[RFC 2/3] fs: Add block_willwrite

Andy Lutomirski
2013-08-05T19:44:41Z

This provides generic support for MADV_WILLWRITE. It creates and maps
buffer heads, but it should not result in anything being marked dirty.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

As described in the 0/0 summary, this may have issues.

fs/buffer.c | 57 +++++++++++++++++++++++++++++++++++++++++++++
include/linux/buffer_head.h | 3 +++
2 files changed, 60 insertions(+)

diff --git a/fs/buffer.c b/fs/buffer.c
index 4d74335..017e822 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2444,6 +2444,63 @@ int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
}
EXPORT_SYMBOL(block_page_mkwrite);

+long block_willwrite(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end,
+ get_block_t get_block)
+{
+ long ret = 0;
+ loff_t size;
+ struct inode *inode = file_inode(vma->vm_file);
+ struct super_block *sb = inode->i_sb;
+
+ for (; start < end; start += PAGE_CACHE_SIZE) {
+ struct page *p;
+ int size_in_page;
+ int tmp = get_user_pages_fast(start, 1, 0, &p);
+ if (tmp == 0)
+ tmp = -EFAULT;
+ if (tmp != 1) {
+ ret = tmp;
+ break;
+ }
+
+ sb_start_pagefault(sb);
+
+ lock_page(p);
+ size = i_size_read(inode);
+ if (WARN_ON_ONCE(p->mapping != inode->i_mapping) ||
+ (page_offset(p) > size)) {
+ ret = -EFAULT; /* A real write would have failed. */
+ goto pagedone_unlock;
+ }
+
+ /* page is partially inside EOF? */
+ if (((p->index + 1) << PAGE_CACHE_SHIFT) > size)
+ size_in_page = size & ~PAGE_CACHE_MASK;
+ else
+ size_in_page = PAGE_CACHE_SIZE;
+
+ tmp = __block_write_begin(p, 0, size_in_page, get_block);
+ if (tmp) {
+ ret = tmp;
+ goto pagedone_unlock;
+ }
+
+ ret += PAGE_CACHE_SIZE;
+
+ /* No need to commit -- we're not writing anything yet. */
+
+ pagedone_unlock:
+ unlock_page(p);
+ sb_end_pagefault(sb);
+ if (ret < 0)
+ break;
+ }
+
+ return ret;
+}
+EXPORT_SYMBOL(block_willwrite);
+
/*
* nobh_write_begin()'s prereads are special: the buffer_heads are freed
* immediately, while under the page lock. So it needs a special end_io
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 91fa9a9..c84639d 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -230,6 +230,9 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
get_block_t get_block);
int block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
get_block_t get_block);
+long block_willwrite(struct vm_area_struct *vma,
+ unsigned long start, unsigned long end,
+ get_block_t get_block);
/* Convert errno to return value from ->page_mkwrite() call */
static inline int block_page_mkwrite_return(int err)
{
--
1.8.3.1