Re: [PATCH] btrfs file write debugging patch

From: Mitch Harder <mitch.harder@sabayonlinux.org>
To: Xin Zhong <thierryzhong@hotmail.com>
Cc: chris.mason@oracle.com, xin.zhong@intel.com, linux-btrfs@vger.kernel.org
Subject: Re: [PATCH] btrfs file write debugging patch
Date: Sat, 5 Mar 2011 10:50:14 -0600	[thread overview]
Message-ID: <AANLkTi=gqurEsoMZv+10Np4duOKq4kZf77mTgu6BfkSC@mail.gmail.com> (raw)
In-Reply-To: <AANLkTimDvTraBapv18iOf8wmUh8nSnUay2etXvjEv77P@mail.gmail.com>

I've constructed a test patch that is currently addressing all the
issues on my system.

The portion of Openmotif that was having issues with page faults works
correctly with this patch, and gcc-4.4.5 builds without issue.

I extracted only the portion of the first patch that corrects the
handling of dirty_pages when copied==0, and incorporated the second
patch that falls back to one-page-at-a-time if there are troubles with
page faults.

--- fs/btrfs/file.c	2011-03-05 07:34:43.025131607 -0600
+++ /usr/src/linux/fs/btrfs/file.c	2011-03-05 07:41:45.001260294 -0600
@@ -1023,8 +1023,20 @@

 		copied = btrfs_copy_from_user(pos, num_pages,
 					   write_bytes, pages, &i);
-		dirty_pages = (copied + offset + PAGE_CACHE_SIZE - 1) >>
-				PAGE_CACHE_SHIFT;
+
+		/*
+		 * if we have trouble faulting in the pages, fall
+		 * back to one page at a time
+		 */
+		if (copied < write_bytes)
+			nrptrs = 1;
+
+		if (copied == 0)
+			dirty_pages = 0;
+		else
+			dirty_pages = (copied + offset +
+				       PAGE_CACHE_SIZE - 1) >>
+				       PAGE_CACHE_SHIFT;

 		if (num_pages > dirty_pages) {
 			if (copied > 0)
