linux-ext4 - Re: [PATCH v1 1/3] mm: split vm_normal_pages for LRU and non-LRU handling

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Message-ID: <07401a0a-6878-6af2-f663-9f0c3c1d88e5@redhat.com>
Date:   Fri, 11 Mar 2022 10:16:08 +0100
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 v1 1/3] mm: split vm_normal_pages for LRU and non-LRU
 handling

On 10.03.22 18:26, Alex Sierra wrote:
> DEVICE_COHERENT pages introduce a subtle distinction in the way
> "normal" pages can be used by various callers throughout the kernel.
> They behave like normal pages for purposes of mapping in CPU page
> tables, and for COW. But they do not support LRU lists, NUMA
> migration or THP. Therefore we split vm_normal_page into two
> functions vm_normal_any_page and vm_normal_lru_page. The latter will
> only return pages that can be put on an LRU list and that support
> NUMA migration, KSM and THP.
> 
> We also introduced a FOLL_LRU flag that adds the same behaviour to
> follow_page and related APIs, to allow callers to specify that they
> expect to put pages on an LRU list.
> 

I still don't see the need for s/vm_normal_page/vm_normal_any_page/. And
as this patch is dominated by that change, I'd suggest (again) to just
drop it as I don't see any value of that renaming. No specifier implies any.

The general idea of this change LGTM.

I wonder how this interacts with the actual DEVICE_COHERENT coherent
series. Is this a preparation? Should it be part of the DEVICE_COHERENT
series?

IOW, should this patch start with

"With DEVICE_COHERENT, we'll soon have vm_normal_pages() return
device-managed anonymous pages that are not LRU pages. Although they
behave like normal pages for purposes of mapping in CPU page, and for
COW, they do not support LRU lists, NUMA migration or THP. [...]"

But then, I'm confused by patch 2 and 3, because it feels more like we'd
already have DEVICE_COHERENT then ("hmm_is_coherent_type").

-- 
Thanks,

David / dhildenb