linux-ext4 - Re: [PATCH v5 01/13] mm: add zone device coherent type memory support

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Date:   Fri, 17 Jun 2022 11:40:39 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Sierra <alex.sierra@....com>, jgg@...dia.com
Cc:     Felix.Kuehling@....com, 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

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 ...

> + * 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?

-- 
Thanks,

David / dhildenb