Re: [PATCH v5 03/16] f2fs: support regular file buffered writes on large folios

[Chao Yu <chao@kernel.org>]

On 10/14/26 14:44, Nanzhe Zhao wrote:
> To avoid the complexity of unlocking a large folio in write_begin,
> preallocate partial blocks for inodes that can use large folios.
> During write_begin, read only the partial head and tail 4K subpages
> that need read-before-write, and skip read I/O for the full middle
> subpages covered by the write.
> 
> Signed-off-by: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
> ---
>   fs/f2fs/data.c | 405 ++++++++++++++++++++++++++++++++++++++++++++++---
>   fs/f2fs/f2fs.h |   1 +
>   fs/f2fs/file.c |  17 ++-
>   3 files changed, 399 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 429fe8998ca3..a1d2f3b7d9df 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -93,6 +93,20 @@ struct bio_post_read_ctx {
>   	block_t fs_blkaddr;
>   };
>   
> +static bool __ffs_mark_subrange_uptodate(struct folio *folio,
> +		struct f2fs_folio_state *ffs, size_t offset, size_t len)
> +{
> +	unsigned int nr_subpages = folio_nr_pages(folio);
> +	unsigned int start, end;
> +
> +	start = offset >> PAGE_SHIFT;
> +	end = (offset + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	end = min(end, nr_subpages);
> +
> +	bitmap_set(ffs->state, start, end - start);
> +	return bitmap_full(ffs->state, nr_subpages);
> +}
> +
>   /*
>    * Update and unlock a bio's pages, and free the bio.
>    *
> @@ -107,6 +121,23 @@ struct bio_post_read_ctx {
>    * called (i.e., I/O error or decryption error, but *not* verity error), and
>    * release the bio's reference to the decompress_io_ctx of the page's cluster.
>    */
> +/*
> + * Update read_pages_pending.
> + */
> +static inline void f2fs_update_read_folio_pending(struct folio *folio, int nr)
> +{
> +	struct f2fs_folio_state *ffs;
> +	unsigned long flags;
> +
> +	if (!f2fs_folio_has_ffs(folio))
> +		return;
> +
> +	ffs = (struct f2fs_folio_state *)folio->private;
> +	spin_lock_irqsave(&ffs->state_lock, flags);
> +	ffs->read_pages_pending += nr;
> +	spin_unlock_irqrestore(&ffs->state_lock, flags);
> +}
> +
>   static void f2fs_finish_read_bio(struct bio *bio, bool in_task)
>   {
>   	struct folio_iter fi;
> @@ -128,7 +159,8 @@ static void f2fs_finish_read_bio(struct bio *bio, bool in_task)
>   		}
>   
>   		if (folio_test_large(folio)) {
> -			struct f2fs_folio_state *ffs = folio->private;
> +			struct f2fs_folio_state *ffs =
> +				(struct f2fs_folio_state *)folio->private;
>   
>   			spin_lock_irqsave(&ffs->state_lock, flags);
>   			ffs->read_pages_pending -= nr_pages;
> @@ -1424,19 +1456,26 @@ static struct bio *f2fs_grab_read_bio(struct inode *inode,
>   
>   /* This can handle encryption stuffs */
>   static void f2fs_submit_page_read(struct inode *inode, struct fsverity_info *vi,
> -				  struct folio *folio, block_t blkaddr,
> -				  blk_opf_t op_flags, bool for_write)
> +				  struct folio *folio, pgoff_t index,
> +				  block_t blkaddr, blk_opf_t op_flags,
> +				  bool for_write)
>   {
>   	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>   	struct bio *bio;
> +	size_t offset = 0;
> +
> +	if (f2fs_folio_has_ffs(folio)) {
> +		offset = offset_in_folio(folio, (loff_t)index << PAGE_SHIFT);
> +		f2fs_update_read_folio_pending(folio, 1);
> +	}
>   
> -	bio = f2fs_grab_read_bio(inode, vi, blkaddr, 1, op_flags, folio->index,
> +	bio = f2fs_grab_read_bio(inode, vi, blkaddr, 1, op_flags, index,
>   				 for_write);
>   
>   	/* wait for GCed page writeback via generic cache */
>   	f2fs_wait_on_block_writeback(inode, blkaddr);
>   
> -	if (!bio_add_folio(bio, folio, PAGE_SIZE, 0))
> +	if (!bio_add_folio(bio, folio, PAGE_SIZE, offset))
>   		f2fs_bug_on(sbi, 1);
>   
>   	inc_cache_count(sbi, F2FS_RD_DATA);
> @@ -1444,6 +1483,39 @@ static void f2fs_submit_page_read(struct inode *inode, struct fsverity_info *vi,
>   	f2fs_submit_read_bio(sbi, bio, DATA);
>   }
>   
> +/*
> + * Synchronously read a single 4K subpage by reusing f2fs_submit_page_read()
> + * so that iostat, trace, blk-crypto and post-read handling are all preserved.
> + * The caller must have already allocated ffs for the folio.
> + */
> +static int f2fs_submit_page_read_sync(struct inode *inode, struct folio *folio,
> +				      pgoff_t index, block_t blkaddr)
> +{
> +	struct f2fs_folio_state *ffs =
> +		(struct f2fs_folio_state *)folio->private;
> +
> +	if (folio_test_large(folio) && !f2fs_folio_has_ffs(folio))
> +		return -EAGAIN;
> +
> +	/* Add bias so end_io does not call folio_end_read(). */
> +	f2fs_update_read_folio_pending(folio, 1);
> +
> +	f2fs_submit_page_read(inode, NULL, folio, index, blkaddr,
> +			      REQ_OP_READ, false);
> +
> +	/* Wait until all bios have completed (pending drops back to our bias). */
> +	while (READ_ONCE(ffs->read_pages_pending) != 1)
> +		f2fs_io_schedule_timeout(DEFAULT_SCHEDULE_TIMEOUT);
> +
> +	/* Remove the bias. */
> +	f2fs_update_read_folio_pending(folio, -1);
> +
> +	if (!f2fs_ffs_test_blk_uptodate(folio, index))
> +		return -EIO;
> +
> +	return 0;
> +}
> +
>   static void __set_data_blkaddr(struct dnode_of_data *dn, block_t blkaddr)
>   {
>   	__le32 *addr = get_dnode_addr(dn->inode, dn->node_entry);
> @@ -1617,8 +1689,8 @@ struct folio *f2fs_get_read_data_folio(struct inode *inode, pgoff_t index,
>   		return folio;
>   	}
>   
> -	f2fs_submit_page_read(inode, f2fs_need_verity(inode, folio->index),
> -			      folio, dn.data_blkaddr, op_flags, for_write);
> +	f2fs_submit_page_read(inode, f2fs_need_verity(inode, index),
> +			      folio, index, dn.data_blkaddr, op_flags, for_write);
>   	return folio;
>   
>   put_err:
> @@ -2780,6 +2852,110 @@ static void ffs_detach_free(struct folio *folio)
>   	kfree(ffs);
>   }
>   
> +bool f2fs_ffs_test_blk_uptodate(const struct folio *folio, pgoff_t index)
> +{
> +	struct f2fs_folio_state *ffs;
> +	unsigned int idx;
> +
> +	f2fs_bug_on(F2FS_F_SB(folio), !folio_contains(folio, index));
> +
> +	if (folio_test_uptodate(folio))
> +		return true;
> +
> +	if (!f2fs_folio_has_ffs(folio))
> +		return false;
> +
> +	ffs = (struct f2fs_folio_state *)folio->private;
> +	idx = index - folio->index;
> +	return test_bit(idx, ffs->state);
> +}
> +
> +static void f2fs_ffs_mark_subrange_uptodate(struct folio *folio, size_t offset,
> +				       size_t len)
> +{
> +	struct f2fs_folio_state *ffs;
> +	unsigned long flags;
> +	bool mark_uptodate = false;
> +
> +	f2fs_bug_on(F2FS_F_SB(folio), offset + len > folio_size(folio));
> +
> +	if (!f2fs_folio_has_ffs(folio)) {
> +		folio_mark_uptodate(folio);
> +		return;
> +	}
> +
> +	ffs = (struct f2fs_folio_state *)folio->private;
> +	spin_lock_irqsave(&ffs->state_lock, flags);
> +	mark_uptodate = __ffs_mark_subrange_uptodate(folio, ffs, offset, len) &&
> +			!ffs->read_pages_pending;
> +	spin_unlock_irqrestore(&ffs->state_lock, flags);
> +	if (mark_uptodate)
> +		folio_mark_uptodate(folio);
> +}
> +
> +static unsigned int __ffs_mark_subrange_dirty(struct folio *folio,
> +			struct f2fs_folio_state *ffs, size_t offset, size_t len)
> +{
> +	unsigned int nr_subpages = folio_nr_pages(folio);
> +	unsigned int start, end, before;
> +
> +	start = offset >> PAGE_SHIFT;
> +	end = (offset + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	end = min(end, nr_subpages);
> +
> +	before = bitmap_weight(ffs->state, 2 * nr_subpages);
> +	bitmap_set(ffs->state, nr_subpages + start, end - start);
> +	return bitmap_weight(ffs->state, 2 * nr_subpages) - before;
> +}
> +
> +static void f2fs_ffs_mark_subrange_dirty(struct folio *folio,
> +				    size_t offset, size_t len)
> +{
> +	struct f2fs_folio_state *ffs;
> +	unsigned long flags;
> +
> +	f2fs_bug_on(F2FS_F_SB(folio), offset + len > folio_size(folio));
> +
> +	if (!f2fs_folio_has_ffs(folio))
> +		return;
> +
> +	ffs = (struct f2fs_folio_state *)folio->private;
> +
> +	spin_lock_irqsave(&ffs->state_lock, flags);
> +	__ffs_mark_subrange_dirty(folio, ffs, offset, len);
> +	spin_unlock_irqrestore(&ffs->state_lock, flags);
> +}
> +
> +static bool find_next_valid_block(const struct folio *folio,
> +				  size_t orig_off, size_t *need_off,
> +				  size_t len)
> +{
> +	size_t start = orig_off;
> +	size_t end = start + len;
> +	size_t head, tail;
> +	pgoff_t index;
> +
> +	if (start & (PAGE_SIZE - 1)) {
> +		head = round_down(start, PAGE_SIZE);
> +		index = folio->index + (head >> PAGE_SHIFT);
> +		if (!f2fs_ffs_test_blk_uptodate(folio, index)) {
> +			*need_off = head;
> +			return true;
> +		}
> +	}
> +
> +	if (end & (PAGE_SIZE - 1)) {
> +		tail = round_down(end, PAGE_SIZE);
> +		index = folio->index + (tail >> PAGE_SHIFT);
> +		if (!f2fs_ffs_test_blk_uptodate(folio, index)) {
> +			*need_off = tail;
> +			return true;
> +		}
> +	}
> +
> +	return false;
> +}
> +
>   static int f2fs_read_data_large_folio(struct inode *inode,
>   		struct fsverity_info *vi,
>   		struct readahead_control *rac, struct folio *folio)
> @@ -4145,6 +4321,147 @@ static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi,
>   	return 0;
>   }
>   
> +/*
> + * Drop the folio lock when a node was changed, then balance the
> + * filesystem and reacquire the lock.
> + *
> + * Return: true if the folio became unusable while unlocked (being
> + * truncated), false if the folio is still usable.
> + */
> +static bool f2fs_balance_fs_folio_locked(struct folio *folio,
> +		struct address_space *mapping, bool node_changed)
> +{
> +	struct inode *inode = folio->mapping->host;
> +	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> +	unsigned int orig_order;
> +	bool had_ffs;
> +
> +	if (!node_changed || IS_NOQUOTA(inode) ||
> +			!has_not_enough_free_secs(sbi, 0, 0))
> +		return false;
> +
> +	had_ffs = f2fs_folio_has_ffs(folio);
> +	orig_order = folio_order(folio);
> +	folio_unlock(folio);
> +	f2fs_balance_fs(sbi, true);
> +	folio_lock(folio);
> +
> +	/*
> +	 * Writeback may have started on the folio during the unlock
> +	 * window, so wait for it before the caller proceeds.
> +	 */
> +	f2fs_folio_wait_writeback(folio, true, true);
> +
> +	if (folio->mapping != mapping)
> +		return true;
> +
> +	if (folio_order(folio) != orig_order)
> +		return true;
> +
> +	/* large folio was released */
> +	if (had_ffs && !f2fs_folio_has_ffs(folio))
> +		return true;
> +
> +	return false;
> +}
> +
> +static int prepare_large_folio_write_begin(struct inode *inode,
> +					  struct folio *folio, loff_t pos,
> +					  unsigned int len)
> +{
> +	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> +	struct f2fs_folio_state *ffs;
> +	size_t ori_off = offset_in_folio(folio, pos);
> +	size_t need_off = ori_off;
> +	int err = 0;
> +
> +	/*
> +	 * when the write did not come through f2fs_write_iter, we preallocate all
> +	 * blocks in the write range here.
> +	 */
> +	if (!is_inode_flag_set(inode, FI_PREALLOCATED_ALL) &&
> +	    !IS_DEVICE_ALIASING(inode)) {
> +		pgoff_t first = F2FS_BYTES_TO_BLK(sbi, pos);
> +		pgoff_t last = F2FS_BLK_ALIGN(sbi, pos + len);
> +		pgoff_t idx;
> +
> +		for (idx = first; idx < last; idx++) {
> +			block_t blkaddr = NULL_ADDR;
> +			bool node_changed = false;
> +
> +			err = __reserve_data_block(inode, idx, &blkaddr,
> +						   &node_changed);
> +			if (err)
> +				return err;
> +			if (f2fs_balance_fs_folio_locked(folio, folio->mapping,
> +						   node_changed))
> +				return -EAGAIN;
> +		}
> +	}
> +
> +	ffs = f2fs_ffs_find_or_alloc(folio);
> +	if (!ffs)
> +		return -ENOMEM;
> +
> +	/* Skip read if the folio is already fully uptodate or the write
> +	 * covers the entire folio.
> +	 */
> +	if (folio_test_uptodate(folio) || len == folio_size(folio))
> +		return 0;
> +
> +	if (!f2fs_verity_in_progress(inode) &&
> +	    !(pos & (PAGE_SIZE - 1)) && (pos + len) >= i_size_read(inode)) {
> +		size_t zoff = ori_off + len;
> +
> +		if (zoff < folio_size(folio))
> +			folio_zero_segment(folio, zoff, folio_size(folio));
> +		return 0;
> +	}
> +
> +	/* Inline data must have been converted before reaching here. */
> +	f2fs_bug_on(sbi, f2fs_has_inline_data(inode));
> +
> +	while (find_next_valid_block(folio, ori_off,
> +				     &need_off, len)) {
> +		struct dnode_of_data dn;
> +		pgoff_t index = folio->index + (need_off >> PAGE_SHIFT);
> +		block_t blkaddr;
> +
> +		if (!f2fs_lookup_read_extent_cache_block(inode, index,
> +							&blkaddr)) {
> +			if (IS_DEVICE_ALIASING(inode))
> +				return -ENODATA;
> +
> +			set_new_dnode(&dn, inode, NULL, NULL, 0);
> +			err = f2fs_get_dnode_of_data(&dn, index, LOOKUP_NODE);
> +			if (err)
> +				return err;
> +			blkaddr = dn.data_blkaddr;
> +			f2fs_put_dnode(&dn);
> +
> +			if (blkaddr == NEW_ADDR) {
> +				size_t off = offset_in_folio(folio,
> +							     (loff_t)index << PAGE_SHIFT);
> +
> +				folio_zero_segment(folio, off, off + PAGE_SIZE);
> +				f2fs_ffs_mark_subrange_uptodate(folio, off,
> +							       PAGE_SIZE);
> +				continue;
> +			}
> +
> +			if (!f2fs_is_valid_blkaddr(sbi, blkaddr,
> +						  DATA_GENERIC_ENHANCE_READ))
> +				return -EFSCORRUPTED;
> +		}
> +
> +		err = f2fs_submit_page_read_sync(inode, folio, index, blkaddr);
> +		if (err)
> +			return err;
> +	}
> +
> +	return 0;
> +}
> +
>   static int f2fs_write_begin(const struct kiocb *iocb,
>   			    struct address_space *mapping,
>   			    loff_t pos, unsigned len, struct folio **foliop,
> @@ -4156,6 +4473,8 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>   	pgoff_t index = pos >> PAGE_SHIFT;
>   	bool need_balance = false;
>   	block_t blkaddr = NULL_ADDR;
> +	fgf_t fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT;
> +	unsigned int blen;
>   	int err = 0;
>   
>   	trace_f2fs_write_begin(inode, pos, len);
> @@ -4203,9 +4522,15 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>   	 * Do not use FGP_STABLE to avoid deadlock.
>   	 * Will wait that below with our IO control.
>   	 */
> -	folio = f2fs_filemap_get_folio(mapping, index,
> -				FGP_LOCK | FGP_WRITE | FGP_CREAT,
> -				mapping_gfp_mask(mapping));
> +	/*
> +	 * Inline data is carried in the first subpage.  Keep the index #0
> +	 * folio order-0 so that the existing inline read/convert paths
> +	 * (prepare_write_begin()) still apply; large folios are used from
> +	 * the second folio on.
> +	 */
> +	fgp |= fgf_set_order(f2fs_has_inline_data(inode) ? PAGE_SIZE : len);
> +	folio = f2fs_filemap_get_folio(mapping, index, fgp,
> +				      mapping_gfp_mask(mapping));
>   	if (IS_ERR(folio)) {
>   		err = PTR_ERR(folio);
>   		goto fail;
> @@ -4215,11 +4540,15 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>   
>   	*foliop = folio;
>   
> +	/* Clamp the write length to the folio size. */
> +	blen = min_t(unsigned int, len,
> +		     folio_size(folio) - offset_in_folio(folio, pos));
> +
>   	if (f2fs_is_atomic_file(inode))
>   		err = prepare_atomic_write_begin(sbi, folio, pos, len,
>   					&blkaddr, &need_balance);
> -	else
> -		err = prepare_write_begin(sbi, folio, pos, len,
> +	else if (!folio_test_large(folio))
> +		err = prepare_write_begin(sbi, folio, pos, blen,
>   					&blkaddr, &need_balance);
>   	if (err)
>   		goto put_folio;
> @@ -4239,7 +4568,19 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>   
>   	f2fs_folio_wait_writeback(folio, false, true);
>   
> -	if (len == folio_size(folio) || folio_test_uptodate(folio))
> +	if (folio_test_large(folio)) {
> +		err = prepare_large_folio_write_begin(inode,
> +					folio, pos, blen);

relocate prepare_large_folio_write_begin() above f2fs_folio_wait_writeback()
like prepare_atomic_write_begin() and prepare_write_begin() did?

Thanks,

> +		if (!err)
> +			return 0;
> +		if (err == -EAGAIN) {
> +			f2fs_folio_put(folio, true);
> +			goto repeat;
> +		}
> +		goto put_folio;
> +	}
> +
> +	if (blen == folio_size(folio) || folio_test_uptodate(folio))
>   		return 0;
>   
>   	if (!(pos & (PAGE_SIZE - 1)) && (pos + len) >= i_size_read(inode) &&
> @@ -4267,7 +4608,7 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>   		 */
>   		f2fs_submit_page_read(inode,
>   				      NULL, /* can't write to fsverity files */
> -				      folio, blkaddr, 0, true);
> +				      folio, index, blkaddr, 0, true);
>   
>   		folio_lock(folio);
>   		if (unlikely(folio->mapping != mapping)) {
> @@ -4299,15 +4640,20 @@ static int f2fs_write_end(const struct kiocb *iocb,
>   	trace_f2fs_write_end(inode, pos, len, copied);
>   
>   	/*
> -	 * This should be come from len == PAGE_SIZE, and we expect copied
> -	 * should be PAGE_SIZE. Otherwise, we treat it with zero copied and
> -	 * let generic_perform_write() try to copy data again through copied=0.
> +	 * If a short copy happens on a folio that isn't uptodate, we treat
> +	 * it with zero copied and let generic_perform_write() try to copy
> +	 * data again through copied=0.
>   	 */
>   	if (!folio_test_uptodate(folio)) {
> -		if (unlikely(copied != len))
> +		if (unlikely(copied != len)) {
>   			copied = 0;
> -		else
> +		} else if (folio_test_large(folio)) {
> +			f2fs_ffs_mark_subrange_uptodate(folio,
> +					offset_in_folio(folio, pos), len);
> +		} else {
> +			/* This should be come from len == PAGE_SIZE */
>   			folio_mark_uptodate(folio);
> +		}
>   	}
>   
>   #ifdef CONFIG_F2FS_FS_COMPRESSION
> @@ -4326,6 +4672,9 @@ static int f2fs_write_end(const struct kiocb *iocb,
>   	if (!copied)
>   		goto unlock_out;
>   
> +	if (folio_test_large(folio))
> +		f2fs_ffs_mark_subrange_dirty(folio, offset_in_folio(folio, pos),
> +								   copied);
>   	folio_mark_dirty(folio);
>   
>   	if (f2fs_is_atomic_file(inode))
> @@ -4382,8 +4731,22 @@ static bool f2fs_dirty_data_folio(struct address_space *mapping,
>   
>   	trace_f2fs_set_page_dirty(folio, DATA);
>   
> -	if (!folio_test_uptodate(folio))
> -		folio_mark_uptodate(folio);
> +	if (!folio_test_uptodate(folio)) {
> +		bool uptodate = true;
> +
> +		if (f2fs_folio_has_ffs(folio)) {
> +			struct f2fs_folio_state *ffs =
> +				(struct f2fs_folio_state *)folio->private;
> +			unsigned long flags;
> +
> +			spin_lock_irqsave(&ffs->state_lock, flags);
> +			uptodate = bitmap_full(ffs->state, folio_nr_pages(folio)) &&
> +				   !ffs->read_pages_pending;
> +			spin_unlock_irqrestore(&ffs->state_lock, flags);
> +		}
> +		if (uptodate)
> +			folio_mark_uptodate(folio);
> +	}
>   	BUG_ON(folio_test_swapcache(folio));
>   
>   	if (filemap_dirty_folio(mapping, folio)) {
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 963cd2cdf7ba..6db23a3c25a9 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -4454,6 +4454,7 @@ int f2fs_write_single_data_page(struct folio *folio, int *submitted,
>   				struct writeback_control *wbc,
>   				enum iostat_type io_type,
>   				int compr_blocks, bool allow_balance);
> +bool f2fs_ffs_test_blk_uptodate(const struct folio *folio, pgoff_t index);
>   void f2fs_write_failed(struct inode *inode, loff_t to);
>   void f2fs_invalidate_folio(struct folio *folio, size_t offset, size_t length);
>   bool f2fs_release_folio(struct folio *folio, gfp_t wait);
> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
> index 1ff505e4ba11..e53cdb4467b4 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -5424,9 +5424,20 @@ static int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *iter,
>   			return ret;
>   	}
>   
> -	/* Do not preallocate blocks that will be written partially in 4KB. */
> -	map.m_lblk = F2FS_BLK_ALIGN(sbi, pos);
> -	map.m_len = F2FS_BYTES_TO_BLK(sbi, pos + count);
> +	if (!dio && mapping_large_folio_support(inode->i_mapping)) {
> +		/*
> +		 * Preallocate all blocks touched by a large-folio buffered write so
> +		 * the regular write_begin path does not need to unlock the folio for
> +		 * f2fs_balance_fs().  Rechecking large-folio state after unlock is
> +		 * unreliable since partial truncation can split the folio.
> +		 */
> +		map.m_lblk = F2FS_BYTES_TO_BLK(sbi, pos);
> +		map.m_len = F2FS_BLK_ALIGN(sbi, pos + count);
> +	} else {
> +		/* Do not preallocate blocks that will be written partially in 4KB. */
> +		map.m_lblk = F2FS_BLK_ALIGN(sbi, pos);
> +		map.m_len = F2FS_BYTES_TO_BLK(sbi, pos + count);
> +	}
>   	if (map.m_len > map.m_lblk)
>   		map.m_len -= map.m_lblk;
>   	else