[PATCH v2 5/9] iomap: Support arbitrarily many blocks per page

[Matthew Wilcox (Oracle)]

Size the uptodate array dynamically to support larger pages in the
page cache.  With a 64kB page, we're only saving 8 bytes per page today,
but with a 2MB maximum page size, we'd have to allocate more than 4kB
per page.  Add a few debugging assertions.

Signed-off-by: Matthew Wilcox (Oracle) <wi...@infradead.org>
Reviewed-by: Dave Chinner <dchin...@redhat.com>
---
 fs/iomap/buffered-io.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 7fc0e02d27b0..9670c096b83e 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -22,18 +22,25 @@
 #include "../internal.h"
 
 /*
- * Structure allocated for each page when block size < PAGE_SIZE to track
- * sub-page uptodate status and I/O completions.
+ * Structure allocated for each page or THP when block size < page size
+ * to track sub-page uptodate status and I/O completions.
  */
 struct iomap_page {
        atomic_t                read_count;
        atomic_t                write_count;
        spinlock_t              uptodate_lock;
-       DECLARE_BITMAP(uptodate, PAGE_SIZE / 512);
+       unsigned long           uptodate[];
 };
 
 static inline struct iomap_page *to_iomap_page(struct page *page)
 {
+       /*
+        * per-block data is stored in the head page.  Callers should
+        * not be dealing with tail pages (and if they are, they can
+        * call thp_head() first.
+        */
+       VM_BUG_ON_PGFLAGS(PageTail(page), page);
+
        if (page_has_private(page))
                return (struct iomap_page *)page_private(page);
        return NULL;
@@ -45,11 +52,13 @@ static struct iomap_page *
 iomap_page_create(struct inode *inode, struct page *page)
 {
        struct iomap_page *iop = to_iomap_page(page);
+       unsigned int nr_blocks = i_blocks_per_page(inode, page);
 
-       if (iop || i_blocks_per_page(inode, page) <= 1)
+       if (iop || nr_blocks <= 1)
                return iop;
 
-       iop = kzalloc(sizeof(*iop), GFP_NOFS | __GFP_NOFAIL);
+       iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
+                       GFP_NOFS | __GFP_NOFAIL);
        spin_lock_init(&iop->uptodate_lock);
        attach_page_private(page, iop);
        return iop;
@@ -59,11 +68,14 @@ static void
 iomap_page_release(struct page *page)
 {
        struct iomap_page *iop = detach_page_private(page);
+       unsigned int nr_blocks = i_blocks_per_page(page->mapping->host, page);
 
        if (!iop)
                return;
        WARN_ON_ONCE(atomic_read(&iop->read_count));
        WARN_ON_ONCE(atomic_read(&iop->write_count));
+       WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
+                       PageUptodate(page));
        kfree(iop);
 }
 
-- 
2.28.0