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[2003:cb:c70b:f900:7b04:69ec:2caf:3b42]) by smtp.gmail.com with ESMTPSA id 11sm21492095wrb.30.2022.02.10.02.53.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 10 Feb 2022 02:53:39 -0800 (PST) Message-ID: <9117b387-3c73-0236-51d1-9e6baf43b34e@redhat.com> Date: Thu, 10 Feb 2022 11:53:38 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH v2 2/3] mm/gup.c: Migrate device coherent pages when pinning instead of failing To: Alistair Popple , akpm@linux-foundation.org, linux-mm@kvack.org Cc: Felix.Kuehling@amd.com, rcampbell@nvidia.com, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, hch@lst.de, jgg@nvidia.com, jglisse@redhat.com, willy@infradead.org, alex.sierra@amd.com, jhubbard@nvidia.com References: From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=HP6LiHyI; spf=none (imf22.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.133.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Stat-Signature: wptw69gyq7o1imcgtsekuqcujbxmkgpp X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: DD73EC000A X-Rspam-User: X-HE-Tag: 1644490422-741745 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > Acked-by: Felix Kuehling > --- > > 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