On 07/29/2011 08:25 PM, Andreas Dilger wrote:
On 2011-07-29, at 11:44 AM, Bernd Schubert wrote:
thanks a lot for sending those patches! From a previous review I still remember ext2fs_mmp_read(), see below.
I will look through the other parts later on.
+errcode_t ext2fs_mmp_read(ext2_filsys fs, blk64_t mmp_blk, void *buf)
+{
+ /* ext2fs_open reserves fd0,1,2 to avoid stdio collision */
+ if (fs->mmp_fd<= 0) {
+ fs->mmp_fd = open(fs->device_name, O_RDWR | O_DIRECT);
+ if (fs->mmp_fd< 0) {
+ retval = EXT2_ET_MMP_OPEN_DIRECT;
+ goto out;
+ }
+ }
+
+ if (ext2fs_llseek(fs->mmp_fd, mmp_blk * fs->blocksize, SEEK_SET) !=
+ mmp_blk * fs->blocksize) {
+ retval = EXT2_ET_LLSEEK_FAILED;
+ goto out;
+ }
+
+ if (read(fs->mmp_fd, fs->mmp_cmp, fs->blocksize) != fs->blocksize) {
+ retval = EXT2_ET_SHORT_READ;
+ goto out;
+ }
While ext2fs_llseek() and read() works fine, I still wonder why the code does not use io_channel_read_blk64() here, similarly as io_channel_write_blk64() write in ext2fs_mmp_write().
One of the reasons for doing a direct open+seek+read here is because previous
versions of e2fsprogs did not allow O_DIRECT reads via io_channel_read_blk64(),
which is critical for the correct MMP functioning. It cannot allow read from
a cached block, either in libext2fs or in the VM, though a cached block on the
disk is OK as long as it is visible to all hosts that are accessing it.
Yeah, I understand that it had to be O_DIRECT, but I didn't know about
(previous) limitation of io_channel_read_blk64(). Maybe you could add a
comment about it to the code about that, if you want to keep it?
Something like
"/* We don't use io_channel_read_blk64() here to make sure blocks are
really not cached */"?
Thanks,
Bernd
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