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Message-ID: <4b6b472d-07b6-0c96-22ed-9a77a878cf61@nvidia.com>
Date: Fri, 11 Feb 2022 18:10:29 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org,
linux-mm@...ck.org
Cc: Felix.Kuehling@....com, 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
Subject: Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when
pinning instead of failing
On 2/6/22 20:26, 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.
>
Hi Alistair and all,
Here's a possible issue (below) that I really should have spotted the
first time around, sorry for this late-breaking review. And maybe it's
actually just my misunderstanding, anyway.
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> Acked-by: Felix Kuehling <Felix.Kuehling@....com>
> ---
>
> 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);
OK...but now gup_flags can no longer be used as a guide for how to
release these pages, right? In other words, up until this point,
FOLL_PIN meant "call unpin_user_page() in order to release". However,
now this page must be released via put_page().
See below...
> + }
> +
> + 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))
> @@ -1924,16 +2003,22 @@ 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]);
...and here, for example, we are still trusting gup_flags to decide how
to release the pages.
This reminds me: out of the many things to monitor, the FOLL_PIN counts
in /proc/vmstat are especially helpful, whenever making changes to code
that deals with this:
nr_foll_pin_acquired
nr_foll_pin_released
...and those should normally be equal to each other when "at rest".
thanks,
--
John Hubbard
NVIDIA
> + 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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