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Message-Id: <30D261EC-6B62-4A83-BA43-29DF560DF193@dilger.ca>
Date: Fri, 12 Sep 2014 13:41:17 -0600
From: Andreas Dilger <adilger@...ger.ca>
To: TR Reardon <thomas_reardon@...mail.com>
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 5:03 PM, TR Reardon <thomas_reardon@...mail.com> wrote:
> diff --git a/misc/e4defrag.c b/misc/e4defrag.c
> index d0eac60..8001182 100644
> --- a/misc/e4defrag.c
> +++ b/misc/e4defrag.c
> @@ -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 );
Lines need to be <= 80 columns. Please run patch through checkpatch.pl.
Why is it opened O_WRONLY, but the permissions are S_IRUSR | S_IWUSR?
I agree it didn't make sense in the old code to have S_IRUSR either,
but I don't think this makes more sense. If the file is write-only
(which is probably correct, unless e4defrag is doing some post-copy
checksum of the data) then S_IWUSR would be enough.
> 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);
Wrap at 80 columns.
This has the same issue with O_WRONLY and S_IRUSR, though it at least
matches the old code.
> + 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;
> }
Shouldn't it reset the timestamp in this case?
Cheers, Andreas
> - goto out;
> }
> -
> +
> /* Allocate space for donor inode */
> orig_group_tmp = orig_group_head;
> do {
> ----------------------------------------
>> From: thomas_reardon@...mail.com
>> To: adilger@...ger.ca
>> CC: darrick.wong@...cle.com; linux-ext4@...r.kernel.org
>> Subject: RE: possible different donor file naming in e4defrag
>> Date: Thu, 11 Sep 2014 18:49:07 -0400
>>
>>> 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 {
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
>> the body of a message to majordomo@...r.kernel.org
>> More majordomo info at http://vger.kernel.org/majordomo-info.html
> <DEFRAG_O_TMPFILE>
Cheers, Andreas
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