ovlfs: cannot read latest data from read only fd

["zhangyi (F)" <yi.zhang@xxxxxxxxxx>]

Hi:
We found that we cannot read the latest data from an opened file(eg: "testfile") in overlayfs
which was opened in read-only mode(this file was originally located in the lowerdir and
mapped in the mergedir, and then was copied to upperdir when someone added some data later).

	For example:

	lower: testfile(empty)
	upper: empty
	work: empty
	merge: empty
	
	mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=work overlayfs merge

	1) fd1 = open("merge/testfile", O_RDONLY);
	2) fd2 = open("merge/testfile", O_WRONLY | O_APPEND);
           then we write some new data to fd2.
	3) read data form fd1.

	expect results: we read data which was written used by fd2.
	reality results: nothing.

We cannot read data from the copied file in the upperdir use fd1, so do we need to fix it?

Thanks,
zhangyi

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