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Date: Mon, 7 Feb 2022 15:19:44 +1100
From: Alistair Popple <apopple@...dia.com>
To: <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
Felix Kuehling <felix.kuehling@....com>
CC: <rcampbell@...dia.com>, <linux-ext4@...r.kernel.org>,
<linux-xfs@...r.kernel.org>, <amd-gfx@...ts.freedesktop.org>,
<dri-devel@...ts.freedesktop.org>, <hch@....de>, <jgg@...dia.com>,
<jglisse@...hat.com>, <willy@...radead.org>, <alex.sierra@....com>,
<jhubbard@...dia.com>
Subject: Re: [PATCH 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing
On Wednesday, 2 February 2022 2:03:01 AM AEDT Felix Kuehling wrote:
>
> Am 2022-02-01 um 02:05 schrieb Alistair Popple:
> > 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@...dia.com>
>
> Thank you for working on this. I have two questions inline.
>
> Other than that, patches 1 and 2 are
>
> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
>
>
> > ---
> > mm/gup.c | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 95 insertions(+), 10 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index f596b93..2cbef54 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1834,6 +1834,60 @@ 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);
>
> Don't you need to check dpage for NULL before the try_grab_page call below?
Yes, thanks for pointing that out. Will fix for v2.
> > +
> > + /*
> > + * 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 (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);
>
> Can't dpage can be NULL here as well?
No - migrate_vma_pages() will clear src_pfn & MIGRATE_PFN_MIGRATE if no
destination page is provided in dst_pfn.
> Regards,
> Felix
>
>
> > + 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.
> > @@ -1861,15 +1915,40 @@ 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);
> > + if (!pages[i]) {
> > + ret = -EBUSY;
> > + break;
> > + }
> > + continue;
> > }
> >
> > + /*
> > + * If we get a movable page, since we are going to be pinning
> > + * these entries, try to move them out if possible.
> > + */
> > if (!is_pinnable_page(head)) {
> > if (PageHuge(head)) {
> > if (!isolate_huge_page(head, &movable_page_list))
> > @@ -1897,16 +1976,22 @@ static long check_and_migrate_movable_pages(unsigned long nr_pages,
> > * If list is empty, and no isolation errors, means that all pages are
> > * in the correct zone.
> > */
> > - if (list_empty(&movable_page_list) && !isolation_error_count)
> > + if (!ret && list_empty(&movable_page_list) && !isolation_error_count)
> > return nr_pages;
> >
> > -unpin_pages:
> > - if (gup_flags & FOLL_PIN) {
> > - unpin_user_pages(pages, nr_pages);
> > - } else {
> > - for (i = 0; i < nr_pages; i++)
> > + for (i = 0; i < nr_pages; i++)
> > + if (!pages[i])
> > + continue;
> > + else if (gup_flags & FOLL_PIN)
> > + unpin_user_page(pages[i]);
> > + else
> > put_page(pages[i]);
> > +
> > + if (ret && !list_empty(&movable_page_list)) {
> > + putback_movable_pages(&movable_page_list);
> > + return ret;
> > }
> > +
> > if (!list_empty(&movable_page_list)) {
> > ret = migrate_pages(&movable_page_list, alloc_migration_target,
> > NULL, (unsigned long)&mtc, MIGRATE_SYNC,
>
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