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

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Date:   Thu, 17 Mar 2022 09:13:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>,
        Felix Kuehling <felix.kuehling@....com>
Cc:     Alex Sierra <alex.sierra@....com>, jgg@...dia.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, 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 17.03.22 03:54, Alistair Popple wrote:
> Felix Kuehling <felix.kuehling@....com> writes:
> 
>> On 2022-03-11 04:16, David Hildenbrand wrote:
>>> 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.
>>
>> OK. If nobody objects, we can adopts that naming convention.
> 
> I'd prefer we avoid the churn too, but I don't think we should make
> vm_normal_page() the equivalent of vm_normal_any_page(). It would mean
> vm_normal_page() would return non-LRU device coherent pages, but to me at least
> device coherent pages seem special and not what I'd expect from a function with
> "normal" in the name.
> 
> So I think it would be better to s/vm_normal_lru_page/vm_normal_page/ and keep
> vm_normal_any_page() (or perhaps call it vm_any_page?). This is basically what
> the previous incarnation of this feature did:
> 
> struct page *_vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
>                             pte_t pte, bool with_public_device);
> #define vm_normal_page(vma, addr, pte) _vm_normal_page(vma, addr, pte, false)
> 
> Except we should add:
> 
> #define vm_normal_any_page(vma, addr, pte) _vm_normal_page(vma, addr, pte, true)
> 

"normal" simply tells us that this is not a special mapping -- IOW, we
want the VM to take a look at the memmap and not treat it like a PFN
map. What we're changing is that we're now also returning non-lru pages.
Fair enough, that's why we introduce vm_normal_lru_page() as a
replacement where we really can only deal with lru pages.

vm_normal_page vs vm_normal_lru_page is good enough. "lru" further
limits what we get via vm_normal_page, that's even how it's implemented.

vm_normal_page vs vm_normal_any_page is confusing IMHO.


-- 
Thanks,

David / dhildenb