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    Subject[PATCH 19/27] CacheFiles: Add a hook to write a single page of data to an inode
    Date
    Add an address space operation to write one single page of data to an inode at
    a page-aligned location (thus permitting the implementation to be highly
    optimised). The data source is a single page.

    This is used by CacheFiles to store the contents of netfs pages into their
    backing file pages.

    Supply a generic implementation for this that uses the write_begin() and
    write_end() address_space operations to bind a copy directly into the page
    cache.

    Hook the Ext2 and Ext3 operations to the generic implementation.

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    fs/ext2/inode.c | 2 ++
    fs/ext3/inode.c | 3 +++
    include/linux/fs.h | 7 ++++++
    mm/filemap.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++
    4 files changed, 73 insertions(+), 0 deletions(-)


    diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
    index b1ab32a..cfa56e6 100644
    --- a/fs/ext2/inode.c
    +++ b/fs/ext2/inode.c
    @@ -796,6 +796,7 @@ const struct address_space_operations ext2_aops = {
    .direct_IO = ext2_direct_IO,
    .writepages = ext2_writepages,
    .migratepage = buffer_migrate_page,
    + .write_one_page = generic_file_buffered_write_one_page,
    };

    const struct address_space_operations ext2_aops_xip = {
    @@ -814,6 +815,7 @@ const struct address_space_operations ext2_nobh_aops = {
    .direct_IO = ext2_direct_IO,
    .writepages = ext2_writepages,
    .migratepage = buffer_migrate_page,
    + .write_one_page = generic_file_buffered_write_one_page,
    };

    /*
    diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
    index bc918d3..435c684 100644
    --- a/fs/ext3/inode.c
    +++ b/fs/ext3/inode.c
    @@ -1780,6 +1780,7 @@ static const struct address_space_operations ext3_ordered_aops = {
    .releasepage = ext3_releasepage,
    .direct_IO = ext3_direct_IO,
    .migratepage = buffer_migrate_page,
    + .write_one_page = generic_file_buffered_write_one_page,
    };

    static const struct address_space_operations ext3_writeback_aops = {
    @@ -1794,6 +1795,7 @@ static const struct address_space_operations ext3_writeback_aops = {
    .releasepage = ext3_releasepage,
    .direct_IO = ext3_direct_IO,
    .migratepage = buffer_migrate_page,
    + .write_one_page = generic_file_buffered_write_one_page,
    };

    static const struct address_space_operations ext3_journalled_aops = {
    @@ -1807,6 +1809,7 @@ static const struct address_space_operations ext3_journalled_aops = {
    .bmap = ext3_bmap,
    .invalidatepage = ext3_invalidatepage,
    .releasepage = ext3_releasepage,
    + .write_one_page = generic_file_buffered_write_one_page,
    };

    void ext3_set_aops(struct inode *inode)
    diff --git a/include/linux/fs.h b/include/linux/fs.h
    index 850d3fc..a3c3369 100644
    --- a/include/linux/fs.h
    +++ b/include/linux/fs.h
    @@ -479,6 +479,11 @@ struct address_space_operations {
    int (*migratepage) (struct address_space *,
    struct page *, struct page *);
    int (*launder_page) (struct page *);
    + /* write the contents of the source page over the page at the specified
    + * index in the target address space (the source page does not need to
    + * be related to the target address space) */
    + int (*write_one_page)(struct address_space *, pgoff_t, struct page *);
    +
    };

    /*
    @@ -1801,6 +1806,8 @@ extern ssize_t generic_file_direct_write(struct kiocb *, const struct iovec *,
    unsigned long *, loff_t, loff_t *, size_t, size_t);
    extern ssize_t generic_file_buffered_write(struct kiocb *, const struct iovec *,
    unsigned long, loff_t, loff_t *, size_t, ssize_t);
    +extern int generic_file_buffered_write_one_page(struct address_space *,
    + pgoff_t, struct page *);
    extern ssize_t do_sync_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos);
    extern ssize_t do_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos);
    extern void do_generic_mapping_read(struct address_space *mapping,
    diff --git a/mm/filemap.c b/mm/filemap.c
    index f22801a..ed52b0b 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -2333,6 +2333,67 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov,
    }
    EXPORT_SYMBOL(generic_file_buffered_write);

    +/**
    + * generic_file_buffered_write_one_page - Write a single page of data to an
    + * inode
    + * @mapping - The address space of the target inode
    + * @index - The target page in the target inode to fill
    + * @source - The data to write into the target page
    + *
    + * Write the data from the source page to the page in the nominated address
    + * space at the @index specified. Note that the file will not be extended if
    + * the page crosses the EOF marker, in which case only the first part of the
    + * page will be written.
    + *
    + * The @source page does not need to have any association with the file or the
    + * target page offset.
    + */
    +int generic_file_buffered_write_one_page(struct address_space *mapping,
    + pgoff_t index,
    + struct page *source)
    +{
    + const struct address_space_operations *a_ops = mapping->a_ops;
    + struct page *page;
    + unsigned len;
    + loff_t isize, pos;
    + void *fsdata;
    + int ret;
    +
    + pos = index;
    + pos <<= PAGE_CACHE_SHIFT;
    +
    + len = PAGE_CACHE_SIZE;
    + isize = i_size_read(mapping->host);
    + if ((isize >> PAGE_CACHE_SHIFT) == index)
    + len = isize & (PAGE_CACHE_SIZE - 1);
    +
    + ret = pagecache_write_begin(NULL, mapping, pos, len,
    + AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
    + if (ret < 0)
    + goto sync;
    +
    + copy_highpage(page, source);
    +
    + ret = pagecache_write_end(NULL, mapping, pos, len, len, page, fsdata);
    + if (ret < 0)
    + goto sync;
    +
    + balance_dirty_pages_ratelimited(mapping);
    + cond_resched();
    +
    +sync:
    + /* the caller must handle O_SYNC themselves, but we handle S_SYNC and
    + * MS_SYNCHRONOUS here */
    + if (unlikely(IS_SYNC(mapping->host)) && !a_ops->writepage)
    + ret = generic_osync_inode(mapping->host, mapping,
    + OSYNC_METADATA | OSYNC_DATA);
    +
    + /* the caller must handle O_DIRECT for themselves */
    +
    + return ret;
    +}
    +EXPORT_SYMBOL(generic_file_buffered_write_one_page);
    +
    static ssize_t
    __generic_file_aio_write_nolock(struct kiocb *iocb, const struct iovec *iov,
    unsigned long nr_segs, loff_t *ppos)


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