Message-ID: <23513e86-0769-4f3f-b90b-22273343a03c@kernel.org>
Date: Thu, 15 Jan 2026 18:08:25 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Jonathan Cameron <jonathan.cameron@...wei.com>
Cc: Li Zhe <lizhe.67@...edance.com>, akpm@...ux-foundation.org,
 ankur.a.arora@...cle.com, fvdl@...gle.com, joao.m.martins@...cle.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org, mhocko@...e.com,
 mjguzik@...il.com, muchun.song@...ux.dev, osalvador@...e.de,
 raghavendra.kt@....com, linux-cxl@...r.kernel.org,
 Davidlohr Bueso <dave@...olabs.net>, Gregory Price <gourry@...rry.net>,
 Dan Williams <dan.j.williams@...el.com>, zhanjie9@...ilicon.com,
 wangzhou1@...ilicon.com
Subject: Re: [PATCH v2 0/8] Introduce a huge-page pre-zeroing mechanism

On 1/15/26 12:57, Jonathan Cameron wrote:
> On Thu, 15 Jan 2026 12:08:03 +0100
> "David Hildenbrand (Red Hat)" <david@...nel.org> wrote:
> 
>> On 1/15/26 10:36, Li Zhe wrote:
>>> On Wed, 14 Jan 2026 18:21:08 +0100, david@...nel.org wrote:
>>>       
>>>>>> But again, I think the main motivation here is "increase application
>>>>>> startup", not optimize that the zeroing happens at specific points in
>>>>>> time during system operation (e.g., when idle etc).
>>>>>>   
>>>>>
>>>>> Framing this as "increase application startup" and merely shifting the
>>>>> overhead to shutdown seems like gaming the problem statement to me.
>>>>> The real problem is total real time spent on it while pages are
>>>>> needed.
>>>>>
>>>>> Support for background zeroing can give you more usable pages provided
>>>>> it has the cpu + ram to do it. If it does not, you are in the worst
>>>>> case in the same spot as with zeroing on free.
>>>>>
>>>>> Let's take a look at some examples.
>>>>>
>>>>> Say there are no free huge pages and you kill a vm + start a new one.
>>>>> On top of that all CPUs are pegged as is. In this case total time is
>>>>> the same for "zero on free" as it is for background zeroing.
>>>>
>>>> Right. If the pages get freed to immediately get allocated again, it
>>>> doesn't really matter who does the freeing. There might be some details,
>>>> of course.
>>>>   
>>>>>
>>>>> Say the system is freshly booted and you start up a vm. There are no
>>>>> pre-zeroed pages available so it suffers at start time no matter what.
>>>>> However, with some support for background zeroing, the machinery could
>>>>> respond to demand and do it in parallel in some capacity, shortening
>>>>> the real time needed.
>>>>
>>>> Just like for init_on_free, I would start with zeroing these pages
>>>> during boot.
>>>>
>>>> init_on_free assures that all pages in the buddy were zeroed out. Which
>>>> greatly simplifies the implementation, because there is no need to track
>>>> what was initialized and what was not.
>>>>
>>>> It's a good question if initialization during that should be done in
>>>> parallel, possibly asynchronously during boot. Reminds me a bit of
>>>> deferred page initialization during boot. But that is rather an
>>>> extension that could be added somewhat transparently on top later.
>>>>
>>>> If ever required we could dynamically enable this setting for a running
>>>> system. Whoever would enable it (flips the magic toggle) would zero out
>>>> all hugetlb pages that are already in the hugetlb allocator as free, but
>>>> not initialized yet.
>>>>
>>>> But again, these are extensions on top of the basic design of having all
>>>> free hugetlb folios be zeroed.
>>>>   
>>>>>
>>>>> Say a little bit of real time passes and you start another vm. With
>>>>> merely zeroing on free there are still no pre-zeroed pages available
>>>>> so it again suffers the overhead. With background zeroing some of the
>>>>> that memory would be already sorted out, speeding up said startup.
>>>>
>>>> The moment they end up in the hugetlb allocator as free folios they
>>>> would have to get initialized.
>>>>
>>>> Now, I am sure there are downsides to this approach (how to speedup
>>>> process exit by parallelizing zeroing, if ever required)? But it sounds
>>>> like being a bit ... simpler without user space changes required. In
>>>> theory :)
>>>
>>> I strongly agree that init_on_free strategy effectively eliminates the
>>> latency incurred during VM creation. However, it appears to introduce
>>> two new issues.
>>>
>>> First, the process that later allocates a page may not be the one that
>>> freed it, raising the question of which process should bear the cost
>>> of zeroing.
>>
>> Right now the cost is payed by the process that allocates a page. If you
>> shift that to the freeing path, it's still the same process, just at a
>> different point in time.
>>
>> Of course, there are exceptions to that: if you have a hugetlb file that
>> is shared by multiple processes (-> process that essentially truncates
>> the file). Or if someone (GUP-pin) holds a reference to a file even after
>> it was truncated (not common but possible).
>>
>> With CoW it would be the process that last unmaps the folio. CoW with
>> hugetlb is fortunately something that is rare (and rather shaky :) ).
>>
>>>
>>> Second, put_page() is executed atomically, making it inappropriate to
>>> invoke clear_page() within that context; off-loading the zeroing to a
>>> workqueue merely reopens the same accounting problem.
>>
>> I thought about this as well. For init_on_free we always invoke it for
>> up to 4MiB folios during put_page() on x86-64.
>>
>> See __folio_put()->__free_frozen_pages()->free_pages_prepare()
>>
>> Where we call kernel_init_pages(page, 1 << order);
>>
>> So surely, for 2 MiB folios (hugetlb) this is not a problem.
>>
>> ... but then, on arm64 with 64k base pages we have 512 MiB folios
>> (managed by the buddy!) where this is apparently not a problem? Or is
>> it and should be fixed?
>>
>> So I would expect once we go up to 1 GiB, we might only reveal more
>> areas where we should have optimized in the first case by dropping
>> the reference outside the spin lock ... and these optimizations would
>> obviously (unless in hugetlb specific code ...) benefit init_on_free
>> setups as well (and page poisoning).
> 
> FWIW I'd be interesting in seeing if we can do the zeroing async and allow
> for hardware offloading. If it happens to be in CXL (and someone
> built the fancy bits) we can ask the device to zero ranges of memory
> for us.  If they built the HDM-DB stuff it's coherent too (came up
> in the Davidlohr's LPC Device-mem talk on HDM-DB + back invalidate
> support)
> +CC linux-cxl and Davidlohr + a few others.
> 
> More locally this sounds like fun for DMA engines, though they are going
> to rapidly eat bandwidth up and so we'll need QoS stuff in place
> to stop them perturbing other workloads.
> 
> Give me a list of 1Gig pages and this stuff becomes much more efficient
> than anything the CPU can do.

Right, and ideally we'd implement any such mechanisms in a way that more 
parts of the kernel can benefit, and not just an unloved in-memory 
file-system that most people just want to get rid of as soon as we can :)

-- 
Cheers

David

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