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From:   =?UTF-8?Q?Andreas_Gr=C3=BCnbacher?= <andreas.gruenbacher@gmail.com>
Date:   Sun, 25 Jul 2021 23:39:51 +0200
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Subject: Re: [PATCH v7] iomap: make inline data support more flexible
To:     Gao Xiang <hsiangkao@linux.alibaba.com>
Cc:     linux-erofs@lists.ozlabs.org,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        "Darrick J . Wong" <djwong@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Huang Jianan <huangjianan@oppo.com>
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Am Fr., 23. Juli 2021 um 19:41 Uhr schrieb Gao Xiang
<hsiangkao@linux.alibaba.com>:
> Add support for reading inline data content into the page cache from
> nonzero page-aligned file offsets.  This enables the EROFS tailpacking
> mode where the last few bytes of the file are stored right after the
> inode.
>
> The buffered write path remains untouched since EROFS cannot be used
> for testing. It'd be better to be implemented if upcoming real users
> care and provide a real pattern rather than leave untested dead code
> around.
>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Darrick J. Wong <djwong@kernel.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Andreas Gruenbacher <andreas.gruenbacher@gmail.com>
> Tested-by: Huang Jianan <huangjianan@oppo.com> # erofs
> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> ---
> v6: https://lore.kernel.org/r/20210722031729.51628-1-hsiangkao@linux.alibaba.com
> changes since v6:
>  - based on Christoph's reply;
>  - update commit message suggested by Darrick;
>  - disable buffered write path until some real fs users.
>
>  fs/iomap/buffered-io.c | 42 ++++++++++++++++++++++++++----------------
>  fs/iomap/direct-io.c   | 10 ++++++----
>  include/linux/iomap.h  | 14 ++++++++++++++
>  3 files changed, 46 insertions(+), 20 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 87ccb3438bec..f351e1f9e3f6 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -205,25 +205,29 @@ struct iomap_readpage_ctx {
>         struct readahead_control *rac;
>  };
>
> -static void
> -iomap_read_inline_data(struct inode *inode, struct page *page,
> -               struct iomap *iomap)
> +static int iomap_read_inline_data(struct inode *inode, struct page *page,
> +               struct iomap *iomap, loff_t pos)

This is completely broken. This function is about filling the page
cache. All the information needed for that is in struct iomap; the
start position of an iomap operation has nothing to do with it.

>  {
> -       size_t size = i_size_read(inode);
> +       size_t size = iomap->length + iomap->offset - pos;
>         void *addr;
>
>         if (PageUptodate(page))
> -               return;
> +               return PAGE_SIZE;
>
> -       BUG_ON(page_has_private(page));
> -       BUG_ON(page->index);
> -       BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> +       /* inline data must start page aligned in the file */
> +       if (WARN_ON_ONCE(offset_in_page(pos)))
> +               return -EIO;
> +       if (WARN_ON_ONCE(!iomap_inline_data_size_valid(iomap)))
> +               return -EIO;
> +       if (WARN_ON_ONCE(page_has_private(page)))
> +               return -EIO;
>
>         addr = kmap_atomic(page);
> -       memcpy(addr, iomap->inline_data, size);
> +       memcpy(addr, iomap_inline_buf(iomap, pos), size);
>         memset(addr + size, 0, PAGE_SIZE - size);
>         kunmap_atomic(addr);
>         SetPageUptodate(page);
> +       return PAGE_SIZE;
>  }
>
>  static inline bool iomap_block_needs_zeroing(struct inode *inode,
> @@ -246,11 +250,8 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
>         unsigned poff, plen;
>         sector_t sector;
>
> -       if (iomap->type == IOMAP_INLINE) {
> -               WARN_ON_ONCE(pos);
> -               iomap_read_inline_data(inode, page, iomap);
> -               return PAGE_SIZE;
> -       }
> +       if (iomap->type == IOMAP_INLINE)
> +               return iomap_read_inline_data(inode, page, iomap, pos);
>
>         /* zero post-eof blocks as the page may be mapped */
>         iop = iomap_page_create(inode, page);
> @@ -589,6 +590,15 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
>         return 0;
>  }
>
> +static int iomap_write_begin_inline(struct inode *inode,
> +               struct page *page, struct iomap *srcmap)
> +{
> +       /* needs more work for the tailpacking case, disable for now */
> +       if (WARN_ON_ONCE(srcmap->offset != 0))
> +               return -EIO;
> +       return iomap_read_inline_data(inode, page, srcmap, 0);
> +}
> +
>  static int
>  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>                 struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> @@ -618,14 +628,14 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
>         }
>
>         if (srcmap->type == IOMAP_INLINE)
> -               iomap_read_inline_data(inode, page, srcmap);
> +               status = iomap_write_begin_inline(inode, page, srcmap);
>         else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
>                 status = __block_write_begin_int(page, pos, len, NULL, srcmap);
>         else
>                 status = __iomap_write_begin(inode, pos, len, flags, page,
>                                 srcmap);
>
> -       if (unlikely(status))
> +       if (unlikely(status < 0))
>                 goto out_unlock;
>
>         *pagep = page;
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 9398b8c31323..a6aaea2764a5 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -378,23 +378,25 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
>                 struct iomap_dio *dio, struct iomap *iomap)
>  {
>         struct iov_iter *iter = dio->submit.iter;
> +       void *dst = iomap_inline_buf(iomap, pos);
>         size_t copied;
>
> -       BUG_ON(pos + length > PAGE_SIZE - offset_in_page(iomap->inline_data));
> +       if (WARN_ON_ONCE(!iomap_inline_data_size_valid(iomap)))
> +               return -EIO;
>
>         if (dio->flags & IOMAP_DIO_WRITE) {
>                 loff_t size = inode->i_size;
>
>                 if (pos > size)
> -                       memset(iomap->inline_data + size, 0, pos - size);
> -               copied = copy_from_iter(iomap->inline_data + pos, length, iter);
> +                       memset(iomap_inline_buf(iomap, size), 0, pos - size);
> +               copied = copy_from_iter(dst, length, iter);
>                 if (copied) {
>                         if (pos + copied > size)
>                                 i_size_write(inode, pos + copied);
>                         mark_inode_dirty(inode);
>                 }
>         } else {
> -               copied = copy_to_iter(iomap->inline_data + pos, length, iter);
> +               copied = copy_to_iter(dst, length, iter);
>         }
>         dio->size += copied;
>         return copied;
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index 479c1da3e221..56b118c6d05c 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -97,6 +97,20 @@ iomap_sector(struct iomap *iomap, loff_t pos)
>         return (iomap->addr + pos - iomap->offset) >> SECTOR_SHIFT;
>  }
>
> +static inline void *iomap_inline_buf(const struct iomap *iomap, loff_t pos)
> +{
> +       return iomap->inline_data - iomap->offset + pos;
> +}
> +
> +/*
> + * iomap->inline_data is a potentially kmapped page, ensure it never crosses a
> + * page boundary.
> + */
> +static inline bool iomap_inline_data_size_valid(const struct iomap *iomap)
> +{
> +       return iomap->length <= PAGE_SIZE - offset_in_page(iomap->inline_data);
> +}
> +
>  /*
>   * When a filesystem sets page_ops in an iomap mapping it returns, page_prepare
>   * and page_done will be called for each page written to.  This only applies to
> --
> 2.24.4
>

Andreas