Commit 914ee295 authored by Xin Zhong's avatar Xin Zhong Committed by Chris Mason
Browse files

Btrfs: pwrite blocked when writing from the mmaped buffer of the same page 

This problem is found in meego testing:
http://bugs.meego.com/show_bug.cgi?id=6672


A file in btrfs is mmaped and the mmaped buffer is passed to pwrite to write to the same page
of the same file. In btrfs_file_aio_write(), the pages is locked by prepare_pages(). So when
btrfs_copy_from_user() is called, page fault happens and the same page needs to be locked again
in filemap_fault(). The fix is to move iov_iter_fault_in_readable() before prepage_pages() to make page
fault happen before pages are locked. And also disable page fault in critical region in
btrfs_copy_from_user().

Reviewed-by: default avatarYan, <Zheng&lt;zheng.z.yan@intel.com>
Signed-off-by: default avatarZhong, Xin <xin.zhong@intel.com>
Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
parent f106e82c

fs/btrfs/file.c

+60 −32
Original line number Diff line number Diff line
@@ -48,30 +48,34 @@ static noinline int btrfs_copy_from_user(loff_t pos, int num_pages, 
					 struct page **prepared_pages,

					 struct iov_iter *i)

{

	size_t copied;

	size_t copied = 0;

	int pg = 0;

	int offset = pos & (PAGE_CACHE_SIZE - 1);

	int total_copied = 0;



	while (write_bytes > 0) {

		size_t count = min_t(size_t,

				     PAGE_CACHE_SIZE - offset, write_bytes);

		struct page *page = prepared_pages[pg];

again:

		if (unlikely(iov_iter_fault_in_readable(i, count)))

			return -EFAULT;



		/* Copy data from userspace to the current page */

		copied = iov_iter_copy_from_user(page, i, offset, count);

		/*

		 * Copy data from userspace to the current page

		 *

		 * Disable pagefault to avoid recursive lock since

		 * the pages are already locked

		 */

		pagefault_disable();

		copied = iov_iter_copy_from_user_atomic(page, i, offset, count);

		pagefault_enable();



		/* Flush processor's dcache for this page */

		flush_dcache_page(page);

		iov_iter_advance(i, copied);

		write_bytes -= copied;

		total_copied += copied;



		/* Return to btrfs_file_aio_write to fault page */

		if (unlikely(copied == 0)) {

			count = min_t(size_t, PAGE_CACHE_SIZE - offset,

				      iov_iter_single_seg_count(i));

			goto again;

			break;

		}



		if (unlikely(copied < PAGE_CACHE_SIZE - offset)) {
@@ -81,7 +85,7 @@ again: 
			offset = 0;

		}

	}

	return 0;

	return total_copied;

}



/*
@@ -854,6 +858,8 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb, 
	unsigned long last_index;

	int will_write;

	int buffered = 0;

	int copied = 0;

	int dirty_pages = 0;



	will_write = ((file->f_flags & O_DSYNC) || IS_SYNC(inode) ||

		      (file->f_flags & O_DIRECT));
@@ -970,7 +976,17 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb, 
		WARN_ON(num_pages > nrptrs);

		memset(pages, 0, sizeof(struct page *) * nrptrs);



		ret = btrfs_delalloc_reserve_space(inode, write_bytes);

		/*

		 * Fault pages before locking them in prepare_pages

		 * to avoid recursive lock

		 */

		if (unlikely(iov_iter_fault_in_readable(&i, write_bytes))) {

			ret = -EFAULT;

			goto out;

		}



		ret = btrfs_delalloc_reserve_space(inode,

					num_pages << PAGE_CACHE_SHIFT);

		if (ret)

			goto out;
@@ -978,37 +994,49 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb, 
				    pos, first_index, last_index,

				    write_bytes);

		if (ret) {

			btrfs_delalloc_release_space(inode, write_bytes);

			btrfs_delalloc_release_space(inode,

					num_pages << PAGE_CACHE_SHIFT);

			goto out;

		}



		ret = btrfs_copy_from_user(pos, num_pages,

		copied = btrfs_copy_from_user(pos, num_pages,

					   write_bytes, pages, &i);

		if (ret == 0) {

		dirty_pages = (copied + PAGE_CACHE_SIZE - 1) >>

					PAGE_CACHE_SHIFT;



		if (num_pages > dirty_pages) {

			if (copied > 0)

				atomic_inc(

					&BTRFS_I(inode)->outstanding_extents);

			btrfs_delalloc_release_space(inode,

					(num_pages - dirty_pages) <<

					PAGE_CACHE_SHIFT);

		}



		if (copied > 0) {

			dirty_and_release_pages(NULL, root, file, pages,

						num_pages, pos, write_bytes);

						dirty_pages, pos, copied);

		}



		btrfs_drop_pages(pages, num_pages);

		if (ret) {

			btrfs_delalloc_release_space(inode, write_bytes);

			goto out;

		}



		if (copied > 0) {

			if (will_write) {

				filemap_fdatawrite_range(inode->i_mapping, pos,

						 pos + write_bytes - 1);

							 pos + copied - 1);

			} else {

			balance_dirty_pages_ratelimited_nr(inode->i_mapping,

							   num_pages);

			if (num_pages <

				balance_dirty_pages_ratelimited_nr(

							inode->i_mapping,

							dirty_pages);

				if (dirty_pages <

				(root->leafsize >> PAGE_CACHE_SHIFT) + 1)

					btrfs_btree_balance_dirty(root, 1);

				btrfs_throttle(root);

			}

		}



		pos += write_bytes;

		num_written += write_bytes;

		pos += copied;

		num_written += copied;



		cond_resched();

	}

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