bio_post_read_ctx *ctx);

	static void decrypt_work(struct work_struct *work)
	{
	struct bio_post_read_ctx *ctx =
	container_of(work, struct bio_post_read_ctx, work);
	struct bio *bio = ctx->bio;

	if (fscrypt_decrypt_bio(bio))
	bio_post_read_processing(ctx);
	else
	__read_end_io(bio);
	}

	static void verity_work(struct work_struct *work)
	{
	struct bio_post_read_ctx *ctx =
	container_of(work, struct bio_post_read_ctx, work);
	struct bio *bio = ctx->bio;

	/*
	* fsverity_verify_bio() may call readahead() again, and although verity
	* will be disabled for that, decryption may still be needed, causing
	* another bio_post_read_ctx to be allocated. So to guarantee that
	* mempool_alloc() never deadlocks we must free the current ctx first.
	* This is safe because verity is the last post-read step.
	*/
	BUILD_BUG_ON(STEP_VERITY + 1 != STEP_MAX);
	mempool_free(ctx, bio_post_read_ctx_pool);
	bio->bi_private = NULL;

	fsverity_verify_bio(bio);

	__read_end_io(bio);
	}

	static void bio_post_read_processing(struct bio_post_read_ctx *ctx)
	{
	/*
	* We use different work queues for decryption and for verity because
	* verity may require reading metadata pages that need decryption, and
	* we shouldn't recurse to the same workqueue.
	*/
	switch (++ctx->cur_step) {
	case STEP_DECRYPT:
	if (ctx->enabled_steps & (1 << STEP_DECRYPT)) {
	INIT_WORK(&ctx->work, decrypt_work);
	fscrypt_enqueue_decrypt_work(&ctx->work);
	return;
	}
	ctx->cur_step++;
	fallthrough;
	case STEP_VERITY:
	if (ctx->enabled_steps & (1 << STEP_VERITY)) {
	INIT_WORK(&ctx->work, verity_work);
	fsverity_enqueue_verify_work(&ctx->work);
	return;
	}
	ctx->cur_step++;
	fallthrough;
	default:
	__read_end_io(ctx->bio);
	}
	}

	static bool bio_post_read_required(struct bio *bio)
	{
	return bio->bi_private && !bio->bi_status;
	}

	/*
	* I/O completion handler for multipage BIOs.
	*
	* The mpage code never puts partial pages into a BIO (except for end-of-file).
	* If a page does not map to a contiguous run of blocks then it simply falls
	* back to block_read_full_folio().
	*
	* Why is this? If a page's completion depends on a number of different BIOs
	* which can complete in any order (or at the same time) then determining the
	* status of that page is hard. See end_buffer_async_read() for the details.
	* There is no point in duplicating all that complexity.
	*/
	static void mpage_end_io(struct bio *bio)
	{
	if (bio_post_read_required(bio)) {
	struct bio_post_read_ctx *ctx = bio->bi_private;

	ctx->cur_step = STEP_INITIAL;
	bio_post_read_processing(ctx);
	return;
	}
	__read_end_io(bio);
	}

	static inline bool ext4_need_verity(const struct inode *inode, pgoff_t idx)
	{
	return fsverity_active(inode) &&
	idx < DIV_ROUND_UP(inode->i_size, PAGE_SIZE);
	}

	static void ext4_set_bio_post_read_ctx(struct bio *bio,
	const struct inode *inode,
	pgoff_t first_idx)
	{
	unsigned int post_read_steps = 0;

	if (fscrypt_inode_uses_fs_layer_crypto(inode))
	post_read_steps \|= 1 << STEP_DECRYPT;

	if (ext4_need_verity(inode, first_idx))
	post_read_steps \|= 1 << STEP_VERITY;

	if (post_read_steps) {
	/* Due to the mempool, this never fails. */
	struct bio_post_read_ctx *ctx =
	mempool_alloc(bio_post_read_ctx_pool, GFP_NOFS);

	ctx->bio = bio;
	ctx->enabled_steps = post_read_steps;
	bio->bi_private = ctx;
	}
	}

	static inline loff_t ext4_readpage_limit(struct inode *inode)
	{
	if (IS_ENABLED(CONFIG_FS_VERITY) && IS_VERITY(inode))
	return inode->i_sb->s_maxbytes;

	return i_size_read(inode);
	}

	int ext4_mpage_readpages(struct inode *inode,
	struct readahead_control rac, struct folio folio)
	{
	struct bio *bio = NULL;
	sector_t last_block_in_bio = 0;

	const unsigned blkbits = inode->i_blkbits;
	const unsigned blocks_per_page = PAGE_SIZE >> blkbits;
	const unsigned blocksize = 1 << blkbits;
	sector_t next_block;
	sector_t block_in_file;
	sector_t last_block;
	sector_t last_block_in_file;
	sector_t blocks[MAX_BUF_PER_PAGE];
	unsigned page_block;
	struct block_device *bdev = inode->i_sb->s_bdev;
	int length;
	unsigned relative_block = 0;
	struct ext4_map_blocks map;
	unsigned int nr_pages = rac ? readahead_count(rac) : 1;

	map.m_pblk = 0;
	map.m_lblk = 0;
	map.m_len = 0;
	map.m_flags = 0;

	for (; nr_pages; nr_pages--) {
	int fully_mapped = 1;
	unsigned first_hole = blocks_per_page;

	if (rac)
	folio = readahead_folio(rac);
	prefetchw(&folio->flags);

	if (folio_buffers(folio))
	goto confused;

	block_in_file = next_block =
	(sector_t)folio->index << (PAGE_SHIFT - blkbits);
	last_block = block_in_file + nr_pages * blocks_per_page;
	last_block_in_file = (ext4_readpage_limit(inode) +
	blocksize - 1) >> blkbits;
	if (last_block > last_block_in_file)
	last_block = last_block_in_file;
	page_block = 0;

	/*
	* Map blocks using the previous result first.
	*/
	if ((map.m_flags & EXT4_MAP_MAPPED) &&
	block_in_file > map.m_lblk &&
	block_in_file < (map.m_lblk + map.m_len)) {
	unsigned map_offset = block_in_file - map.m_lblk;
	unsigned last = map.m_len - map_offset;

	for (relative_block = 0; ; relative_block++) {
	if (relative_block == last) {
	/* needed? */
	map.m_flags &= ~EXT4_MAP_MAPPED;
	break;
	}
	if (page_block == blocks_per_page)
	break;
	blocks[page_block] = map.m_pblk + map_offset +
	relative_block;
	page_block++;
	block_in_file++;
	}
	}

	/*
	* Then do more ext4_map_blocks() calls until we are
	* done with this folio.
	*/
	while (page_block < blocks_per_page) {
	if (block_in_file < last_block) {
	map.m_lblk = block_in_file;
	map.m_len = last_block - block_in_file;

	if (ext4_map_blocks(NULL, inode, &map, 0) < 0) {
	set_error_page:
	folio_set_error(folio);
	folio_zero_segment(folio, 0,
	folio_size(folio));
	folio_unlock(folio);
	goto next_page;
	}
	}
	if ((map.m_flags & EXT4_MAP_MAPPED) == 0) {
	fully_mapped = 0;
	if (first_hole == blocks_per_page)
	first_hole = page_block;
	page_block++;
	block_in_file++;
	continue;
	}
	if (first_hole != blocks_per_page)
	goto confused; /* hole -> non-hole */

	/* Contiguous blocks? */
	if (page_block && blocks[page_block-1] != map.m_pblk-1)
	goto confused;
	for (relative_block = 0; ; relative_block++) {
	if (relative_block == map.m_len) {
	/* needed? */
	map.m_flags &= ~EXT4_MAP_MAPPED;
	break;
	} else if (page_block == blocks_per_page)
	break;
	blocks[page_block] = map.m_pblk+relative_block;
	page_block++;
	block_in_file++;
	}
	}
	if (first_hole != blocks_per_page) {
	folio_zero_segment(folio, first_hole << blkbits,
	folio_size(folio));
	if (first_hole == 0) {
	if (ext4_need_verity(inode, folio->index) &&
	!fsverity_verify_folio(folio))
	goto set_error_page;
	folio_end_read(folio, true);
	continue;
	}
	} else if (fully_mapped) {
	folio_set_mappedtodisk(folio);
	}

	/*
	* This folio will go to BIO. Do we need to send this
	* BIO off first?
	*/
	if (bio && (last_block_in_bio != blocks[0] - 1 \|\|
	!fscrypt_mergeable_bio(bio, inode, next_block))) {
	submit_and_realloc:
	submit_bio(bio);
	bio = NULL;
	}
	if (bio == NULL) {
	/*
	* bio_alloc will _always_ be able to allocate a bio if
	* __GFP_DIRECT_RECLAIM is set, see bio_alloc_bioset().
	*/
	bio = bio_alloc(bdev, bio_max_segs(nr_pages),
	REQ_OP_READ, GFP_KERNEL);
	fscrypt_set_bio_crypt_ctx(bio, inode, next_block,
	GFP_KERNEL);
	ext4_set_bio_post_read_ctx(bio, inode, folio->index);
	bio->bi_iter.bi_sector = blocks[0] << (blkbits - 9);
	bio->bi_end_io = mpage_end_io;
	if (rac)
	bio->bi_opf \|= REQ_RAHEAD;
	}

	length = first_hole << blkbits;
	if (!bio_add_folio(bio, folio, length, 0))
	goto submit_and_realloc;

	if (((map.m_flags & EXT4_MAP_BOUNDARY) &&
	(relative_block == map.m_len)) \|\|
	(first_hole != blocks_per_page)) {
	submit_bio(bio);
	bio = NULL;
	} else
	last_block_in_bio = blocks[blocks_per_page - 1];
	continue;
	confused:
	if (bio) {
	submit_bio(bio);
	bio = NULL;
	}
	if (!folio_test_uptodate(folio))
	block_read_full_folio(folio, ext4_get_block);
	else
	folio_unlock(folio);
	next_page:
	; /* A label shall be followed by a statement until C23 */
	}
	if (bio)
	submit_bio(bio);
	return 0;
	}

	int __init ext4_init_post_read_processing(void)
	{
	bio_post_read_ctx_cache = KMEM_CACHE(bio_post_read_ctx, SLAB_RECLAIM_ACCOUNT);

	if (!bio_post_read_ctx_cache)
	goto fail;
	bio_post_read_ctx_pool =
	mempool_create_slab_pool(NUM_PREALLOC_POST_READ_CTXS,
	bio_post_read_ctx_cache);
	if (!bio_post_read_ctx_pool)
	goto fail_free_cache;
	return 0;

	fail_free_cache:
	kmem_cache_destroy(bio_post_read_ctx_cache);
	fail:
	return -ENOMEM;
	}

	void ext4_exit_post_read_processing(void)
	{
	mempool_destroy(bio_post_read_ctx_pool);
	kmem_cache_destroy(bio_post_read_ctx_cache);
	}