Re: [PATCH] mm,mremap: Bail out earlier in mremap_to under map pressure

On Thu, Feb 28, 2019 at 12:06 AM Vlastimil Babka [off-list ref] wrote:

On 2/27/19 10:32 PM, Oscar Salvador wrote:

quoted

On Tue, Feb 26, 2019 at 02:04:28PM -0800, Andrew Morton wrote:

quoted

How is this going to affect existing userspace which is aware of the
current behaviour?

Well, current behavior is not really predictable.
Our customer was "surprised" that the call to mremap() failed, but the regions
got unmapped nevertheless.
They found it the hard way when they got a segfault when trying to write to those
regions when cleaning up.

As I said in the changelog, the possibility for false positives exists, due to
the fact that we might get rid of several vma's when unmapping, but I do not
expect existing userspace applications to start failing.
Should be that the case, we can revert the patch, it is not that it adds a lot
of churn.

Hopefully the only program that would start failing would be a LTP test
testing the current behavior near the limit (if such test exists). And
that can be adjusted.

IMO the original behavior is itself probably not a big issue because
if userspace wanted to mremap over something, it was prepared to lose
the "over something" mapping anyway. So it does seem to be a stretch
to call the behavior a "bug". Still I agree with the patch that mremap
should not leave any side effects after returning error.

thanks,

 - Joel