From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:58345 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726135AbfAaSLL (ORCPT ); Thu, 31 Jan 2019 13:11:11 -0500 Date: Thu, 31 Jan 2019 13:11:09 -0500 From: Brian Foster Subject: Re: [PATCH 04/11] xfs: don't try to map blocks beyond i_size in writeback Message-ID: <20190131181108.GF36239@bfoster> References: <20190131075524.4769-1-hch@lst.de> <20190131075524.4769-5-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190131075524.4769-5-hch@lst.de> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Christoph Hellwig Cc: linux-xfs@vger.kernel.org On Thu, Jan 31, 2019 at 08:55:17AM +0100, Christoph Hellwig wrote: > We already shortcut xfs_map_blocks for COW mappings, but there is just > as little reason to start writeback beyond i_size in the data fork. > > Note that this has to be just an optimization as hole punches can unmaps > block just like truncate, and we need to handle that case further down > in the low-level code. > > Signed-off-by: Christoph Hellwig > --- > fs/xfs/xfs_aops.c | 34 ++++++++++++++-------------------- > 1 file changed, 14 insertions(+), 20 deletions(-) > > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c > index 8bfb62d8776f..9c2a1947d5dd 100644 > --- a/fs/xfs/xfs_aops.c > +++ b/fs/xfs/xfs_aops.c > @@ -348,6 +348,20 @@ xfs_map_blocks( > if (XFS_FORCED_SHUTDOWN(mp)) > return -EIO; > > + /* > + * If the offset is beyond the inode size, we know that we raced with > + * truncate. No point in doing calling any lower level code, just > + * return a hole so that the writeback code skips writeback for the > + * rest of the file. > + */ > + if (offset > i_size_read(inode)) { > + wpc->imap.br_startoff = offset_fsb; > + wpc->imap.br_blockcount = end_fsb - offset_fsb; > + wpc->imap.br_startblock = HOLESTARTBLOCK; > + wpc->imap.br_state = XFS_EXT_NORM; > + return 0; > + } > + The code looks fine, but I don't see any more value in this code than the similar code down in xfs_iomap_write_allocate(). The comment implies this skips writeback for the rest of the file, but AFACT the higher level page->index code in xfs_do_writepage() already does that. All these checks do is skip the remaining blocks in the current page. When you consider that we're most likely sending an I/O in the latter case either way, I'm curious why we'd bother to keep this around at all. Brian > /* > * COW fork blocks can overlap data fork blocks even if the blocks > * aren't shared. COW I/O always takes precedent, so we must always > @@ -388,26 +402,6 @@ xfs_map_blocks( > xfs_iunlock(ip, XFS_ILOCK_SHARED); > > wpc->fork = XFS_COW_FORK; > - > - /* > - * Truncate can race with writeback since writeback doesn't > - * take the iolock and truncate decreases the file size before > - * it starts truncating the pages between new_size and old_size. > - * Therefore, we can end up in the situation where writeback > - * gets a CoW fork mapping but the truncate makes the mapping > - * invalid and we end up in here trying to get a new mapping. > - * bail out here so that we simply never get a valid mapping > - * and so we drop the write altogether. The page truncation > - * will kill the contents anyway. > - */ > - if (offset > i_size_read(inode)) { > - wpc->imap.br_blockcount = end_fsb - offset_fsb; > - wpc->imap.br_startoff = offset_fsb; > - wpc->imap.br_startblock = HOLESTARTBLOCK; > - wpc->imap.br_state = XFS_EXT_NORM; > - return 0; > - } > - > goto allocate_blocks; > } > > -- > 2.20.1 >