On 07.02.22 05:26, Alistair Popple wrote:
> Currently any attempts to pin a device coherent page will fail. This is
> because device coherent pages need to be managed by a device driver, and
> pinning them would prevent a driver from migrating them off the device.
>
> However this is no reason to fail pinning of these pages. These are
> coherent and accessible from the CPU so can be migrated just like
> pinning ZONE_MOVABLE pages. So instead of failing all attempts to pin
> them first try migrating them out of ZONE_DEVICE.
>
> Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx>
> Acked-by: Felix Kuehling <Felix.Kuehling@xxxxxxx>
> ---
>
> Changes for v2:
>
> - Added Felix's Acked-by
> - Fixed missing check for dpage == NULL
>
> mm/gup.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 95 insertions(+), 10 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 56d9577..5e826db 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1861,6 +1861,60 @@ struct page *get_dump_page(unsigned long addr)
>
> #ifdef CONFIG_MIGRATION
> /*
> + * Migrates a device coherent page back to normal memory. Caller should have a
> + * reference on page which will be copied to the new page if migration is
> + * successful or dropped on failure.
> + */
> +static struct page *migrate_device_page(struct page *page,
> + unsigned int gup_flags)
> +{
> + struct page *dpage;
> + struct migrate_vma args;
> + unsigned long src_pfn, dst_pfn = 0;
> +
> + lock_page(page);
> + src_pfn = migrate_pfn(page_to_pfn(page)) | MIGRATE_PFN_MIGRATE;
> + args.src = &src_pfn;
> + args.dst = &dst_pfn;
> + args.cpages = 1;
> + args.npages = 1;
> + args.vma = NULL;
> + migrate_vma_setup(&args);
> + if (!(src_pfn & MIGRATE_PFN_MIGRATE))
> + return NULL;
> +
> + dpage = alloc_pages(GFP_USER | __GFP_NOWARN, 0);
> +
> + /*
> + * get/pin the new page now so we don't have to retry gup after
> + * migrating. We already have a reference so this should never fail.
> + */
> + if (dpage && WARN_ON_ONCE(!try_grab_page(dpage, gup_flags))) {
> + __free_pages(dpage, 0);
> + dpage = NULL;
> + }
> +
> + if (dpage) {
> + lock_page(dpage);
> + dst_pfn = migrate_pfn(page_to_pfn(dpage));
> + }
> +
> + migrate_vma_pages(&args);
> + if (src_pfn & MIGRATE_PFN_MIGRATE)
> + copy_highpage(dpage, page);
> + migrate_vma_finalize(&args);
> + if (dpage && !(src_pfn & MIGRATE_PFN_MIGRATE)) {
> + if (gup_flags & FOLL_PIN)
> + unpin_user_page(dpage);
> + else
> + put_page(dpage);
> + dpage = NULL;
> + }
> +
> + return dpage;
> +}
> +
> +/*
> * Check whether all pages are pinnable, if so return number of pages. If some
> * pages are not pinnable, migrate them, and unpin all pages. Return zero if
> * pages were migrated, or if some pages were not successfully isolated.
> @@ -1888,15 +1942,40 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
> continue;
> prev_head = head;
> /*
> - * If we get a movable page, since we are going to be pinning
> - * these entries, try to move them out if possible.
> + * Device coherent pages are managed by a driver and should not
> + * be pinned indefinitely as it prevents the driver moving the
> + * page. So when trying to pin with FOLL_LONGTERM instead try
> + * migrating page out of device memory.
> */
> if (is_dev_private_or_coherent_page(head)) {
> + /*
> + * device private pages will get faulted in during gup
> + * so it shouldn't be possible to see one here.
> + */
> WARN_ON_ONCE(is_device_private_page(head));
> - ret = -EFAULT;
> - goto unpin_pages;
> + WARN_ON_ONCE(PageCompound(head));
> +
> + /*
> + * migration will fail if the page is pinned, so convert
> + * the pin on the source page to a normal reference.
> + */
> + if (gup_flags & FOLL_PIN) {
> + get_page(head);
> + unpin_user_page(head);
> + }
> +
> + pages[i] = migrate_device_page(head, gup_flags);
For ordinary migrate_pages(), we'll unpin all pages and return 0 so the
caller will retry pinning by walking the page tables again.
Why can't we apply the same mechanism here? This "let's avoid another
walk" looks unnecessary complicated to me, but I might be wrong.
--
Thanks,
David / dhildenb
