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    Subject[PATCH 1 of 2] block_page_mkwrite() Implementation V2

    Generic page_mkwrite functionality.

    Filesystems that make use of the VM ->page_mkwrite() callout will generally use
    the same core code to implement it. There are several tricky truncate-related
    issues that we need to deal with here as we cannot take the i_mutex as we
    normally would for these paths. These issues are not documented anywhere yet
    so block_page_mkwrite() seems like the best place to start.

    Version 2:

    - read inode size only once
    - more comments explaining implementation restrictions

    Signed-Off-By: Dave Chinner <dgc@sgi.com>

    ---
    fs/buffer.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
    include/linux/buffer_head.h | 2 +
    2 files changed, 49 insertions(+)

    Index: 2.6.x-xfs-new/fs/buffer.c
    ===================================================================
    --- 2.6.x-xfs-new.orig/fs/buffer.c 2007-03-17 10:55:32.291414968 +1100
    +++ 2.6.x-xfs-new/fs/buffer.c 2007-03-19 08:13:54.519909087 +1100
    @@ -2194,6 +2194,52 @@ int generic_commit_write(struct file *fi
    return 0;
    }

    +/*
    + * block_page_mkwrite() is not allowed to change the file size as it gets
    + * called from a page fault handler when a page is first dirtied. Hence we must
    + * be careful to check for EOF conditions here. We set the page up correctly
    + * for a written page which means we get ENOSPC checking when writing into
    + * holes and correct delalloc and unwritten extent mapping on filesystems that
    + * support these features.
    + *
    + * We are not allowed to take the i_mutex here so we have to play games to
    + * protect against truncate races as the page could now be beyond EOF. Because
    + * vmtruncate() writes the inode size before removing pages, once we have the
    + * page lock we can determine safely if the page is beyond EOF. If it is not
    + * beyond EOF, then the page is guaranteed safe against truncation until we
    + * unlock the page.
    + */
    +int
    +block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
    + get_block_t get_block)
    +{
    + struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
    + unsigned long end;
    + loff_t size;
    + int ret = -EINVAL;
    +
    + lock_page(page);
    + size = i_size_read(inode);
    + if ((page->mapping != inode->i_mapping) ||
    + ((page->index << PAGE_CACHE_SHIFT) > size)) {
    + /* page got truncated out from underneath us */
    + goto out_unlock;
    + }
    +
    + /* page is wholly or partially inside EOF */
    + if (((page->index + 1) << PAGE_CACHE_SHIFT) > size)
    + end = size & ~PAGE_CACHE_MASK;
    + else
    + end = PAGE_CACHE_SIZE;
    +
    + ret = block_prepare_write(page, 0, end, get_block);
    + if (!ret)
    + ret = block_commit_write(page, 0, end);
    +
    +out_unlock:
    + unlock_page(page);
    + return ret;
    +}

    /*
    * nobh_prepare_write()'s prereads are special: the buffer_heads are freed
    @@ -2997,6 +3043,7 @@ EXPORT_SYMBOL(__brelse);
    EXPORT_SYMBOL(__wait_on_buffer);
    EXPORT_SYMBOL(block_commit_write);
    EXPORT_SYMBOL(block_prepare_write);
    +EXPORT_SYMBOL(block_page_mkwrite);
    EXPORT_SYMBOL(block_read_full_page);
    EXPORT_SYMBOL(block_sync_page);
    EXPORT_SYMBOL(block_truncate_page);
    Index: 2.6.x-xfs-new/include/linux/buffer_head.h
    ===================================================================
    --- 2.6.x-xfs-new.orig/include/linux/buffer_head.h 2007-03-17 10:55:32.135435539 +1100
    +++ 2.6.x-xfs-new/include/linux/buffer_head.h 2007-03-17 10:55:32.567378573 +1100
    @@ -206,6 +206,8 @@ int cont_prepare_write(struct page*, uns
    int generic_cont_expand(struct inode *inode, loff_t size);
    int generic_cont_expand_simple(struct inode *inode, loff_t size);
    int block_commit_write(struct page *page, unsigned from, unsigned to);
    +int block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
    + get_block_t get_block);
    void block_sync_page(struct page *);
    sector_t generic_block_bmap(struct address_space *, sector_t, get_block_t *);
    int generic_commit_write(struct file *, struct page *, unsigned, unsigned);
    -
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