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

[Vlastimil Babka <vbabka@suse.cz>]

On 11/30/20 11:37 AM, Mohamed Alzayat wrote:
> 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.

Thanks for bisecting! Let's CC people involved in that commit. All important 
should be in the quoted conversation above.

Vlastimil

> 
>>
>> > 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
>> >
>>
>>
>