From: Jason Gunthorpe <jgg@ziepe.ca>
To: Jerome Glisse <jglisse@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-rdma@vger.kernel.org,
Leon Romanovsky <leonro@mellanox.com>,
Doug Ledford <dledford@redhat.com>,
Artemy Kovalyov <artemyko@mellanox.com>,
Moni Shoua <monis@mellanox.com>,
Mike Marciniszyn <mike.marciniszyn@intel.com>,
Kaike Wan <kaike.wan@intel.com>,
Dennis Dalessandro <dennis.dalessandro@intel.com>,
linux-mm@kvack.org
Subject: Re: [PATCH v4 0/1] Use HMM for ODP v4
Date: Wed, 22 May 2019 19:43:20 -0300 [thread overview]
Message-ID: <20190522224320.GB15389@ziepe.ca> (raw)
In-Reply-To: <20190522214917.GA20179@redhat.com>
On Wed, May 22, 2019 at 05:49:18PM -0400, Jerome Glisse wrote:
> > > > So why is mm suddenly guarenteed valid? It was a bug report that
> > > > triggered the race the mmget_not_zero is fixing, so I need a better
> > > > explanation why it is now safe. From what I see the hmm_range_fault
> > > > is doing stuff like find_vma without an active mmget??
> > >
> > > So the mm struct can not go away as long as we hold a reference on
> > > the hmm struct and we hold a reference on it through both hmm_mirror
> > > and hmm_range struct. So struct mm can not go away and thus it is
> > > safe to try to take its mmap_sem.
> >
> > This was always true here, though, so long as the umem_odp exists the
> > the mm has a grab on it. But a grab is not a get..
> >
> > The point here was the old code needed an mmget() in order to do
> > get_user_pages_remote()
> >
> > If hmm does not need an external mmget() then fine, we delete this
> > stuff and rely on hmm.
> >
> > But I don't think that is true as we have:
> >
> > CPU 0 CPU1
> > mmput()
> > __mmput()
> > exit_mmap()
> > down_read(&mm->mmap_sem);
> > hmm_range_dma_map(range, device,..
> > ret = hmm_range_fault(range, block);
> > if (hmm->mm == NULL || hmm->dead)
> > mmu_notifier_release()
> > hmm->dead = true
> > vma = find_vma(hmm->mm, start);
> > .. rb traversal .. while (vma) remove_vma()
> >
> > *goes boom*
> >
> > I think this is violating the basic constraint of the mm by acting on
> > a mm's VMA's without holding a mmget() to prevent concurrent
> > destruction.
> >
> > In other words, mmput() destruction does not respect the mmap_sem - so
> > holding the mmap sem alone is not enough locking.
> >
> > The unlucked hmm->dead simply can't save this. Frankly every time I
> > look a struct with 'dead' in it, I find races like this.
> >
> > Thus we should put the mmget_notzero back in.
>
> So for some reason i thought exit_mmap() was setting the mm_rb
> to empty node and flushing vmacache so that find_vma() would
> fail.
It would still be racy without locks.
> Note that right before find_vma() there is also range->valid
> check which will also intercept mm release.
There is no locking on range->valid so it is just moves the race
around. You can't solve races with unlocked/non-atomic variables.
> Anyway the easy fix is to get ref on mm user in range_register.
Yes a mmget_not_zero inside range_register would be fine.
How do you want to handle that patch?
> > I saw some other funky looking stuff in hmm as well..
> >
> > > Hence it is safe to take mmap_sem and it is safe to call in hmm, if
> > > mm have been kill it will return EFAULT and this will propagate to
> > > RDMA.
> >
> > > As per_mm i removed the per_mm->mm = NULL from release so that it is
> > > always safe to use that field even in face of racing mm "killing".
> >
> > Yes, that certainly wasn't good.
> >
> > > > > - * An array of the pages included in the on-demand paging umem.
> > > > > - * Indices of pages that are currently not mapped into the device will
> > > > > - * contain NULL.
> > > > > + * An array of the pages included in the on-demand paging umem. Indices
> > > > > + * of pages that are currently not mapped into the device will contain
> > > > > + * 0.
> > > > > */
> > > > > - struct page **page_list;
> > > > > + uint64_t *pfns;
> > > >
> > > > Are these actually pfns, or are they mangled with some shift? (what is range->pfn_shift?)
> > >
> > > They are not pfns they have flags (hence range->pfn_shift) at the
> > > bottoms i just do not have a better name for this.
> >
> > I think you need to have a better name then
>
> Suggestion ? i have no idea for a better name, it has pfn value
> in it.
pfn_flags?
Jason
next prev parent reply other threads:[~2019-05-22 22:43 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20190411181314.19465-1-jglisse@redhat.com>
[not found] ` <20190506195657.GA30261@ziepe.ca>
[not found] ` <20190521205321.GC3331@redhat.com>
[not found] ` <20190522005225.GA30819@ziepe.ca>
[not found] ` <20190522174852.GA23038@redhat.com>
2019-05-22 19:22 ` [PATCH v4 0/1] Use HMM for ODP v4 Jason Gunthorpe
2019-05-22 21:49 ` Jerome Glisse
2019-05-22 22:43 ` Jason Gunthorpe [this message]
2019-05-22 20:12 ` Jason Gunthorpe
2019-05-22 21:12 ` Ralph Campbell
2019-05-22 22:06 ` Jerome Glisse
2019-05-22 22:04 ` Jerome Glisse
2019-05-22 22:39 ` Jason Gunthorpe
2019-05-22 22:42 ` Jerome Glisse
2019-05-22 22:52 ` Jason Gunthorpe
[not found] ` <20190522235737.GD15389@ziepe.ca>
[not found] ` <20190523150432.GA5104@redhat.com>
[not found] ` <20190523154149.GB12159@ziepe.ca>
[not found] ` <20190523155207.GC5104@redhat.com>
[not found] ` <20190523163429.GC12159@ziepe.ca>
[not found] ` <20190523173302.GD5104@redhat.com>
[not found] ` <20190523175546.GE12159@ziepe.ca>
[not found] ` <20190523182458.GA3571@redhat.com>
[not found] ` <20190523191038.GG12159@ziepe.ca>
2019-05-24 6:40 ` Christoph Hellwig
2019-05-24 12:44 ` RFC: Run a dedicated hmm.git for 5.3 Jason Gunthorpe
2019-05-24 16:27 ` Daniel Vetter
2019-05-24 16:53 ` Jason Gunthorpe
2019-05-24 16:59 ` Daniel Vetter
2019-05-25 22:52 ` Andrew Morton
2019-05-27 19:12 ` Jason Gunthorpe
2019-06-06 15:25 ` Jason Gunthorpe
2019-06-06 19:53 ` Stephen Rothwell
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