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cifs: destage any unwritten data to the server before calling copychunk_write

Ronnie Sahlberg requested to merge lsahlber/centos-stream-9:1997373 into main

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1997373 commit f5d0f921ea362636e4a2efb7c38d1ead373a8700

because the copychunk_write might cover a region of the file that has not yet been sent to the server and thus fail.

A simple way to reproduce this is: truncate -s 0 /mnt/testfile; strace -f -o x -ttT xfs_io -i -f -c 'pwrite 0k 128k' -c 'fcollapse 16k 24k' /mnt/testfile

the issue is that the 'pwrite 0k 128k' becomes rearranged on the wire with the 'fcollapse 16k 24k' due to write-back caching.

fcollapse is implemented in cifs.ko as a SMB2 IOCTL(COPYCHUNK_WRITE) call and it will fail serverside since the file is still 0b in size serverside until the writes have been destaged. To avoid this we must ensure that we destage any unwritten data to the server before calling COPYCHUNK_WRITE.

Reported-by: Xiaoli Feng <xifeng@redhat.com>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
(cherry picked from commit f5d0f921ea362636e4a2efb7c38d1ead373a8700)

Signed-off-by: Ronnie Sahlberg lsahlber@redhat.com

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