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    Subject[PATCH 10/22] 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).

    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 prepare_write() and
    commit_write() address_space operations to bound 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 | 95 ++++++++++++++++++++++++++++++++++++++++++++++++++++
    4 files changed, 107 insertions(+), 0 deletions(-)

    diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
    index 0079b2c..b3e4b50 100644
    --- a/fs/ext2/inode.c
    +++ b/fs/ext2/inode.c
    @@ -695,6 +695,7 @@ 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 = {
    @@ -713,6 +714,7 @@ 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 de4e316..93809eb 100644
    --- a/fs/ext3/inode.c
    +++ b/fs/ext3/inode.c
    @@ -1713,6 +1713,7 @@ 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 = {
    @@ -1727,6 +1728,7 @@ 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 = {
    @@ -1740,6 +1742,7 @@ 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 bf35441..38f67ac 100644
    --- a/include/linux/fs.h
    +++ b/include/linux/fs.h
    @@ -434,6 +434,11 @@ 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 *);
    +
    };

    struct backing_dev_info;
    @@ -1669,6 +1674,8 @@ 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 3a61923..21aeee9 100644
    --- a/mm/filemap.c
    +++ b/mm/filemap.c
    @@ -2016,6 +2016,101 @@ zero_length_segment:
    }
    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 pagevec lru_pvec;
    + struct page *page, *cached_page = NULL;
    + unsigned psize;
    + loff_t isize;
    + long status = 0;
    +
    + pagevec_init(&lru_pvec, 0);
    +
    + page = __grab_cache_page(mapping, index, &cached_page, &lru_pvec);
    + if (!page) {
    + BUG_ON(cached_page);
    + return -ENOMEM;
    + }
    +
    + psize = PAGE_CACHE_SIZE;
    + isize = i_size_read(mapping->host);
    + if ((isize >> PAGE_SHIFT) == index)
    + psize = isize & (PAGE_SIZE - 1);
    +
    + status = a_ops->prepare_write(NULL, page, 0, psize);
    + if (unlikely(status)) {
    + isize = i_size_read(mapping->host);
    +
    + if (status != AOP_TRUNCATED_PAGE)
    + unlock_page(page);
    + page_cache_release(page);
    + if (status == AOP_TRUNCATED_PAGE)
    + goto sync;
    +
    + /* prepare_write() may have instantiated a few blocks outside
    + * i_size. Trim these off again.
    + */
    + if ((1ULL << (index + 1)) > isize)
    + vmtruncate(mapping->host, isize);
    + goto sync;
    + }
    +
    + copy_highpage(page, source);
    + flush_dcache_page(page);
    +
    + status = a_ops->commit_write(NULL, page, 0, psize);
    + if (status == AOP_TRUNCATED_PAGE) {
    + page_cache_release(page);
    + goto sync;
    + }
    +
    + if (status > 0)
    + status = 0;
    +
    + unlock_page(page);
    + mark_page_accessed(page);
    + page_cache_release(page);
    + if (status < 0)
    + return status;
    +
    + balance_dirty_pages_ratelimited(mapping);
    + cond_resched();
    +
    +sync:
    + if (cached_page)
    + page_cache_release(cached_page);
    +
    + /* 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)
    + status = generic_osync_inode(mapping->host, mapping,
    + OSYNC_METADATA | OSYNC_DATA);
    +
    + /* the caller must handle O_DIRECT for themselves */
    +
    + pagevec_lru_add(&lru_pvec);
    + return status;
    +}
    +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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