The other filesystems such as ext3 do not have this problem since they are using generic_file_buffered_write(). And this problem is fixed back in 2007 for generic_file_buffered_write(): http://lkml.org/lkml/2007/2/4/26 I am very new to btrfs. I am wondering why btrfs has it's own write routine? Thanks! -----Original Message----- From: Zhong, Xin Sent: Tuesday, December 07, 2010 5:25 PM To: linux-btrfs@xxxxxxxxxxxxxxx Cc: Zhong, Xin Subject: [PATCH] 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(). Signed-off-by: Xin Zhong <xin.zhong@xxxxxxxxx> --- fs/btrfs/file.c | 23 ++++++++++++++++++----- 1 files changed, 18 insertions(+), 5 deletions(-) diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index c1faded..805f2ee 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -57,11 +57,15 @@ static noinline int btrfs_copy_from_user(loff_t pos, int num_pages, 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); @@ -974,6 +978,15 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb, if (ret) goto out; + /* + * 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 = prepare_pages(root, file, pages, num_pages, pos, first_index, last_index, write_bytes); -- 1.6.2.2 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
