[PATCH] do_generic_file_read / readahead adjustments

This does the following three things:

 (1) Makes the functions in mm/readahead.c only use struct file* to pass to
     readpage(). address_mapping* and file_ra_state* are used instead to keep
     track of readahead stuff.

 (2) Adds a new function do_generic_mapping_read() that is similar to
     do_generic_file_read(), except that it uses a mapping pointer and a
     readahead state pointer to access a file. The file* is only used to pass
     to readpage().

 (3) Turns do_generic_file_read() into an inline function in linux/fs.h that
     simply wraps do_generic_mapping_read().

This should mean that it is no longer necessary to have a struct file to
access a file in this manner. Just an inode or address space should be
sufficient.

It also means alternate read-ahead structures can be maintained.

The reason I want this is that I'm writing a general cache manager for
filesystems such as AFS, NFSv4, and Lustre. Block devices are made available
to the "cache manager" by means of a filesystem that can be mounted. I'm
storing meta data in an inode in the cache, but to scan this at the moment I
need to gain a "struct file" to use with do_generic_file_read().

This involves either creating a dummy dentry and struct file (which will cause
Al Viro to come looking for me with a shotgun), or to use an extra auxilliary
filesystem mounted with do_kern_mount(), neither of which are particularly
appealing.

This patch is the alternative... it provides a function that I can pass an
address_space to. This also allows me to make use of readahead semantics
without having to reinvent them for myself.

include/linux/fs.h

@@ -1251,7 +1251,8 @@ extern ssize_t do_sync_write(struct file *filp, const char *buf, size_t len, lof
 ssize_t generic_file_write_nolock(struct file *file, const struct iovec *iov,
 				unsigned long nr_segs, loff_t *ppos);
 extern ssize_t generic_file_sendfile(struct file *, struct file *, loff_t *, size_t);
-extern void do_generic_file_read(struct file *, loff_t *, read_descriptor_t *, read_actor_t);
+extern void do_generic_mapping_read(struct address_space *, struct file_ra_state *, struct file *,
+				    loff_t *, read_descriptor_t *, read_actor_t);
 extern ssize_t generic_file_direct_IO(int rw, struct file *file,
 	const struct iovec *iov, loff_t offset, unsigned long nr_segs);
 extern int generic_direct_IO(int rw, struct inode *inode, const struct iovec 
@@ -1268,6 +1269,18 @@ extern int generic_file_open(struct inode * inode, struct file * filp);
 extern int generic_vm_writeback(struct page *page,
 				struct writeback_control *wbc);
 
+static inline void do_generic_file_read(struct file * filp, loff_t *ppos,
+					read_descriptor_t * desc,
+					read_actor_t actor)
+{
+	do_generic_mapping_read(filp->f_dentry->d_inode->i_mapping,
+				&filp->f_ra,
+				filp,
+				ppos,
+				desc,
+				actor);
+}
+
 extern struct file_operations generic_ro_fops;
 
 extern int vfs_readlink(struct dentry *, char *, int, const char *);

include/linux/mm.h

@@ -513,11 +513,18 @@ int write_one_page(struct page *page, int wait);
 /* readahead.c */
 #define VM_MAX_READAHEAD	128	/* kbytes */
 #define VM_MIN_READAHEAD	16	/* kbytes (includes current page) */
-int do_page_cache_readahead(struct file *file,
+int do_page_cache_readahead(struct address_space *mapping, struct file *filp,
 			unsigned long offset, unsigned long nr_to_read);
-void page_cache_readahead(struct file *file, unsigned long offset);
-void page_cache_readaround(struct file *file, unsigned long offset);
-void handle_ra_miss(struct file *file);
+void page_cache_readahead(struct address_space *mapping, 
+			  struct file_ra_state *ra,
+			  struct file *filp,
+			  unsigned long offset);
+void page_cache_readaround(struct address_space *mapping, 
+			   struct file_ra_state *ra,
+			   struct file *filp,
+			   unsigned long offset);
+void handle_ra_miss(struct address_space *mapping, 
+		    struct file_ra_state *ra);
 
 /* Do stack extension */
 extern int expand_stack(struct vm_area_struct * vma, unsigned long address);

kernel/ksyms.c

@@ -229,7 +229,7 @@ EXPORT_SYMBOL(block_truncate_page);
 EXPORT_SYMBOL(generic_block_bmap);
 EXPORT_SYMBOL(generic_file_read);
 EXPORT_SYMBOL(generic_file_sendfile);
-EXPORT_SYMBOL(do_generic_file_read);
+EXPORT_SYMBOL(do_generic_mapping_read);
 EXPORT_SYMBOL(generic_file_write);
 EXPORT_SYMBOL(generic_file_write_nolock);
 EXPORT_SYMBOL(generic_file_mmap);

mm/filemap.c

@@ -570,10 +570,15 @@ void mark_page_accessed(struct page *page)
  *
  * This is really ugly. But the goto's actually try to clarify some
  * of the logic when it comes to error handling etc.
+ * - note the struct file * is only passed for the use of readpage
  */
-void do_generic_file_read(struct file * filp, loff_t *ppos, read_descriptor_t * desc, read_actor_t actor)
+void do_generic_mapping_read(struct address_space *mapping,
+			     struct file_ra_state *ra,
+			     struct file * filp,
+			     loff_t *ppos,
+			     read_descriptor_t * desc,
+			     read_actor_t actor)
 {
-	struct address_space *mapping = filp->f_dentry->d_inode->i_mapping;
 	struct inode *inode = mapping->host;
 	unsigned long index, offset;
 	struct page *cached_page;
@@ -598,7 +603,7 @@ void do_generic_file_read(struct file * filp, loff_t *ppos, read_descriptor_t *
 				break;
 		}
 
-		page_cache_readahead(filp, index);
+		page_cache_readahead(mapping, ra, filp, index);
 
 		nr = nr - offset;
 
@@ -610,7 +615,7 @@ void do_generic_file_read(struct file * filp, loff_t *ppos, read_descriptor_t *
 		page = radix_tree_lookup(&mapping->page_tree, index);
 		if (!page) {
 			read_unlock(&mapping->page_lock);
-			handle_ra_miss(filp);
+			handle_ra_miss(mapping,ra);
 			goto no_cached_page;
 		}
 		page_cache_get(page);
@@ -946,9 +951,9 @@ ssize_t generic_file_sendfile(struct file *out_file, struct file *in_file,
 }
 
 static ssize_t
-do_readahead(struct file *file, unsigned long index, unsigned long nr)
+do_readahead(struct address_space *mapping, struct file *filp,
+	     unsigned long index, unsigned long nr)
 {
-	struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
 	unsigned long max;
 	unsigned long active;
 	unsigned long inactive;
@@ -962,7 +967,7 @@ do_readahead(struct file *file, unsigned long index, unsigned long nr)
 	if (nr > max)
 		nr = max;
 
-	do_page_cache_readahead(file, index, nr);
+	do_page_cache_readahead(mapping, filp, index, nr);
 	return 0;
 }
 
@@ -975,10 +980,11 @@ asmlinkage ssize_t sys_readahead(int fd, loff_t offset, size_t count)
 	file = fget(fd);
 	if (file) {
 		if (file->f_mode & FMODE_READ) {
+			struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
 			unsigned long start = offset >> PAGE_CACHE_SHIFT;
 			unsigned long end = (offset + count - 1) >> PAGE_CACHE_SHIFT;
 			unsigned long len = end - start + 1;
-			ret = do_readahead(file, start, len);
+			ret = do_readahead(mapping, file, start, len);
 		}
 		fput(file);
 	}
@@ -999,6 +1005,7 @@ struct page * filemap_nopage(struct vm_area_struct * area, unsigned long address
 	int error;
 	struct file *file = area->vm_file;
 	struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
+	struct file_ra_state *ra = &file->f_ra;
 	struct inode *inode = mapping->host;
 	struct page *page;
 	unsigned long size, pgoff, endoff;
@@ -1031,7 +1038,7 @@ struct page * filemap_nopage(struct vm_area_struct * area, unsigned long address
 	 */
 	if (VM_SequentialReadHint(area)) {
 		did_readahead = 1;
-		page_cache_readahead(area->vm_file, pgoff);
+		page_cache_readahead(mapping, ra, file, pgoff);
 	}
 
 	/*
@@ -1040,7 +1047,7 @@ struct page * filemap_nopage(struct vm_area_struct * area, unsigned long address
 	 */
 	if ((pgoff < size) && !VM_RandomReadHint(area)) {
 		did_readahead = 1;
-		page_cache_readaround(file, pgoff);
+		page_cache_readaround(mapping, ra, file, pgoff);
 	}
 
 	/*
@@ -1050,7 +1057,7 @@ struct page * filemap_nopage(struct vm_area_struct * area, unsigned long address
 	page = find_get_page(mapping, pgoff);
 	if (!page) {
 		if (did_readahead) {
-			handle_ra_miss(file);
+			handle_ra_miss(mapping,ra);
 			did_readahead = 0;
 		}
 		goto no_cached_page;

mm/madvise.c

@@ -80,7 +80,7 @@ static long madvise_willneed(struct vm_area_struct * vma,
 	if ((vma->vm_mm->rss + (end - start)) > rlim_rss)
 		return error;
 
-	do_page_cache_readahead(file, start, end - start);
+	do_page_cache_readahead(file->f_dentry->d_inode->i_mapping, file, start, end - start);
 	return 0;
 }
 

mm/readahead.c

@@ -22,18 +22,18 @@ struct backing_dev_info default_backing_dev_info = {
 /*
  * Return max readahead size for this inode in number-of-pages.
  */
-static inline unsigned long get_max_readahead(struct file *file)
+static inline unsigned long get_max_readahead(struct file_ra_state *ra)
 {
-	return file->f_ra.ra_pages;
+	return ra->ra_pages;
 }
 
-static inline unsigned long get_min_readahead(struct file *file)
+static inline unsigned long get_min_readahead(struct file_ra_state *ra)
 {
 	return (VM_MIN_READAHEAD * 1024) / PAGE_CACHE_SIZE;
 }
 
 static int
-read_pages(struct file *file, struct address_space *mapping,
+read_pages(struct address_space *mapping, struct file *filp,
 		struct list_head *pages, unsigned nr_pages)
 {
 	unsigned page_idx;
@@ -48,7 +48,7 @@ read_pages(struct file *file, struct address_space *mapping,
 		struct page *page = list_entry(pages->prev, struct page, list);
 		list_del(&page->list);
 		if (!add_to_page_cache(page, mapping, page->index)) {
-			mapping->a_ops->readpage(file, page);
+			mapping->a_ops->readpage(filp, page);
 			if (!pagevec_add(&lru_pvec, page))
 				__pagevec_lru_add(&lru_pvec);
 		} else {
@@ -134,10 +134,11 @@ read_pages(struct file *file, struct address_space *mapping,
  *
  * Returns the number of pages which actually had IO started against them.
  */
-int do_page_cache_readahead(struct file *file,
-			unsigned long offset, unsigned long nr_to_read)
+int do_page_cache_readahead(struct address_space *mapping,
+			    struct file *filp,
+			    unsigned long offset,
+			    unsigned long nr_to_read)
 {
-	struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
 	struct inode *inode = mapping->host;
 	struct page *page;
 	unsigned long end_index;	/* The last page we want to read */
@@ -181,7 +182,7 @@ int do_page_cache_readahead(struct file *file,
 	 * will then handle the error.
 	 */
 	if (ret) {
-		read_pages(file, mapping, &page_pool, ret);
+		read_pages(mapping, filp, &page_pool, ret);
 		blk_run_queues();
 	}
 	BUG_ON(!list_empty(&page_pool));
@@ -216,9 +217,9 @@ check_ra_success(struct file_ra_state *ra, pgoff_t attempt,
  * page_cache_readahead is the main function.  If performs the adaptive
  * readahead window size management and submits the readahead I/O.
  */
-void page_cache_readahead(struct file *file, unsigned long offset)
+void page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
+			  struct file *filp, unsigned long offset)
 {
-	struct file_ra_state *ra = &file->f_ra;
 	unsigned max;
 	unsigned min;
 	unsigned orig_next_size;
@@ -239,11 +240,11 @@ void page_cache_readahead(struct file *file, unsigned long offset)
 	if (ra->next_size == -1UL)
 		goto out;	/* Maximally shrunk */
 
-	max = get_max_readahead(file);
+	max = get_max_readahead(ra);
 	if (max == 0)
 		goto out;	/* No readahead */
 
-	min = get_min_readahead(file);
+	min = get_min_readahead(ra);
 	orig_next_size = ra->next_size;
 
 	if (ra->next_size == 0 && offset == 0) {
@@ -316,7 +317,8 @@ void page_cache_readahead(struct file *file, unsigned long offset)
 		ra->ahead_start = 0;		/* Invalidate these */
 		ra->ahead_size = 0;
 
-		actual = do_page_cache_readahead(file, offset, ra->size);
+		actual = do_page_cache_readahead(mapping, filp, offset,
+						 ra->size);
 		check_ra_success(ra, ra->size, actual, orig_next_size);
 	} else {
 		/*
@@ -327,7 +329,7 @@ void page_cache_readahead(struct file *file, unsigned long offset)
 		if (ra->ahead_start == 0) {
 			ra->ahead_start = ra->start + ra->size;
 			ra->ahead_size = ra->next_size;
-			actual = do_page_cache_readahead(file,
+			actual = do_page_cache_readahead(mapping, filp,
 					ra->ahead_start, ra->ahead_size);
 			check_ra_success(ra, ra->ahead_size,
 					actual, orig_next_size);
@@ -342,12 +344,11 @@ void page_cache_readahead(struct file *file, unsigned long offset)
  * but somewhat ascending.  So readaround favours pages beyond the target one.
  * We also boost the window size, as it can easily shrink due to misses.
  */
-void page_cache_readaround(struct file *file, unsigned long offset)
+void page_cache_readaround(struct address_space *mapping, struct file_ra_state *ra,
+			   struct file *filp, unsigned long offset)
 {
-	struct file_ra_state *ra = &file->f_ra;
-
 	if (ra->next_size != -1UL) {
-		const unsigned long min = get_min_readahead(file) * 2;
+		const unsigned long min = get_min_readahead(ra) * 2;
 		unsigned long target;
 		unsigned long backward;
 
@@ -365,7 +366,7 @@ void page_cache_readaround(struct file *file, unsigned long offset)
 			target = 0;
 		else
 			target -= backward;
-		page_cache_readahead(file, target);
+		page_cache_readahead(mapping, ra, filp, target);
 	}
 }
 
@@ -383,10 +384,9 @@ void page_cache_readaround(struct file *file, unsigned long offset)
  * that the readahead window size will stabilise around the maximum level at
  * which there is no thrashing.
  */
-void handle_ra_miss(struct file *file)
+void handle_ra_miss(struct address_space *mapping, struct file_ra_state *ra)
 {
-	struct file_ra_state *ra = &file->f_ra;
-	const unsigned long min = get_min_readahead(file);
+	const unsigned long min = get_min_readahead(ra);
 
 	if (ra->next_size == -1UL) {
 		ra->next_size = min;