linux-kernel - Re: [PATCH v4 19/25] proc/task_mmu: Ignore ZONE

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Message-ID: <37rxl2bjda3psdknhboexhbg3hahf5ifmublp5fw7ltdoyqllc@udbz76jklmnu>
Date: Thu, 19 Dec 2024 10:11:32 +1100
From: Alistair Popple <apopple@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, dan.j.williams@...el.com, 
	linux-mm@...ck.org, lina@...hilina.net, zhang.lyra@...il.com, 
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Subject: Re: [PATCH v4 19/25] proc/task_mmu: Ignore ZONE_DEVICE pages

On Tue, Dec 17, 2024 at 11:31:25PM +0100, David Hildenbrand wrote:
> On 17.12.24 06:13, Alistair Popple wrote:
> > The procfs mmu files such as smaps currently ignore device dax and fs
> > dax pages because these pages are considered special. To maintain
> > existing behaviour once these pages are treated as normal pages and
> > returned from vm_normal_page() add tests to explicitly skip them.
> > 
> > Signed-off-by: Alistair Popple <apopple@...dia.com>
> > ---
> >   fs/proc/task_mmu.c | 18 ++++++++++++++----
> >   1 file changed, 14 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 38a5a3e..c9b227a 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -801,6 +801,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
> >   	if (pte_present(ptent)) {
> >   		page = vm_normal_page(vma, addr, ptent);
> > +		if (page && (is_device_dax_page(page) || is_fsdax_page(page)))
> 
> This "is_device_dax_page(page) || is_fsdax_page(page)" is a common theme
> here, likely we should have a special helper?

Sounds good, will add is_dax_page() if there are enough callers left after any
review comments.
 
> But, don't we actually want to include them in the smaps output now? I think
> we want.

I'm not an expert in what callers of vm_normal_page() think of as a "normal"
page. So my philosphy here was to ensure anything calling vm_normal_page()
didn't accidentally start seeing DAX pages, either by checking existing filters
(lots of callers already call vma_is_special_huge() or some equivalent) or
explicitly filtering them out in the hope someone smarter than me could tell me
it was unneccssary.

That stategy seems to have worked, and so I agree we likely do want them in
smaps. I just didn't want to silently do it without this kind of discussion
first.

> The rmap code will indicate these pages in /proc/meminfo, per-node info, in
> the memcg ... as "Mapped:" etc.
> 
> So likely we just want to also indicate them here, or is there any downsides
> we know of?

I don't know of any, and I think it makes sense to also indicate them so will
drop this check in the respin.

> -- 
> Cheers,
> 
> David / dhildenb
>