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Date:   Wed, 18 Oct 2023 04:41:44 +0900
From:   Ryusuke Konishi <konishi.ryusuke@...il.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, gfs2@...ts.linux.dev,
        linux-nilfs@...r.kernel.org, linux-ntfs-dev@...ts.sourceforge.net,
        ntfs3@...ts.linux.dev, ocfs2-devel@...ts.linux.dev,
        reiserfs-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v2 01/27] buffer: Return bool from grow_dev_folio()

On Tue, Oct 17, 2023 at 5:11 AM Matthew Wilcox (Oracle) wrote:
>
> Rename grow_dev_page() to grow_dev_folio() and make it return a bool.
> Document what that bool means; it's more subtle than it first appears.
> Also rename the 'failed' label to 'unlock' beacuse it's not exactly
> 'failed'.  It just hasn't succeeded.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  fs/buffer.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index b33cc74e9649..dec41d84044b 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1024,24 +1024,26 @@ static sector_t folio_init_buffers(struct folio *folio,
>  }
>
>  /*
> - * Create the page-cache page that contains the requested block.
> + * Create the page-cache folio that contains the requested block.
>   *
>   * This is used purely for blockdev mappings.
> + *
> + * Returns false if we have a 'permanent' failure.  Returns true if
> + * we succeeded, or the caller should retry.
>   */
> -static int
> -grow_dev_page(struct block_device *bdev, sector_t block,
> +static bool grow_dev_folio(struct block_device *bdev, sector_t block,
>               pgoff_t index, int size, int sizebits, gfp_t gfp)
>  {
>         struct inode *inode = bdev->bd_inode;
>         struct folio *folio;
>         struct buffer_head *bh;
>         sector_t end_block;
> -       int ret = 0;
> +       bool ret;
>
>         folio = __filemap_get_folio(inode->i_mapping, index,
>                         FGP_LOCK | FGP_ACCESSED | FGP_CREAT, gfp);
>         if (IS_ERR(folio))
> -               return PTR_ERR(folio);
> +               return false;
>
>         bh = folio_buffers(folio);
>         if (bh) {
> @@ -1050,14 +1052,17 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>                                         (sector_t)index << sizebits, size);
>                         goto done;
>                 }
> +
> +               /* Caller should retry if this call fails */
> +               ret = true;
>                 if (!try_to_free_buffers(folio))
> -                       goto failed;
> +                       goto unlock;
>         }
>
> -       ret = -ENOMEM;
> +       ret = false;
>         bh = folio_alloc_buffers(folio, size, gfp | __GFP_ACCOUNT);
>         if (!bh)
> -               goto failed;
> +               goto unlock;
>
>         /*
>          * Link the folio to the buffers and initialise them.  Take the
> @@ -1070,19 +1075,19 @@ grow_dev_page(struct block_device *bdev, sector_t block,
>                         (sector_t)index << sizebits, size);
>         spin_unlock(&inode->i_mapping->private_lock);
>  done:
> -       ret = (block < end_block) ? 1 : -ENXIO;
> -failed:
> +       ret = block < end_block;
> +unlock:
>         folio_unlock(folio);
>         folio_put(folio);
>         return ret;
>  }
>
>  /*
> - * Create buffers for the specified block device block's page.  If
> - * that page was dirty, the buffers are set dirty also.
> + * Create buffers for the specified block device block's folio.  If
> + * that folio was dirty, the buffers are set dirty also.
>   */
> -static int
> -grow_buffers(struct block_device *bdev, sector_t block, int size, gfp_t gfp)
> +static bool grow_buffers(struct block_device *bdev, sector_t block,
> +               int size, gfp_t gfp)
>  {
>         pgoff_t index;
>         int sizebits;
> @@ -1099,11 +1104,11 @@ grow_buffers(struct block_device *bdev, sector_t block, int size, gfp_t gfp)
>                         "device %pg\n",
>                         __func__, (unsigned long long)block,
>                         bdev);
> -               return -EIO;
> +               return false;
>         }
>
> -       /* Create a page with the proper size buffers.. */
> -       return grow_dev_page(bdev, block, index, size, sizebits, gfp);
> +       /* Create a folio with the proper size buffers.. */
> +       return grow_dev_folio(bdev, block, index, size, sizebits, gfp);
>  }
>
>  static struct buffer_head *
> --
> 2.40.1

This changes the return type of grow_buffers() from "int"  to "bool".
But, it seems that the caller, __getblk_slow(), has not changed the
type of the variable "ret" that receives its return value:

        for (;;) {
                struct buffer_head *bh;
                int ret;

                bh = __find_get_block(bdev, block, size);
                if (bh)
                        return bh;

                ret = grow_buffers(bdev, block, size, gfp);
                if (ret < 0)
                        return NULL;
        }

So, it looks like the error check immediately after calling
grow_buffers() will not branch like before.
Is this okay ?   Or, am I missing some other changes?

Also, there is a typo in the changelog: "beacuse" -> "because".

Regards,
Ryusuke Konishi

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