Re: [syzbot] general protection fault in PageHeadHuge

[Peter Xu]

 On Thu, Sep 29, 2022 at 09:29:54PM -0400, Peter Xu wrote:
> Hi, Mike,
> 
> On Thu, Sep 29, 2022 at 04:33:53PM -0700, Mike Kravetz wrote:
> > I added some TLB flushing to hugetlb_mcopy_atomic_pte, but it did not make
> > any difference.  Suggestions would be appreciated as cache/tlb/???  flushing
> > issues take me a while to figure out.
> 
> It seems the UFFDIO_COPY for hugetlb is the major issue here, in that for
> private mappings we don't inject the page cache.
> 
> I think it makes sense in that e.g. we don't want to allow a private
> mapping to be able to write to the page cache.  But afaict that's not
> correct because UFFDIO_COPY resolves exactly page faults in page cache
> layer for file backed memories.  So what we should do is inject page cache
> but mark the page RO, waiting for a coming CoW if needed.
> 
> I'll attach one patch fix that will start to inject the page into page
> cache for UFFDIO_COPY+hugetlb even if mapping is private.  Another test
> patch is also added because otherwise the private hugetlb selftest won't
> work after the fix applied - in the selftest we used to use DONTNEED to
> drop the private mapping, but IMHO that's not enough, we need to drop the
> page cache too (after the fix).  I've also have the test patch attached.
> 
> Feel free to try out with the two patches applied.  It started to work for
> me for current issue.
> 
> I didn't yet post them out yet because after I applied the two patches I
> found other issues - the reserved pages are messed up and leaked.  I'll
> keep looking tomorrow on the leak issue, but please also let me know if you
> figured anything suspecious as I know you're definitely must more fluent on
> the reservation code.
> 
> And that's not the only issue I found - shmem can have other issues
> regarding private mappings; shmem does it right on the page cache insertion
> but not the rest I think..  I'll look into them one by one.  It's quite
> interesting to dig multiple things out of the write check symptons..

The patches..

-- 
Peter Xu