linux-kernel - Re: [PATCH] mm/hugetlb: optionally pre-zero hugetlb pages

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Message-ID: <4d5c9ad7-6898-4f14-8974-126306e62db7@oracle.com>
Date: Tue, 3 Dec 2024 14:02:08 +0000
From: Joao Martins <joao.m.martins@...cle.com>
To: Mateusz Guzik <mjguzik@...il.com>, Frank van der Linden <fvdl@...gle.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org,
        Muchun Song <muchun.song@...ux.dev>, Miaohe Lin <linmiaohe@...wei.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>, Peter Xu <peterx@...hat.com>,
        linux-kernel@...r.kernel.org, Ankur Arora <ankur.a.arora@...cle.com>
Subject: Re: [PATCH] mm/hugetlb: optionally pre-zero hugetlb pages

On 02/12/2024 21:58, Mateusz Guzik wrote:
> On Mon, Dec 02, 2024 at 08:20:58PM +0000, Frank van der Linden wrote:
>> Fresh hugetlb pages are zeroed out when they are faulted in,
>> just like with all other page types. This can take up a good
>> amount of time for larger page sizes (e.g. around 40
>> milliseconds for a 1G page on a recent AMD-based system).
>>
>> This normally isn't a problem, since hugetlb pages are typically
>> mapped by the application for a long time, and the initial
>> delay when touching them isn't much of an issue.
>>
>> However, there are some use cases where a large number of hugetlb
>> pages are touched when an application (such as a VM backed by these
>> pages) starts. For 256 1G pages and 40ms per page, this would take
>> 10 seconds, a noticeable delay.
> 
> The current huge page zeroing code is not that great to begin with.
> 
> There was a patchset posted some time ago to remedy at least some of it:
> https://lore.kernel.org/all/20230830184958.2333078-1-ankur.a.arora@oracle.com/
> 
> but it apparently fell through the cracks.
> 

It didn't fell through the cracks for sure

Just had a detour into preempt=auto before resuming the main work. But that
seems to be done in the last merge window with the lazy preempt stuff. I think
Ankur was planning on following that series above soon-ish.

Adding him here, such that he keeps me honest :)