Re: [f2fs-dev] [PATCH v2 03/14] f2fs: support regular file buffered writes on large folios

[Daeho Jeong <daeho43@gmail.com>]

On Mon, Sep 14, 2026 at 9:24 PM Nanzhe Zhao via Linux-f2fs-devel
<linux-f2fs-devel@lists.sourceforge.net> 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 | 300 +++++++++++++++++++++++++++++++++++++++++++++++--
>  fs/f2fs/f2fs.h |   1 +
>  fs/f2fs/file.c |  17 ++-
>  3 files changed, 304 insertions(+), 14 deletions(-)
>
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index f14044e8a6f4..a490df4f50b3 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -1233,6 +1233,35 @@ 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 = folio->private;
> +
> +       /* 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_folio);
> @@ -2570,6 +2599,114 @@ 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 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);
> +}
> +
> +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 void f2fs_ffs_mark_subrange_dirty(struct folio *folio,
> +                                   size_t offset, size_t len)
> +{
> +       struct f2fs_folio_state *ffs;
> +       unsigned int nr_subpages, start, end;
> +       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;
> +       nr_subpages = folio_nr_pages(folio);
> +       start = offset >> PAGE_SHIFT;
> +       end = (offset + len + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +       end = min(end, nr_subpages);
> +
> +       spin_lock_irqsave(&ffs->state_lock, flags);
> +       bitmap_set(ffs->state, nr_subpages + start, end - start);
> +       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 - 1, 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)
> @@ -3927,6 +4064,110 @@ static int prepare_atomic_write_begin(struct f2fs_sb_info *sbi,
>         return 0;
>  }
>
> +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;
> +
> +       len = min_t(unsigned int, len, folio_size(folio) - ori_off);
> +
> +       /*
> +        * When folio minimum order is non-zero, the fsverity
> +        * page_cache_write() path enters f2fs_write_begin() via
> +        * aops->write_begin without going through f2fs_write_iter(),
> +        * so preallocation from f2fs_write_iter() is skipped.  In that
> +        * case, if FI_PREALLOCATED_ALL is not set, we must preallocate
> +        * the write blocks here.
> +        */
> +       if (!is_inode_flag_set(inode, FI_PREALLOCATED_ALL)) {
> +               struct f2fs_map_blocks map = {};
> +
> +               map.m_lblk = F2FS_BYTES_TO_BLK(sbi, pos);
> +               map.m_len = F2FS_BLK_ALIGN(sbi, pos + len) - map.m_lblk;
> +
> +               if (!IS_DEVICE_ALIASING(inode))
> +                       map.m_may_create = true;
> +               map.m_seg_type = NO_CHECK_TYPE;
> +
> +               err = f2fs_map_blocks(inode, &map, F2FS_GET_BLOCK_PRE_AIO);
> +               if (err)
> +                       return err;
> +       }

Hi Nanzhe,

This runs with the folio locked, but f2fs_map_blocks() with m_may_create
calls f2fs_balance_fs(), right? If foreground GC then needs this
folio, it can deadlock.

The comment only mentions the fsverity path, but the condition is just
!FI_PREALLOCATED_ALL, so regular writes can get here too: Maybe, short writes
(fault_in fails), partial preallocation on -ENOSPC/-EDQUOT, and DIO
falling back to buffered I/O. On the fsverity path, FI_PREALLOCATED_ALL
is never set, so it always runs this with the folio locked.

How about only reserving blocks here, without balancing (e.g.
f2fs_reserve_block() and collecting node_changed)? f2fs_write_begin()
could then do the balance after unlocking and putting the folio, and
retry with "goto repeat".

Thanks,

> +
> +       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;
> +               bool get_dn = false;
> +
> +               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;
> +                       get_dn = true;
> +                       blkaddr = dn.data_blkaddr;
> +
> +                       if (blkaddr == NEW_ADDR) {
> +                               size_t off = offset_in_folio(folio,
> +                                                            index << PAGE_SHIFT);
> +
> +                               folio_zero_segment(folio, off, off + PAGE_SIZE);
> +                               f2fs_ffs_mark_subrange_uptodate(folio, off,
> +                                                              PAGE_SIZE);
> +                               goto out;
> +                       }
> +
> +                       if (!f2fs_is_valid_blkaddr(sbi, blkaddr,
> +                                                 DATA_GENERIC_ENHANCE_READ)) {
> +                               err = -EFSCORRUPTED;
> +                               goto out;
> +                       }
> +               }
> +
> +               err = f2fs_submit_page_read_sync(inode, folio, index, blkaddr);
> +out:
> +               if (get_dn)
> +                       f2fs_put_dnode(&dn);
> +               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,
> @@ -3938,6 +4179,7 @@ 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;
>         int err = 0;
>
>         trace_f2fs_write_begin(inode, pos, len);
> @@ -3985,9 +4227,15 @@ 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;
> @@ -4000,7 +4248,7 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>         if (f2fs_is_atomic_file(inode))
>                 err = prepare_atomic_write_begin(sbi, folio, pos, len,
>                                         &blkaddr, &need_balance);
> -       else
> +       else if (!folio_test_large(folio))
>                 err = prepare_write_begin(sbi, folio, pos, len,
>                                         &blkaddr, &need_balance);
>         if (err)
> @@ -4021,6 +4269,14 @@ static int f2fs_write_begin(const struct kiocb *iocb,
>
>         f2fs_folio_wait_writeback(folio, DATA, false, true);
>
> +       if (folio_test_large(folio)) {
> +               err = prepare_large_folio_write_begin(inode,
> +                                       folio, pos, len);
> +               if (!err)
> +                       return 0;
> +               goto put_folio;
> +       }
> +
>         if (len == folio_size(folio) || folio_test_uptodate(folio))
>                 return 0;
>
> @@ -4081,15 +4337,20 @@ 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
> @@ -4108,6 +4369,9 @@ 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))
> @@ -4170,8 +4434,22 @@ 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 326382ede1c3..8eccca6e5d27 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -4392,6 +4392,7 @@ 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 b0bdc7a977b9..9071bd23e57b 100644
> --- a/fs/f2fs/file.c
> +++ b/fs/f2fs/file.c
> @@ -5412,9 +5412,20 @@ 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
> --
> 2.43.0
>
>
>
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