linux-ext4 - RE: possible different donor file naming in e4defrag

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Date:	Thu, 11 Sep 2014 18:49:07 -0400
From:	TR Reardon <thomas_reardon@...mail.com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: RE: possible different donor file naming in e4defrag

> On Sep 11, 2014, at 1:48 PM, TR Reardon <thomas_reardon@...mail.com> wrote:
>> Picking this back up. How would O_TMPFILE avoid races? It definitely avoids the unwanted mtime/atime update, but then the existing "<filename>.defrag" pseudo-lock file would no longer be available. How could you use O_TMPFILE and still avoid multiple defrag? If this isn't possible, then resetting the parent times on unlink(tmpfile), as you suggest, is the simplest way out of this.
>
> Looking at this the opposite way - what are the chances that there
> will be concurrent defrags on the same file? Basically no chance at
> all. So long as it doesn't explode (the kernel would need to protect
> against this anyway to avoid malicious apps), the worst case is that
> there will be some extra defragmentation done in a very rare case.
>
> Conversely, creating a temporary filename and then resetting the
> parent directory timestamp is extra work for every file defragmented,
> and is racy because e4defrag may "reset" the time to before the temp
> file was created, but clobber a legitimate timestamp update in the
> directory from some other concurrent update. That timestamp update
> is always going to be racy, even if e4defrag tries to be careful.
>
> Cheers, Andreas


Thanks, well described.

So a simple attempt with O_TMPFILE first, then revert to original behavior, like below?


diff --git a/misc/e4defrag.c b/misc/e4defrag.c
index d0eac60..8001182 100644
--- a/misc/e4defrag.c
+++ b/misc/e4defrag.c
@@ -40,6 +40,7 @@
 #include <sys/stat.h>
 #include <sys/statfs.h>
 #include <sys/vfs.h>
+#include <libgen.h>
 
 /* A relatively new ioctl interface ... */
 #ifndef EXT4_IOC_MOVE_EXT
@@ -1526,31 +1527,36 @@ static int file_defrag(const char *file, const struct stat64 *buf,
 
 	/* Create donor inode */
 	memset(tmp_inode_name, 0, PATH_MAX + 8);
-	sprintf(tmp_inode_name, "%.*s.defrag",
-				(int)strnlen(file, PATH_MAX), file);
-	donor_fd = open64(tmp_inode_name, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR);
+	/* Try O_TMPFILE first, to avoid changing directory mtime */
+	sprintf(tmp_inode_name, "%.*s", (int)strnlen(file, PATH_MAX), file);
+	donor_fd = open64(dirname(tmp_inode_name), O_TMPFILE | O_WRONLY | O_EXCL, S_IRUSR | S_IWUSR );
 	if (donor_fd < 0) {
-		if (mode_flag & DETAIL) {
-			PRINT_FILE_NAME(file);
-			if (errno == EEXIST)
-				PRINT_ERR_MSG_WITH_ERRNO(
-				"File is being defraged by other program");
-			else
-				PRINT_ERR_MSG_WITH_ERRNO(NGMSG_FILE_OPEN);
+		sprintf(tmp_inode_name, "%.*s.defrag",
+					(int)strnlen(file, PATH_MAX), file);
+		donor_fd = open64(tmp_inode_name, O_WRONLY | O_CREAT | O_EXCL, S_IRUSR);
+		if (donor_fd < 0) {
+			if (mode_flag & DETAIL) {
+				PRINT_FILE_NAME(file);
+				if (errno == EEXIST)
+					PRINT_ERR_MSG_WITH_ERRNO(
+					"File is being defraged by other program");
+				else
+					PRINT_ERR_MSG_WITH_ERRNO(NGMSG_FILE_OPEN);
+			}
+			goto out;
 		}
-		goto out;
-	}
 
-	/* Unlink donor inode */
-	ret = unlink(tmp_inode_name);
-	if (ret < 0) {
-		if (mode_flag & DETAIL) {
-			PRINT_FILE_NAME(file);
-			PRINT_ERR_MSG_WITH_ERRNO("Failed to unlink");
+		/* Unlink donor inode */
+		ret = unlink(tmp_inode_name);
+		if (ret < 0) {
+			if (mode_flag & DETAIL) {
+				PRINT_FILE_NAME(file);
+				PRINT_ERR_MSG_WITH_ERRNO("Failed to unlink");
+			}
+			goto out;
 		}
-		goto out;
 	}
-
+	
 	/* Allocate space for donor inode */
 	orig_group_tmp = orig_group_head;
 	do {

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