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Subject[PATCH 06/14] CacheFiles: Add a hook to write a single page of data to an inode [try #2]
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 6bf1395..1b1f288 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -433,6 +433,11 @@ 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 7b96487..5e419a2 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2032,6 +2032,101 @@ 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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