linux-kernel - Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

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Date:   Tue, 14 Jan 2020 13:00:01 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Ralph Campbell <rcampbell@...dia.com>
CC:     "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        Jerome Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ben Skeggs <bskeggs@...hat.com>, Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH v6 5/6] nouveau: use new mmu interval notifiers

On Mon, Jan 13, 2020 at 02:47:02PM -0800, Ralph Campbell wrote:
>  void
>  nouveau_svmm_fini(struct nouveau_svmm **psvmm)
>  {
>  	struct nouveau_svmm *svmm = *psvmm;
> +	struct mmu_interval_notifier *mni;
> +
>  	if (svmm) {
>  		mutex_lock(&svmm->mutex);
> +		while (true) {
> +			mni = mmu_interval_notifier_find(svmm->mm,
> +					&nouveau_svm_mni_ops, 0UL, ~0UL);
> +			if (!mni)
> +				break;
> +			mmu_interval_notifier_put(mni);

Oh, now I really don't like the name 'put'. It looks like mni is
refcounted here, and it isn't. put should be called 'remove_deferred'

And then you also need a way to barrier this scheme on driver unload.

> +		}
>  		svmm->vmm = NULL;
>  		mutex_unlock(&svmm->mutex);
> -		mmu_notifier_put(&svmm->notifier);

While here it was actually a refcount.

> +static void nouveau_svmm_do_unmap(struct mmu_interval_notifier *mni,
> +				 const struct mmu_notifier_range *range)
> +{
> +	struct svmm_interval *smi =
> +		container_of(mni, struct svmm_interval, notifier);
> +	struct nouveau_svmm *svmm = smi->svmm;
> +	unsigned long start = mmu_interval_notifier_start(mni);
> +	unsigned long last = mmu_interval_notifier_last(mni);

This whole algorithm only works if it is protected by the read side of
the interval tree lock. Deserves at least a comment if not an
assertion too.

>  static int nouveau_range_fault(struct nouveau_svmm *svmm,
>  			       struct nouveau_drm *drm, void *data, u32 size,
> -			       u64 *pfns, struct svm_notifier *notifier)
> +			       u64 *pfns, u64 start, u64 end)
>  {
>  	unsigned long timeout =
>  		jiffies + msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
>  	/* Have HMM fault pages within the fault window to the GPU. */
>  	struct hmm_range range = {
> -		.notifier = &notifier->notifier,
> -		.start = notifier->notifier.interval_tree.start,
> -		.end = notifier->notifier.interval_tree.last + 1,
> +		.start = start,
> +		.end = end,
>  		.pfns = pfns,
>  		.flags = nouveau_svm_pfn_flags,
>  		.values = nouveau_svm_pfn_values,
> +		.default_flags = 0,
> +		.pfn_flags_mask = ~0UL,
>  		.pfn_shift = NVIF_VMM_PFNMAP_V0_ADDR_SHIFT,
>  	};
> -	struct mm_struct *mm = notifier->notifier.mm;
> +	struct mm_struct *mm = svmm->mm;
>  	long ret;
>  
>  	while (true) {
>  		if (time_after(jiffies, timeout))
>  			return -EBUSY;
>  
> -		range.notifier_seq = mmu_interval_read_begin(range.notifier);
> -		range.default_flags = 0;
> -		range.pfn_flags_mask = -1UL;
>  		down_read(&mm->mmap_sem);

mmap sem doesn't have to be held for the interval search, and again we
have lifetime issues with the membership here.

> +		ret = nouveau_svmm_interval_find(svmm, &range);
> +		if (ret) {
> +			up_read(&mm->mmap_sem);
> +			return ret;
> +		}
> +		range.notifier_seq = mmu_interval_read_begin(range.notifier);
>  		ret = hmm_range_fault(&range, 0);
>  		up_read(&mm->mmap_sem);
>  		if (ret <= 0) {

I'm still not sure this is a better approach than what ODP does. It
looks very expensive on the fault path..

Jason