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

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Message-ID: <20220317132543.GW11336@nvidia.com>
Date:   Thu, 17 Mar 2022 10:25:43 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alistair Popple <apopple@...dia.com>,
        Felix Kuehling <felix.kuehling@....com>,
        Alex Sierra <alex.sierra@....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 Thu, Mar 17, 2022 at 09:13:50AM +0100, David Hildenbrand wrote:
> 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.

This naming makes sense to me.

Jason