On Thu, Oct 17, 2024 at 12:51:08PM +1100, Alistair Popple wrote:
>
> Matthew Brost <matthew.brost@xxxxxxxxx> writes:
>
> > On Wed, Oct 16, 2024 at 03:00:08PM +1100, Alistair Popple wrote:
> >>
> >> Matthew Brost <matthew.brost@xxxxxxxxx> writes:
> >>
> >> > Avoid multiple CPU page faults to the same device page racing by trying
> >> > to lock the page in do_swap_page before taking an extra reference to the
> >> > page. This prevents scenarios where multiple CPU page faults each take
> >> > an extra reference to a device page, which could abort migration in
> >> > folio_migrate_mapping. With the device page being locked in
> >> > do_swap_page, the migrate_vma_* functions need to be updated to avoid
> >> > locking the fault_page argument.
> >> >
> >> > Prior to this change, a livelock scenario could occur in Xe's (Intel GPU
> >> > DRM driver) SVM implementation if enough threads faulted the same device
> >> > page.
> >> >
> >> > Cc: Philip Yang <Philip.Yang@xxxxxxx>
> >> > Cc: Felix Kuehling <felix.kuehling@xxxxxxx>
> >> > Cc: Christian König <christian.koenig@xxxxxxx>
> >> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> > Suggessted-by: Simona Vetter <simona.vetter@xxxxxxxx>
> >> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> >> > ---
> >> > mm/memory.c | 13 ++++++---
> >> > mm/migrate_device.c | 69 ++++++++++++++++++++++++++++++---------------
> >> > 2 files changed, 56 insertions(+), 26 deletions(-)
> >> >
> >> > diff --git a/mm/memory.c b/mm/memory.c
> >> > index 2366578015ad..b72bde782611 100644
> >> > --- a/mm/memory.c
> >> > +++ b/mm/memory.c
> >> > @@ -4252,10 +4252,15 @@ 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);
> >> > + ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> >> > + put_page(vmf->page);
> >> > + unlock_page(vmf->page);
> >>
> >> I don't think my previous review of this change has really been
> >> addressed. Why don't we just install the migration entry here? Seems
> >> like it would be a much simpler way of solving this.
> >>
> >
> > I should have mentioned this in the cover-letter, I haven't got around
> > to trying that out yet. Included this existing version for correctness
> > but I also think this is not strickly required to merge this series as
> > our locking in migrate_to_ram only relies on the core MM locks so
> > some thread would eventually win the race and make forward progress.
> >
> > So I guess just ignore this patch and will send an updated version
> > individually with installing a migration entry in do_swap_page. If for
> > some reason that doesn't work, I'll respond here explaining why.
>
> That would be great. I have a fairly strong preference for doing that
> instead of adding more special cases for the fault page in the migration
> code. And if we can't do that it would be good to understand
> why. Thanks.
>
I've looked into this and actually prefer the approach in this patch.
Consider the scenario where we install a migration entry, but
migrate_to_ram fails. How do we handle this?
We don't know where migrate_to_ram failed. Was migrate_device_finalize
called, removing the migration PTE? Do we need to special-case failures
in migrate_to_ram to prevent migrate_device_finalize from removing the
faulting page's migration entry? Should we check for a migration entry
after migrate_to_ram and remove it if it exists?
Now, if migrate_to_ram succeeds, it seems the migration entry should be
removed in migrate_device_finalize since the new page is only available
there. We could return the new page in migrate_to_ram, but that feels
messy.
Additionally, the page lock needs to be held across migrate_to_ram, as
this patch does, so we'll require some special handling in
migrate_device_finalize to avoid unlocking the faulting page.
Finally, installing a migration entry is non-trivial, while taking a
page reference under a lock is straightforward.
Given all this, I prefer to keep this patch as it is.
Matt
> - Alistair
>
> > Matt
> >
> >> > + } 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 f163c2131022..2477d39f57be 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, 0);
> >> >
> >> > 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++) {
> >> > @@ -824,7 +828,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >> >
> >> > if (!page) {
> >> > if (dst) {
> >> > - folio_unlock(dst);
> >> > + if (fault_folio != dst)
> >> > + folio_unlock(dst);
> >> > folio_put(dst);
> >> > }
> >> > continue;
> >> > @@ -834,14 +839,16 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >> >
> >> > if (!(src_pfns[i] & MIGRATE_PFN_MIGRATE) || !dst) {
> >> > if (dst) {
> >> > - folio_unlock(dst);
> >> > + if (fault_folio != dst)
> >> > + folio_unlock(dst);
> >> > folio_put(dst);
> >> > }
> >> > dst = src;
> >> > }
> >> >
> >> > remove_migration_ptes(src, dst, 0);
> >> > - folio_unlock(src);
> >> > + if (fault_folio != src)
> >> > + folio_unlock(src);
> >> >
> >> > if (folio_is_zone_device(src))
> >> > folio_put(src);
> >> > @@ -849,7 +856,8 @@ void migrate_device_finalize(unsigned long *src_pfns,
> >> > folio_putback_lru(src);
> >> >
> >> > if (dst != src) {
> >> > - folio_unlock(dst);
> >> > + if (fault_folio != dst)
> >> > + folio_unlock(dst);
> >> > if (folio_is_zone_device(dst))
> >> > folio_put(dst);
> >> > else
> >> > @@ -857,6 +865,22 @@ 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);
> >> >
> >> > /**
> >> > @@ -872,7 +896,8 @@ 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);
> >>
>
