[RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages)

[Kasireddy, Vivek]

Hi Alistair, David, Jason,

> >>>>>> Right, the "the zero pages are changed into writable pages" in your
> >>>>>> above comment just might not apply, because there won't be any
> page
> >>>>>> replacement (hopefully :) ).
> >>>>
> >>>>> If the page replacement does not happen when there are new writes
> to the
> >>>>> area where the hole previously existed, then would we still get an
> >>>> invalidate
> >>>>> when this happens? Is there any other way to get notified when the
> zeroed
> >>>>> page is written to if the invalidate does not get triggered?
> >>>>
> >>>> What David is saying is that memfd does not use the zero page
> >>>> optimization for hole punches. Any access to the memory, including
> >>>> read-only access through hmm_range_fault() will allocate unique
> >>>> pages. Since there is no zero page and no zero-page replacement there
> >>>> is no issue with invalidations.
> >>
> >>> It looks like even with hmm_range_fault(), the invalidate does not get
> >>> triggered when the hole is refilled with new pages because of writes.
> >>> This is probably because hmm_range_fault() does not fault in any pages
> >>> that get invalidated later when writes occur.
> >> hmm_range_fault() returns the current content of the VMAs, or it
> >> faults. If it returns pages then it came from one of these two places.
> >> If your VMA is incoherent with what you are doing then you have
> >> bigger
> >> problems, or maybe you found a bug.
> 
> Note it will only fault in pages if HMM_PFN_REQ_FAULT is specified. You
> are setting that however you aren't setting HMM_PFN_REQ_WRITE which is
> what would trigger a fault to bring in the new pages. Does setting that
> fix the issue you are seeing?
No, adding HMM_PFN_REQ_WRITE still doesn't help in fixing the issue.
Although, I do not have THP enabled (or built-in), shmem does not evict
the pages after hole punch as noted in the comment in shmem_fallocate():
                if ((u64)unmap_end > (u64)unmap_start)
                        unmap_mapping_range(mapping, unmap_start,
                                            1 + unmap_end - unmap_start, 0);
                shmem_truncate_range(inode, offset, offset + len - 1);
                /* No need to unmap again: hole-punching leaves COWed pages */

As a result, the pfn is still valid and the pte is pte_present() and pte_write().
This is the reason why adding in HMM_PFN_REQ_WRITE does not help;
because, it fails the below condition in hmm_pte_need_fault():
        if ((pfn_req_flags & HMM_PFN_REQ_WRITE) &&
            !(cpu_flags & HMM_PFN_WRITE))
                return HMM_NEED_FAULT | HMM_NEED_WRITE_FAULT;

If I force it to read-fault or write-fault (by hacking hmm_pte_need_fault()),
it gets indefinitely stuck in the do while loop in hmm_range_fault().
AFAIU, unless there is a way to fault-in zero pages (or any scratch pages)
after hole punch that get invalidated because of writes, I do not see how
using hmm_range_fault() can help with my use-case. 

Thanks,
Vivek

> 
> >>> The above log messages are seen immediately after the hole is punched.
> As
> >>> you can see, hmm_range_fault() returns the pfns of old pages and not
> zero
> >>> pages. And, I see the below messages (with patch #2 in this series
> applied)
> >>> as the hole is refilled after writes:
> >> I don't know what you are doing, but it is something wrong or you've
> >> found a bug in the memfds.
> >
> >
> > Maybe THP is involved? I recently had to dig that out for an internal
> > discussion:
> >
> > "Currently when truncating shmem file, if the range is partial of THP
> > (start or end is in the middle of THP), the pages actually will just get
> > cleared rather than being freed unless the range cover the whole THP.
> > Even though all the subpages are truncated (randomly or sequentially),
> > the THP may still be kept in page cache.  This might be fine for some
> > usecases which prefer preserving THP."
> >
> > My recollection is that this behavior was never changed.
> >
> > https://lore.kernel.org/all/1575420174-19171-1-git-send-email-
> yang.shi at linux.alibaba.com/