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

Kasireddy, Vivek
2023-08-28T04:47:28Z

Hi Alistair,

>
> > >
> > >> >> > > > 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():
> > >> >> > >
> > >> >> > > This is the source of all your problems.
> > >> >> > >
> > >> >> > > Things that are mm-centric are supposed to track the VMAs and
> > >> changes
> > >> >> to
> > >> >> > > the PTEs. If you do something in userspace and it doesn't cause
> the
> > >> >> > > CPU page tables to change then it certainly shouldn't cause any
> > mmu
> > >> >> > > notifiers or hmm_range_fault changes.
> > >> >> > I am not doing anything out of the blue in the userspace. I think the
> > >> >> behavior
> > >> >> > I am seeing with shmem (where an invalidation event
> > >> >> (MMU_NOTIFY_CLEAR)
> > >> >> > does occur because of a hole punch but the PTEs don't really get
> > >> updated)
> > >> >> > can arguably be considered an optimization.
> > >> >>
> > >> >> Your explanations don't make sense.
> > >> >>
> > >> >> If MMU_NOTIFER_CLEAR was sent but the PTEs were left present
> then:
> > >> >>
> > >> >> > > There should still be an invalidation notifier at some point when
> the
> > >> >> > > CPU tables do eventually change, whenever that is. Missing that
> > >> >> > > notification would be a bug.
> > >> >> > I clearly do not see any notification getting triggered (from both
> > >> >> shmem_fault()
> > >> >> > and hugetlb_fault()) when the PTEs do get updated as the hole is
> > refilled
> > >> >> > due to writes. Are you saying that there needs to be an invalidation
> > >> event
> > >> >> > (MMU_NOTIFY_CLEAR?) dispatched at this point?
> > >> >>
> > >> >> You don't get to get shmem_fault in the first place.
> > >> > What I am observing is that even after MMU_NOTIFY_CLEAR (hole
> > punch)
> > >> is sent,
> > >> > hmm_range_fault() finds that the PTEs associated with the hole are still
> > >> pte_present().
> > >> > I think it remains this way as long as there are reads on the hole. Once
> > >> there are
> > >> > writes, it triggers shmem_fault() which results in PTEs getting updated
> > but
> > >> without
> > >> > any notification.
> > >>
> > >> Oh wait, this is shmem. The read from hmm_range_fault() (assuming
> you
> > >> specified HMM_PFN_REQ_FAULT) will trigger shmem_fault() due to the
> > >> missing PTE.
> > > When running one of the udmabuf subtests (introduced in the third patch
> > of
> > > this series), I see that MMU_NOTIFY_CLEAR is sent when a hole is
> punched.
> > > As a response, hmm_range_fault() is called from the udmabuf invalidate
> > callback,
> >
> > Actually I'm suprised that works. If you've setup an interval notifier
> > and are updating the notifier sequence numbers correctly I would expect
> > hmm_range_fault() to return -EBUSY until
> > mmu_notifier_invalidate_range_end() is called.
> >
> > It might be helpful to post the code you're testing with somewhere but
> > are you calling mmu_interval_read_begin() to start the critical section
> > and mmu_interval_set_seq() to update the sequence in another notifier?
> > I'm not at all convinced calling hmm_range_fault() from a notifier can
> > be made to work though.
Turns out, calling hmm_range_fault() from the invalidate callback was indeed
a problem and the reason why new pages were not faulted-in. In other words,
it looks like the invalidate callback is not the right place to invoke hmm_range_fault()
as the PTEs may not have been cleared.

> That could be part of the problem. I mean the way hmm_range_fault()
> is invoked from the invalidate callback is probably incorrect as you are
> suggesting. Anyway, here is the code I am testing with:
> static bool invalidate_udmabuf(struct mmu_interval_notifier *mn,
> const struct mmu_notifier_range *range_mn,
> unsigned long cur_seq)
> {
> struct udmabuf_vma_range *range =
> container_of(mn, struct udmabuf_vma_range, range_mn);
> struct udmabuf *ubuf = range->ubuf;
> struct hmm_range hrange = {0};
> unsigned long *pfns, num_pages, timeout;
> int i, ret;
>
> printk("invalidate; start = %lu, end = %lu\n",
> range->start, range->end);
>
> hrange.notifier = mn;
> hrange.default_flags = HMM_PFN_REQ_FAULT;
> hrange.start = max(range_mn->start, range->start);
> hrange.end = min(range_mn->end, range->end);
> num_pages = (hrange.end - hrange.start) >> PAGE_SHIFT;
>
> pfns = kmalloc_array(num_pages, sizeof(*pfns), GFP_KERNEL);
> if (!pfns)
> return true;
>
> printk("invalidate; num pages = %lu\n", num_pages);
>
> hrange.hmm_pfns = pfns;
> timeout = jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
> do {
> hrange.notifier_seq = mmu_interval_read_begin(mn);
>
> mmap_read_lock(ubuf->vmm_mm);
> ret = hmm_range_fault(&hrange);
> mmap_read_unlock(ubuf->vmm_mm);
> if (ret) {
> if (ret == -EBUSY && !time_after(jiffies, timeout))
> continue;
> break;
> }
>
> if (mmu_interval_read_retry(mn, hrange.notifier_seq))
> continue;
> } while (ret);
>
> if (!ret) {
> for (i = 0; i < num_pages; i++) {
> printk("hmm returned page = %p; pfn = %lu\n",
> hmm_pfn_to_page(pfns[i]),
> pfns[i] & ~HMM_PFN_FLAGS);
> }
> }
> return true;
> }
>
Doing the above from a wq worker func (scheduled after invalidate event)
instead of the invalidate callback lets hmm_range_fault() fault-in new pages.
What this means is that, at-least in my use-case, getting MMU_NOTIFY_CLEAR
indicates that the invalidation is still ongoing and that it is not done yet.
Sorry for the confusion.

Thanks,
Vivek

> static const struct mmu_interval_notifier_ops udmabuf_invalidate_ops = {
> .invalidate = invalidate_udmabuf,
> };
>
> >
> > > to walk over the PTEs associated with the hole. When this happens, I
> > noticed that
> > > the below function returns HMM_PFN_VALID | HMM_PFN_WRITE for all
> > the
> > > PTEs associated with the hole.
> > > static inline unsigned long pte_to_hmm_pfn_flags(struct hmm_range
> > *range,
> > > pte_t pte)
> > > {
> > > if (pte_none(pte) || !pte_present(pte) || pte_protnone(pte))
> > > return 0;
> > > return pte_write(pte) ? (HMM_PFN_VALID | HMM_PFN_WRITE) :
> > HMM_PFN_VALID;
> > > }
> > >
> > > As a result, hmm_pte_need_fault() always returns 0 and shmem_fault()
> > > never gets triggered despite specifying HMM_PFN_REQ_FAULT |
> > HMM_PFN_REQ_WRITE.
> > > And, the set of PFNs returned by hmm_range_fault() are the same ones
> > > that existed before the hole was punched.
> > >
> > >> Subsequent writes will just upgrade PTE permissions
> > >> assuming the read didn't map them RW to begin with. If you want to
> > >> actually see the hole with hmm_range_fault() don't specify
> > >> HMM_PFN_REQ_FAULT (or _WRITE).
> > >>
> > >> >>
> > >> >> If they were marked non-prsent during the CLEAR then the shadow
> side
> > >> >> remains non-present until it gets its own fault.
> > >> >>
> > >> >> If they were made non-present without an invalidation then that is a
> > >> >> bug.
> > >> >>
> > >> >> > > hmm_range_fault() is the correct API to use if you are working
> with
> > >> >> > > notifiers. Do not hack something together using pin_user_pages.
> > >> >>
> > >> >> > I noticed that hmm_range_fault() does not seem to be working as
> > >> expected
> > >> >> > given that it gets stuck(hangs) while walking hugetlb pages.
> > >> >>
> > >> >> You are the first to report that, it sounds like a serious bug. Please
> > >> >> try to fix it.
> > >> >>
> > >> >> > Regardless, as I mentioned above, the lack of notification when PTEs
> > >> >> > do get updated due to writes is the crux of the issue
> > >> >> > here. Therefore, AFAIU, triggering an invalidation event or some
> > >> >> > other kind of notification would help in fixing this issue.
> > >> >>
> > >> >> You seem to be facing some kind of bug in the mm, it sounds pretty
> > >> >> serious, and it almost certainly is a missing invalidation.
> > >> >>
> > >> >> Basically, anything that changes a PTE must eventually trigger an
> > >> >> invalidation. It is illegal to change a PTE from one present value to
> > >> >> another present value without invalidation notification.
> > >> >>
> > >> >> It is not surprising something would be missed here.
> > >> > As you suggest, it looks like the root-cause of this issue is the missing
> > >> > invalidation notification when the PTEs are changed from one present
> > >>
> > >> I don't think there's a missing invalidation here. You say you're seeing
> > >> the MMU_NOTIFY_CLEAR when hole punching which is when the PTE is
> > >> cleared. When else do you expect a notification?
> > > Oh, given that we are finding PTEs that are still pte_present() even after
> > > MMU_NOTIFY_CLEAR is sent, the theory is that another
> > MMU_NOTIFY_CLEAR
> > > needs to be sent after the PTEs are updated when new pages are faulted-
> in.
> > >
> > > However, it just occurred to me that maybe the behavior I am seeing is not
> > > unexpected as it might be a timing issue that has to do with when the
> PTEs
> > > are walked. Let me explain. Here is what shmem does when a hole is
> > punched:
> > > 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);
> > >
> > > IIUC, the invalidate callback is called from unmap_mapping_range() but
> > > the page removal does not happen until shmem_truncate_range() gets
> > > called. So, if I were to call hmm_range_fault() after
> > shmem_truncate_range(),
> > > I might see different results as the PTEs would probably no longer be
> > present.
> > > In order to test this theory, I would have to schedule a wq thread func
> from
> > the
> > > invalidate callback (to walk the PTEs after a slight delay). I'll try this out
> > when
> > > I get a chance after addressing some of the locking concerns associated
> with
> > > pairing static/dynamic dmabuf exporters and importers.
> >
> > That sounds plausible. The PTE will actually be cleared in
> > unmap_mapping_range() after the mmu notifier is called. I'm curious how
> > hmm_range_fault() passes though.
> >
> > > Thanks,
> > > Vivek
> > >
> > >>
> > >> > value to another. I'd like to fix this issue eventually but I first need to
> > >> > focus on addressing udmabuf page migration (out of movable zone)
> > >> > and also look into the locking concerns Daniel mentioned about pairing
> > >> > static and dynamic dmabuf exporters and importers.
> > >> >
> > >> > Thanks,
> > >> > Vivek