From: Alistair Popple <apopple@nvidia.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: <linux-mm@kvack.org>, <nouveau@lists.freedesktop.org>,
<bskeggs@redhat.com>, <akpm@linux-foundation.org>,
<linux-doc@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<kvm-ppc@vger.kernel.org>, <dri-devel@lists.freedesktop.org>,
<jhubbard@nvidia.com>, <rcampbell@nvidia.com>,
<jglisse@redhat.com>, <jgg@nvidia.com>, <daniel@ffwll.ch>,
<willy@infradead.org>
Subject: Re: [PATCH v6 5/8] mm: Device exclusive memory access
Date: Mon, 22 Mar 2021 21:27:46 +1100 [thread overview]
Message-ID: <6616451.iqfUG9VtI1@nvdebian> (raw)
In-Reply-To: <20210315074245.GC4136862@infradead.org>
On Monday, 15 March 2021 6:42:45 PM AEDT Christoph Hellwig wrote:
> > +Not all devices support atomic access to system memory. To support atomic
> > +operations to a shared virtual memory page such a device needs access to
that
> > +page which is exclusive of any userspace access from the CPU. The
> > +``make_device_exclusive_range()`` function can be used to make a memory
range
> > +inaccessible from userspace.
>
> s/Not all devices/Some devices/ ?
I will reword this. What I was trying to convey is that devices may have
features which allow for atomics to be implemented with SW assistance.
> > static inline int mm_has_notifiers(struct mm_struct *mm)
> > @@ -528,7 +534,17 @@ static inline void mmu_notifier_range_init_migrate(
> > {
> > mmu_notifier_range_init(range, MMU_NOTIFY_MIGRATE, flags, vma, mm,
> > start, end);
> > - range->migrate_pgmap_owner = pgmap;
> > + range->owner = pgmap;
> > +}
> > +
> > +static inline void mmu_notifier_range_init_exclusive(
> > + struct mmu_notifier_range *range, unsigned int flags,
> > + struct vm_area_struct *vma, struct mm_struct *mm,
> > + unsigned long start, unsigned long end, void *owner)
> > +{
> > + mmu_notifier_range_init(range, MMU_NOTIFY_EXCLUSIVE, flags, vma, mm,
> > + start, end);
> > + range->owner = owner;
>
> Maybe just replace mmu_notifier_range_init_migrate with a
> mmu_notifier_range_init_owner helper that takes the owner but does
> not hard code a type?
Ok. That does result in a function which takes a fair number of arguments, but
I guess that's no worse than multiple functions hard coding the different
types and it does result in less code overall.
> > }
> > + } else if (is_device_exclusive_entry(entry)) {
> > + page = pfn_swap_entry_to_page(entry);
> > +
> > + get_page(page);
> > + rss[mm_counter(page)]++;
> > +
> > + if (is_writable_device_exclusive_entry(entry) &&
> > + is_cow_mapping(vm_flags)) {
> > + /*
> > + * COW mappings require pages in both
> > + * parent and child to be set to read.
> > + */
> > + entry = make_readable_device_exclusive_entry(
> > + swp_offset(entry));
> > + pte = swp_entry_to_pte(entry);
> > + if (pte_swp_soft_dirty(*src_pte))
> > + pte = pte_swp_mksoft_dirty(pte);
> > + if (pte_swp_uffd_wp(*src_pte))
> > + pte = pte_swp_mkuffd_wp(pte);
> > + set_pte_at(src_mm, addr, src_pte, pte);
> > + }
>
> Just cosmetic, but I wonder if should factor this code block into
> a little helper.
In that case there are arguably are other bits of this function which should
be refactored into helpers as well. Unless you feel strongly about it I would
like to leave this as is and put together a future series to fix this and a
couple of other areas I've noticed that could do with some refactoring/clean
ups.
> > +
> > +static bool try_to_protect_one(struct page *page, struct vm_area_struct
*vma,
> > + unsigned long address, void *arg)
> > +{
> > + struct mm_struct *mm = vma->vm_mm;
> > + struct page_vma_mapped_walk pvmw = {
> > + .page = page,
> > + .vma = vma,
> > + .address = address,
> > + };
> > + struct ttp_args *ttp = (struct ttp_args *) arg;
>
> This cast should not be needed.
>
> > + return ttp.valid && (!page_mapcount(page) ? true : false);
>
> This can be simplified to:
>
> return ttp.valid && !page_mapcount(page);
>
> > + npages = get_user_pages_remote(mm, start, npages,
> > + FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
> > + pages, NULL, NULL);
> > + for (i = 0; i < npages; i++, start += PAGE_SIZE) {
> > + if (!trylock_page(pages[i])) {
> > + put_page(pages[i]);
> > + pages[i] = NULL;
> > + continue;
> > + }
> > +
> > + if (!try_to_protect(pages[i], mm, start, arg)) {
> > + unlock_page(pages[i]);
> > + put_page(pages[i]);
> > + pages[i] = NULL;
> > + }
>
> Should the trylock_page go into try_to_protect to simplify the loop
> a little? Also I wonder if we need make_device_exclusive_range or
> should just open code the get_user_pages_remote + try_to_protect
> loop in the callers, as that might allow them to also deduct other
> information about the found pages.
This function has evolved over time and putting the trylock_page into
try_to_protect does simplify things nicely. I'm not sure what other
information a caller could deduct through open coding though, but I guess in
some circumstances it might be possible for callers to skip
get_user_pages_remote() which might be a future improvement.
The main reason it looks like this was simply to keep it looking fairly
similar to how hmm_range_fault() and migrate_vma() are used with an array of
pages (or pfns) which are filled out from the given address range.
> Otherwise looks good:
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
>
Thanks.
next prev parent reply other threads:[~2021-03-22 10:27 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-12 8:38 [PATCH v6 0/8] Add support for SVM atomics in Nouveau Alistair Popple
2021-03-12 8:38 ` [PATCH v6 1/8] mm: Remove special swap entry functions Alistair Popple
2021-03-15 7:27 ` Christoph Hellwig
2021-03-22 9:20 ` Alistair Popple
2021-03-12 8:38 ` [PATCH v6 2/8] mm/swapops: Rework swap entry manipulation code Alistair Popple
2021-03-12 8:38 ` [PATCH v6 3/8] mm/rmap: Split try_to_munlock from try_to_unmap Alistair Popple
2021-03-15 7:28 ` Christoph Hellwig
2021-03-12 8:38 ` [PATCH v6 4/8] mm/rmap: Split migration into its own function Alistair Popple
2021-03-12 8:38 ` [PATCH v6 5/8] mm: Device exclusive memory access Alistair Popple
2021-03-15 7:42 ` Christoph Hellwig
2021-03-22 10:27 ` Alistair Popple [this message]
2021-03-12 8:38 ` [PATCH v6 6/8] mm: Selftests for exclusive device memory Alistair Popple
2021-03-12 8:38 ` [PATCH v6 7/8] nouveau/svm: Refactor nouveau_range_fault Alistair Popple
2021-03-12 8:38 ` [PATCH v6 8/8] nouveau/svm: Implement atomic SVM access Alistair Popple
2021-03-15 7:51 ` Christoph Hellwig
2021-03-22 9:27 ` Alistair Popple
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