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[2003:d8:2f04:2500:cdb0:9b78:d423:43f]) by smtp.gmail.com with ESMTPSA id b5-20020a05600c4e0500b0039aef592ca0sm18064551wmq.35.2022.06.21.05.25.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Jun 2022 05:25:26 -0700 (PDT) Message-ID: <643c44e7-48be-375b-c7ab-6a30b5ee2937@redhat.com> Date: Tue, 21 Jun 2022 14:25:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support To: Alistair Popple Cc: Felix Kuehling , "Sierra Guiza, Alejandro (Alex)" , jgg@nvidia.com, linux-mm@kvack.org, 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, jglisse@redhat.com, willy@infradead.org, akpm@linux-foundation.org References: <20220531200041.24904-1-alex.sierra@amd.com> <20220531200041.24904-2-alex.sierra@amd.com> <3ac89358-2ce0-7d0d-8b9c-8b0e5cc48945@redhat.com> <02ed2cb7-3ad3-8ffc-6032-04ae1853e234@amd.com> <7605beee-0a76-4ee9-e950-17419630f2cf@redhat.com> <6aef4b7f-0ced-08cd-1f0c-50c22996aa41@redhat.com> <65987ab8-426d-e533-0295-069312b4f751@amd.com> <34e94bdb-675a-5d5c-6137-8aa1ee658d49@redhat.com> <87letq6wb5.fsf@nvdebian.thelocal> From: David Hildenbrand Organization: Red Hat In-Reply-To: <87letq6wb5.fsf@nvdebian.thelocal> 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 ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655814330; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=65Njp+ThqlVdqFDxuhaHtcra47G9ZdLFWICrTgPdMDI=; b=Cn53X35RHCbD6zjhi2eyZpbRUO0npylEylkhl1cwjChD9PRRz6W4x/bEsWIA4TSOk+txTj D/yb+LaPzN/YF6WDET0ofTebBo1UkqqY8UhKtVtEfAU6SBQe9ZpVNEVMTW9WygiKkSz86I LTGSLSJFkLfNyRgHPG3YCu3eRg1mZk8= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655814330; a=rsa-sha256; cv=none; b=nDio1ZyrYBQSwPPYNp7Pr/fOkzIY/nwsR6bWoCDxYwEitr7rrvCR3EncyDXShfVUNhMi0M L3yVGKr0+ZoNmjLB+/Aeiy3W9gI38ro75h8WGTmCf3vHLS3KPq7mRdnuK+JPjr3nN2zwZO t3Gf1NP3M0rXWeGeZ+IZ08H/7CJ6Il0= ARC-Authentication-Results: i=1; imf20.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=LrlmT0ZB; spf=none (imf20.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: oritfa1wqtywufgpbgqpnuwfj9qy888s Authentication-Results: imf20.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=LrlmT0ZB; spf=none (imf20.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-Rspamd-Queue-Id: 2205D1C0021 X-Rspamd-Server: rspam02 X-Rspam-User: X-HE-Tag: 1655814329-675148 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 21.06.22 13:55, Alistair Popple wrote: > > David Hildenbrand writes: > >> On 21.06.22 13:25, Felix Kuehling wrote: >>> >>> Am 6/17/22 um 23:19 schrieb David Hildenbrand: >>>> On 17.06.22 21:27, Sierra Guiza, Alejandro (Alex) wrote: >>>>> On 6/17/2022 12:33 PM, David Hildenbrand wrote: >>>>>> On 17.06.22 19:20, Sierra Guiza, Alejandro (Alex) wrote: >>>>>>> On 6/17/2022 4:40 AM, David Hildenbrand wrote: >>>>>>>> On 31.05.22 22:00, Alex Sierra wrote: >>>>>>>>> Device memory that is cache coherent from device and CPU point of view. >>>>>>>>> This is used on platforms that have an advanced system bus (like CAPI >>>>>>>>> or CXL). Any page of a process can be migrated to such memory. However, >>>>>>>>> no one should be allowed to pin such memory so that it can always be >>>>>>>>> evicted. >>>>>>>>> >>>>>>>>> Signed-off-by: Alex Sierra >>>>>>>>> Acked-by: Felix Kuehling >>>>>>>>> Reviewed-by: Alistair Popple >>>>>>>>> [hch: rebased ontop of the refcount changes, >>>>>>>>> removed is_dev_private_or_coherent_page] >>>>>>>>> Signed-off-by: Christoph Hellwig >>>>>>>>> --- >>>>>>>>> include/linux/memremap.h | 19 +++++++++++++++++++ >>>>>>>>> mm/memcontrol.c | 7 ++++--- >>>>>>>>> mm/memory-failure.c | 8 ++++++-- >>>>>>>>> mm/memremap.c | 10 ++++++++++ >>>>>>>>> mm/migrate_device.c | 16 +++++++--------- >>>>>>>>> mm/rmap.c | 5 +++-- >>>>>>>>> 6 files changed, 49 insertions(+), 16 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h >>>>>>>>> index 8af304f6b504..9f752ebed613 100644 >>>>>>>>> --- a/include/linux/memremap.h >>>>>>>>> +++ b/include/linux/memremap.h >>>>>>>>> @@ -41,6 +41,13 @@ struct vmem_altmap { >>>>>>>>> * A more complete discussion of unaddressable memory may be found in >>>>>>>>> * include/linux/hmm.h and Documentation/vm/hmm.rst. >>>>>>>>> * >>>>>>>>> + * MEMORY_DEVICE_COHERENT: >>>>>>>>> + * Device memory that is cache coherent from device and CPU point of view. This >>>>>>>>> + * is used on platforms that have an advanced system bus (like CAPI or CXL). A >>>>>>>>> + * driver can hotplug the device memory using ZONE_DEVICE and with that memory >>>>>>>>> + * type. Any page of a process can be migrated to such memory. However no one >>>>>>>> Any page might not be right, I'm pretty sure. ... just thinking about special pages >>>>>>>> like vdso, shared zeropage, ... pinned pages ... >>>>>> Well, you cannot migrate long term pages, that's what I meant :) >>>>>> >>>>>>>>> + * should be allowed to pin such memory so that it can always be evicted. >>>>>>>>> + * >>>>>>>>> * MEMORY_DEVICE_FS_DAX: >>>>>>>>> * Host memory that has similar access semantics as System RAM i.e. DMA >>>>>>>>> * coherent and supports page pinning. In support of coordinating page >>>>>>>>> @@ -61,6 +68,7 @@ struct vmem_altmap { >>>>>>>>> enum memory_type { >>>>>>>>> /* 0 is reserved to catch uninitialized type fields */ >>>>>>>>> MEMORY_DEVICE_PRIVATE = 1, >>>>>>>>> + MEMORY_DEVICE_COHERENT, >>>>>>>>> MEMORY_DEVICE_FS_DAX, >>>>>>>>> MEMORY_DEVICE_GENERIC, >>>>>>>>> MEMORY_DEVICE_PCI_P2PDMA, >>>>>>>>> @@ -143,6 +151,17 @@ static inline bool folio_is_device_private(const struct folio *folio) >>>>>>>> In general, this LGTM, and it should be correct with PageAnonExclusive I think. >>>>>>>> >>>>>>>> >>>>>>>> However, where exactly is pinning forbidden? >>>>>>> Long-term pinning is forbidden since it would interfere with the device >>>>>>> memory manager owning the >>>>>>> device-coherent pages (e.g. evictions in TTM). However, normal pinning >>>>>>> is allowed on this device type. >>>>>> I don't see updates to folio_is_pinnable() in this patch. >>>>> Device coherent type pages should return true here, as they are pinnable >>>>> pages. >>>> That function is only called for long-term pinnings in try_grab_folio(). >>>> >>>>>> So wouldn't try_grab_folio() simply pin these pages? What am I missing? >>>>> As far as I understand this return NULL for long term pin pages. >>>>> Otherwise they get refcount incremented. >>>> I don't follow. >>>> >>>> You're saying >>>> >>>> a) folio_is_pinnable() returns true for device coherent pages >>>> >>>> and that >>>> >>>> b) device coherent pages don't get long-term pinned >>>> >>>> >>>> Yet, the code says >>>> >>>> struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags) >>>> { >>>> if (flags & FOLL_GET) >>>> return try_get_folio(page, refs); >>>> else if (flags & FOLL_PIN) { >>>> struct folio *folio; >>>> >>>> /* >>>> * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a >>>> * right zone, so fail and let the caller fall back to the slow >>>> * path. >>>> */ >>>> if (unlikely((flags & FOLL_LONGTERM) && >>>> !is_pinnable_page(page))) >>>> return NULL; >>>> ... >>>> return folio; >>>> } >>>> } >>>> >>>> >>>> What prevents these pages from getting long-term pinned as stated in this patch? >>> >>> Long-term pinning is handled by __gup_longterm_locked, which migrates >>> pages returned by __get_user_pages_locked that cannot be long-term >>> pinned. try_grab_folio is OK to grab the pages. Anything that can't be >>> long-term pinned will be migrated afterwards, and >>> __get_user_pages_locked will be retried. The migration of >>> DEVICE_COHERENT pages was implemented by Alistair in patch 5/13 >>> ("mm/gup: migrate device coherent pages when pinning instead of failing"). >> >> Thanks. >> >> __gup_longterm_locked()->check_and_migrate_movable_pages() >> >> Which checks folio_is_pinnable() and doesn't do anything if set. >> >> Sorry to be dense here, but I don't see how what's stated in this patch >> works without adjusting folio_is_pinnable(). > > Ugh, I think you might be right about try_grab_folio(). > > We didn't update folio_is_pinnable() to include device coherent pages > because device coherent pages are pinnable. It is really just > FOLL_LONGTERM that we want to prevent here. > > For normal PUP that is done by my change in > check_and_migrate_movable_pages() which migrates pages being pinned with > FOLL_LONGTERM. But I think I incorrectly assumed we would take the > pte_devmap() path in gup_pte_range(), which we don't for coherent pages. > So I think the check in try_grab_folio() needs to be: I think I said it already (and I might be wrong without reading the code), but folio_is_pinnable() is *only* called for long-term pinnings. It should actually be called folio_is_longterm_pinnable(). That's where that check should go, no? -- Thanks, David / dhildenb