Correctly lock pages for .readpages()

Unlike the .readpage() callback which is passed a single locked page
to be populated.  The .readpages() callback is passed a list of unlocked
pages which are all marked for read-ahead (PG_readahead set).  It is
the responsibly of .readpages() to ensure to pages are properly locked
before being populated.

Prior to this change the requested read-ahead pages would be updated
outside of the page lock which is unsafe.  The unlocked pages would then
be unlocked again which is harmless but should have been immediately
detected as bug.  Unfortunately, newer kernels failed detect this issue
because the check is done with a VM_BUG_ON which is disabled by default.
Luckily, the old Debian Lenny 2.6.26 kernel caught this because it
simply uses a BUG_ON.

The straight forward fix for this is to update the .readpages() callback
to use the read_cache_pages() helper function.  The helper function will
ensure that each page in the list is properly locked before it is passed
to the .readpage() callback.  In addition resolving the bug, this results
in a nice simplification of the existing code.

The downside to this change is that instead of passing one large read
request to the dmu multiple smaller ones are submitted.  All of these
requests however are marked for readahead so the lower layers should
issue a large I/O regardless.  Thus most of the request should hit the
ARC cache.

Futher optimization of this code can be done in the future is a perform
analysis determines it to be worthwhile.  But for the moment, it is
preferable that code be correct and understandable.

Signed-off-by: Brian Behlendorf <[email protected]>
Closes #355

1 files changed, 13 insertions, 54 deletions

diff --git a/module/zfs/zpl_file.c b/module/zfs/zpl_file.c
index 7eaf65c6e..53921d1a7 100644
--- a/module/zfs/zpl_file.c
+++ b/module/zfs/zpl_file.c
@@ -260,60 +260,6 @@ zpl_mmap(struct file *filp, struct vm_area_struct *vma)
 	return (error);
 }
 
-static struct page **
-pages_vector_from_list(struct list_head *pages, unsigned nr_pages)
-{
-	struct page **pl;
-	struct page *t;
-	unsigned page_idx;
-
-	pl = kmalloc(sizeof(*pl) * nr_pages, GFP_NOFS);
-	if (!pl)
-		return ERR_PTR(-ENOMEM);
-
-	page_idx = 0;
-	list_for_each_entry_reverse(t, pages, lru) {
-		pl[page_idx] = t;
-		page_idx++;
-	}
-
-	return pl;
-}
-
-static int
-zpl_readpages(struct file *file, struct address_space *mapping,
-	struct list_head *pages, unsigned nr_pages)
-{
-	struct inode *ip;
-	struct page  **pl;
-	struct page  *p, *n;
-	int          error;
-
-	ip = mapping->host;
-
-	pl = pages_vector_from_list(pages, nr_pages);
-	if (IS_ERR(pl))
-		return PTR_ERR(pl);
-
-	error = -zfs_getpage(ip, pl, nr_pages);
-	if (error)
-		goto error;
-
-	list_for_each_entry_safe_reverse(p, n, pages, lru) {
-
-		list_del(&p->lru);
-
-		flush_dcache_page(p);
-		SetPageUptodate(p);
-		unlock_page(p);
-		page_cache_release(p);
-	}
-
-error:
-	kfree(pl);
-	return error;
-}
-
 /*
  * Populate a page with data for the Linux page cache.  This function is
  * only used to support mmap(2).  There will be an identical copy of the
@@ -349,6 +295,19 @@ zpl_readpage(struct file *filp, struct page *pp)
 	return error;
 }
 
+/*
+ * Populate a set of pages with data for the Linux page cache.  This
+ * function will only be called for read ahead and never for demand
+ * paging.  For simplicity, the code relies on read_cache_pages() to
+ * correctly lock each page for IO and call zpl_readpage().
+ */
+static int
+zpl_readpages(struct file *filp, struct address_space *mapping,
+	struct list_head *pages, unsigned nr_pages)
+{
+	return (read_cache_pages(mapping, pages, zpl_readpage, filp));
+}
+
 int
 zpl_putpage(struct page *pp, struct writeback_control *wbc, void *data)
 {