take offset of start position into account when calculating page count.

Signed-off-by: Yan, Zheng <[email protected]>
---
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 66836d8..ef94e46 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -970,8 +970,8 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
                size_t write_bytes = min(iov_iter_count(&i),
                                         nrptrs * (size_t)PAGE_CACHE_SIZE -
                                         offset);
-               size_t num_pages = (write_bytes + PAGE_CACHE_SIZE - 1) >>
-                                       PAGE_CACHE_SHIFT;
+               size_t num_pages = (write_bytes + offset +
+                                   PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
 
                WARN_ON(num_pages > nrptrs);
                memset(pages, 0, sizeof(struct page *) * nrptrs);
@@ -1001,8 +1001,8 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
 
                copied = btrfs_copy_from_user(pos, num_pages,
                                           write_bytes, pages, &i);
-               dirty_pages = (copied + PAGE_CACHE_SIZE - 1) >>
-                                       PAGE_CACHE_SHIFT;
+               dirty_pages = (copied + offset + PAGE_CACHE_SIZE - 1) >>
+                               PAGE_CACHE_SHIFT;
 
                if (num_pages > dirty_pages) {
                        if (copied > 0)
--
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