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Message-ID: <87msk5our1.fsf@nvdebian.thelocal>
Date: Thu, 19 Sep 2024 01:10:49 +1000
From: Alistair Popple <apopple@...dia.com>
To: Matthew Brost <matthew.brost@...el.com>
Cc: intel-xe@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org, simona.vetter@...ll.ch,
Philip.Yang@....com, akpm@...ux-foundation.org, felix.kuehling@....com,
christian.koenig@....com
Subject: Re: [PATCH 1/1] mm/migrate: Trylock device page in do_swap_page
Matthew Brost <matthew.brost@...el.com> writes:
> On Wed, Sep 11, 2024 at 02:53:31PM +1000, Alistair Popple wrote:
>>
>> Matthew Brost <matthew.brost@...el.com> writes:
>>
>> I haven't seen the same in the NVIDIA UVM driver (out-of-tree, I know)
>
> Still a driver.
Indeed, and I'm happy to answer any questions about our implementation.
>> but theoretically it seems like it should be possible. However we
>> serialize migrations of the same virtual address range to avoid these
>> kind of issues as they can happen the other way too (ie. multiple
>> threads trying to migrate to GPU).
>>
>> So I suspect what happens in UVM is that one thread wins and installs
>> the migration entry while the others fail to get the driver migration
>> lock and bail out sufficiently early in the fault path to avoid the
>> live-lock.
>>
>
> I had to try hard to show this, doubt an actual user could trigger this.
>
> I wrote a test which kicked 8 threads, each thread did a pthread join,
> and then tried to read the same page. This repeats in loop for like 512
> pages or something. I needed an exclusive lock in migrate_to_ram vfunc
> for it to livelock. Without an exclusive lock I think on average I saw
> about 32k retries (i.e. migrate_to_ram calls on the same page) before a
> thread won this race.
>
> From reading UVM, pretty sure if you tried hard enough you could trigger
> a livelock given it appears you take excluvise locks in migrate_to_ram.
Yes, I suspect you're correct here and that we just haven't tried hard
enough to trigger it.
>> > Cc: Philip Yang <Philip.Yang@....com>
>> > Cc: Felix Kuehling <felix.kuehling@....com>
>> > Cc: Christian König <christian.koenig@....com>
>> > Cc: Andrew Morton <akpm@...ux-foundation.org>
>> > Suggessted-by: Simona Vetter <simona.vetter@...ll.ch>
>> > Signed-off-by: Matthew Brost <matthew.brost@...el.com>
>> > ---
>> > mm/memory.c | 13 +++++++---
>> > mm/migrate_device.c | 60 +++++++++++++++++++++++++++++++--------------
>> > 2 files changed, 50 insertions(+), 23 deletions(-)
>> >
>> > diff --git a/mm/memory.c b/mm/memory.c
>> > index 3c01d68065be..bbd97d16a96a 100644
>> > --- a/mm/memory.c
>> > +++ b/mm/memory.c
>> > @@ -4046,10 +4046,15 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>> > * Get a page reference while we know the page can't be
>> > * freed.
>> > */
>> > - get_page(vmf->page);
>> > - pte_unmap_unlock(vmf->pte, vmf->ptl);
>> > - ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
>> > - put_page(vmf->page);
>> > + if (trylock_page(vmf->page)) {
>> > + get_page(vmf->page);
>> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
>>
>> This is all beginning to look a lot like migrate_vma_collect_pmd(). So
>> rather than do this and then have to pass all this context
>> (ie. fault_page) down to the migrate_vma_* functions could we instead
>> just do what migrate_vma_collect_pmd() does here? Ie. we already have
>> the PTL and the page lock so there's no reason we couldn't just setup
>> the migration entry prior to calling migrate_to_ram().
>>
>> Obviously calling migrate_vma_setup() would show the page as not
>> migrating, but drivers could easily just fill in the src_pfn info after
>> calling migrate_vma_setup().
>>
>> This would eliminate the whole fault_page ugliness.
>>
>
> This seems like it would work and agree it likely be cleaner. Let me
> play around with this and see what I come up with. Multi-tasking a bit
> so expect a bit of delay here.
>
> Thanks for the input,
> Matt
>
>> > + ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
>> > + put_page(vmf->page);
>> > + unlock_page(vmf->page);
>> > + } else {
>> > + pte_unmap_unlock(vmf->pte, vmf->ptl);
>> > + }
>> > } else if (is_hwpoison_entry(entry)) {
>> > ret = VM_FAULT_HWPOISON;
>> > } else if (is_pte_marker_entry(entry)) {
>> > diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>> > index 6d66dc1c6ffa..049893a5a179 100644
>> > --- a/mm/migrate_device.c
>> > +++ b/mm/migrate_device.c
>> > @@ -60,6 +60,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> > struct mm_walk *walk)
>> > {
>> > struct migrate_vma *migrate = walk->private;
>> > + struct folio *fault_folio = migrate->fault_page ?
>> > + page_folio(migrate->fault_page) : NULL;
>> > struct vm_area_struct *vma = walk->vma;
>> > struct mm_struct *mm = vma->vm_mm;
>> > unsigned long addr = start, unmapped = 0;
>> > @@ -88,11 +90,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> >
>> > folio_get(folio);
>> > spin_unlock(ptl);
>> > - if (unlikely(!folio_trylock(folio)))
>> > + if (unlikely(fault_folio != folio &&
>> > + !folio_trylock(folio)))
>> > return migrate_vma_collect_skip(start, end,
>> > walk);
>> > ret = split_folio(folio);
>> > - folio_unlock(folio);
>> > + if (fault_folio != folio)
>> > + folio_unlock(folio);
>> > folio_put(folio);
>> > if (ret)
>> > return migrate_vma_collect_skip(start, end,
>> > @@ -192,7 +196,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> > * optimisation to avoid walking the rmap later with
>> > * try_to_migrate().
>> > */
>> > - if (folio_trylock(folio)) {
>> > + if (fault_folio == folio || folio_trylock(folio)) {
>> > bool anon_exclusive;
>> > pte_t swp_pte;
>> >
>> > @@ -204,7 +208,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>> >
>> > if (folio_try_share_anon_rmap_pte(folio, page)) {
>> > set_pte_at(mm, addr, ptep, pte);
>> > - folio_unlock(folio);
>> > + if (fault_folio != folio)
>> > + folio_unlock(folio);
>> > folio_put(folio);
>> > mpfn = 0;
>> > goto next;
>> > @@ -363,6 +368,8 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
>> > unsigned long npages,
>> > struct page *fault_page)
>> > {
>> > + struct folio *fault_folio = fault_page ?
>> > + page_folio(fault_page) : NULL;
>> > unsigned long i, restore = 0;
>> > bool allow_drain = true;
>> > unsigned long unmapped = 0;
>> > @@ -427,7 +434,8 @@ static unsigned long migrate_device_unmap(unsigned long *src_pfns,
>> > remove_migration_ptes(folio, folio, false);
>> >
>> > src_pfns[i] = 0;
>> > - folio_unlock(folio);
>> > + if (fault_folio != folio)
>> > + folio_unlock(folio);
>> > folio_put(folio);
>> > restore--;
>> > }
>> > @@ -536,6 +544,8 @@ int migrate_vma_setup(struct migrate_vma *args)
>> > return -EINVAL;
>> > if (args->fault_page && !is_device_private_page(args->fault_page))
>> > return -EINVAL;
>> > + if (args->fault_page && !PageLocked(args->fault_page))
>> > + return -EINVAL;
>> >
>> > memset(args->src, 0, sizeof(*args->src) * nr_pages);
>> > args->cpages = 0;
>> > @@ -799,19 +809,13 @@ void migrate_vma_pages(struct migrate_vma *migrate)
>> > }
>> > EXPORT_SYMBOL(migrate_vma_pages);
>> >
>> > -/*
>> > - * migrate_device_finalize() - complete page migration
>> > - * @src_pfns: src_pfns returned from migrate_device_range()
>> > - * @dst_pfns: array of pfns allocated by the driver to migrate memory to
>> > - * @npages: number of pages in the range
>> > - *
>> > - * Completes migration of the page by removing special migration entries.
>> > - * Drivers must ensure copying of page data is complete and visible to the CPU
>> > - * before calling this.
>> > - */
>> > -void migrate_device_finalize(unsigned long *src_pfns,
>> > - unsigned long *dst_pfns, unsigned long npages)
>> > +static void __migrate_device_finalize(unsigned long *src_pfns,
>> > + unsigned long *dst_pfns,
>> > + unsigned long npages,
>> > + struct page *fault_page)
>> > {
>> > + struct folio *fault_folio = fault_page ?
>> > + page_folio(fault_page) : NULL;
>> > unsigned long i;
>> >
>> > for (i = 0; i < npages; i++) {
>> > @@ -838,7 +842,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
>> > src = page_folio(page);
>> > dst = page_folio(newpage);
>> > remove_migration_ptes(src, dst, false);
>> > - folio_unlock(src);
>> > + if (fault_folio != src)
>> > + folio_unlock(src);
>> >
>> > if (is_zone_device_page(page))
>> > put_page(page);
>> > @@ -854,6 +859,22 @@ void migrate_device_finalize(unsigned long *src_pfns,
>> > }
>> > }
>> > }
>> > +
>> > +/*
>> > + * migrate_device_finalize() - complete page migration
>> > + * @src_pfns: src_pfns returned from migrate_device_range()
>> > + * @dst_pfns: array of pfns allocated by the driver to migrate memory to
>> > + * @npages: number of pages in the range
>> > + *
>> > + * Completes migration of the page by removing special migration entries.
>> > + * Drivers must ensure copying of page data is complete and visible to the CPU
>> > + * before calling this.
>> > + */
>> > +void migrate_device_finalize(unsigned long *src_pfns,
>> > + unsigned long *dst_pfns, unsigned long npages)
>> > +{
>> > + return __migrate_device_finalize(src_pfns, dst_pfns, npages, NULL);
>> > +}
>> > EXPORT_SYMBOL(migrate_device_finalize);
>> >
>> > /**
>> > @@ -869,7 +890,8 @@ EXPORT_SYMBOL(migrate_device_finalize);
>> > */
>> > void migrate_vma_finalize(struct migrate_vma *migrate)
>> > {
>> > - migrate_device_finalize(migrate->src, migrate->dst, migrate->npages);
>> > + __migrate_device_finalize(migrate->src, migrate->dst, migrate->npages,
>> > + migrate->fault_page);
>> > }
>> > EXPORT_SYMBOL(migrate_vma_finalize);
>>
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