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Date: Tue, 21 Jun 2022 13:25:32 +0200 From: Felix Kuehling <felix.kuehling@....com> To: David Hildenbrand <david@...hat.com>, "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>, jgg@...dia.com Cc: linux-mm@...ck.org, 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, jglisse@...hat.com, apopple@...dia.com, willy@...radead.org, akpm@...ux-foundation.org Subject: Re: [PATCH v5 01/13] mm: add zone device coherent type memory support 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 <alex.sierra@....com> >>>>>> Acked-by: Felix Kuehling <Felix.Kuehling@....com> >>>>>> Reviewed-by: Alistair Popple <apopple@...dia.com> >>>>>> [hch: rebased ontop of the refcount changes, >>>>>> removed is_dev_private_or_coherent_page] >>>>>> Signed-off-by: Christoph Hellwig <hch@....de> >>>>>> --- >>>>>> 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"). Regards, Felix > > I am probably missing something important. > P.S.: I'm on vacation and looking at a tiny screen. Hope I didn't miss anything myself.
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