Re: Potential bug in soft-dirty bits (with test case)

[Mohamed Alzayat <alzayat@mpi-sws.org>]

On Fri, Nov 27, 2020 at 5:40 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 11/25/20 3:15 PM, Mohamed Alzayat wrote:
> > Hi Everyone,
> >
> > I have noticed a change in the synchrony of updating the soft-dirty
> > bits in recent kernel versions (5.6+). More precisely, up to kernel
> > v5.5, the soft-dirty bits as parsed from /proc/pid/pagemap accurately
> > capture the dirtied pages. Recently, I started testing on kernels v5.6
> > - v5.9, and I noticed that the soft-dirty bits are not immediately
> > updated.
> >
> > I have prepared a short test that repeatedly causes at least one
> > memory page to be dirtied, then scans /proc/pid/pagemap counting the
> > soft-dirty bits. The test fails if this count is zero. In my
> > observation, this test fails once in every 10-20 trials. The test
> > defaults to 100 trials and can be found at
> > https://gitlab.mpi-sws.org/-/snippets/1696
> >
> > Is this non-synchronous propagation of soft dirty bits intended? If
>
> AFAIK, not. The tracking is done by write-protecting the pages to cause a page
> fault, so it should be quite synchronous update of page table entries, and
> reading pagemap is a page table walk of those very entries.
>
> But as you have the test, it should be possible to git bisect it? Just do enough
> trials to be sure enough that no fail means indeed a "good" kernel.

Thanks for confirming, Vlastimil!

The first bad commit is: 0758cd8304942292e95a0f750c374533db378b32
asm-generic/tlb: avoid potential double flush
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0758cd8304942292e95a0f750c374533db378b32

Reverting this commit solves the problem, but this might not be the
right way of fixing it.



>
> > yes, is there a way to force the soft-dirty bits to be propagated to
> > the page map entries immediately, or is there an alternative interface
> > that has the synchronous behavior?
> >
> > Thanks in advance,
> > Mohamed Alzayat
> >
>
>