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

Kasireddy, Vivek
2023-08-24T06:48:32Z

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.
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;
}

static const struct mmu_interval_notifier_ops udmabuf_invalidate_ops = {
.invalidate = invalidate_udmabuf,
};

Thanks,
Vivek

>
> > 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