[Dazuko-devel] PATCH 1/3 implement memory mapped writing

[Lino Sanfilippo]

The following patches apply against dazukofs-3.1.4-rc2.

1. Implement mmaped writing for dazukofs:

In writepage() we look for the corresponding lower page and create it if it
does not exist.
Then we copy the content of the upper page to the lower page and mark it 
dirty.
This causes marked pages to be written to disk as soon as the pages are 
synced (i.e
due to an msync() or sync()).

Note that this patch also removes some of the functions that do also cause
writepage() to be called, like generic_file_aio_[read|write] and 
splice_read() since
mmap has not been tested in conjunction with these functions - also 
writebegin()
should not be called any more now.

With this the only address space operations that we should need are 
readpage()
and writepage().








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diff -Nurp dazukofs-3.1.4-rc2/file.c dazukofs-3.1.4-rc2-patch1/file.c
--- dazukofs-3.1.4-rc2/file.c   2010-10-16 18:03:49.000000000 +0200
+++ dazukofs-3.1.4-rc2-patch1/file.c    2010-11-02 11:56:01.000000000 +0100
@@ -319,7 +319,7 @@ static int dazukofs_mmap(struct file *fi
        if (!lower_file->f_op || !lower_file->f_op->mmap)
                return -ENODEV;
 
-       return generic_file_readonly_mmap(file, vm);
+       return generic_file_mmap(file, vm);
 }
 
 /**
@@ -342,9 +342,7 @@ static int dazukofs_mmap(struct file *fi
 const struct file_operations dazukofs_main_fops = {
        .llseek         = dazukofs_llseek,
        .read           = dazukofs_read,
-       .aio_read       = generic_file_aio_read,
        .write          = dazukofs_write,
-       .aio_write      = generic_file_aio_write,
        .readdir        = dazukofs_readdir,
        .unlocked_ioctl = dazukofs_unlocked_ioctl,
 #ifdef CONFIG_COMPAT
@@ -356,7 +354,6 @@ const struct file_operations dazukofs_ma
        .release        = dazukofs_release,
        .fsync          = dazukofs_fsync,
        .fasync         = dazukofs_fasync,
-       .splice_read    = generic_file_splice_read,
 };
 
 /**
@@ -392,5 +389,4 @@ const struct file_operations dazukofs_di
        .release        = dazukofs_release,
        .fsync          = dazukofs_fsync,
        .fasync         = dazukofs_fasync,
-       .splice_read    = generic_file_splice_read,
 };
diff -Nurp dazukofs-3.1.4-rc2/mmap.c dazukofs-3.1.4-rc2-patch1/mmap.c
--- dazukofs-3.1.4-rc2/mmap.c   2010-05-30 12:58:22.000000000 +0200
+++ dazukofs-3.1.4-rc2-patch1/mmap.c    2010-11-02 12:00:45.000000000 +0100
@@ -27,11 +27,6 @@
 
 #include "dazukofs_fs.h"
 
-static int dazukofs_writepage(struct page *page, struct writeback_control *wbc)
-{
-       /* mmap read-only */
-       return -EINVAL;
-}
 
 /**
  * Description: Called by the VM to read a page from backing store. The page
@@ -98,14 +93,66 @@ out:
        return err;
 }
 
-int dazukofs_write_begin(struct file *f, struct address_space *mapping,
-                        loff_t pos, unsigned len, unsigned flags,
-                        struct page **pagep, void **fsdata)
+
+
+/**
+ * Called if a page fault occured due to reading or writing to upper  mmaped 
+ * file. We get the lower page and mark it dirty, we DONT call lower 
+ * writepage() yet. Instead we let this be done by msync() or other kernel 
+ * facilities (pdflush) that try to write pages to disc. 
+ */
+static int dazukofs_writepage(struct page *page, struct writeback_control *wbc)
 {
-       /* mmap read-only */
-       return -EINVAL;
+       struct inode *inode = page->mapping->host;
+       struct inode *lower_inode = get_lower_inode(inode);
+       struct page *lower_page;
+       char *page_data;
+       char *lower_page_data;
+       int rv = 0;
+
+       lower_page = grab_cache_page(lower_inode->i_mapping, page->index);
+
+       if (!lower_page) {
+               printk(KERN_ERR "dazukofs: Error getting lower page.\n");
+               rv = -ENOMEM;
+               goto fail;
+       }
+       
+       page_data = (char *) kmap(page);
+       if (!page_data) {
+               rv = -ENOMEM;
+               printk(KERN_ERR "dazukofs: Error mapping page.\n");
+               unlock_page(lower_page);
+               goto fail;
+       }
+
+       lower_page_data = (char *) kmap(lower_page);
+       if (!lower_page_data) {
+               rv = -ENOMEM;
+               printk(KERN_ERR "dazukofs: Error mapping lower page.\n");
+               kunmap(page);
+               unlock_page(lower_page);
+               goto fail;
+       }
+
+       memcpy(lower_page_data, page_data, PAGE_CACHE_SIZE);
+
+       kunmap(page);
+       kunmap(lower_page);
+
+       /* Mark lower page dirty. Dont call lower writepage() yet. */
+       set_page_dirty(lower_page);
+       unlock_page(lower_page);
+       SetPageUptodate(page);
+
+fail:
+       unlock_page(page);
+
+       return rv;
 }
 
+
+
 /**
  * Unused operations:
  *   - sync_page
@@ -126,5 +173,4 @@ int dazukofs_write_begin(struct file *f,
 const struct address_space_operations dazukofs_aops = {
        .writepage      = dazukofs_writepage,
        .readpage       = dazukofs_readpage,
-       .write_begin    = dazukofs_write_begin,
 };