linux-kernel - Re: [PATCH] mm/gup: restore the ability to pin more than 2GB at a time

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Message-ID: <6b6b8477-921a-40b4-b689-36080981a134@nvidia.com>
Date: Wed, 30 Oct 2024 10:49:45 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Vlastimil Babka <vbabka@...e.cz>, Christoph Hellwig <hch@...radead.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
 linux-stable@...r.kernel.org, Vivek Kasireddy <vivek.kasireddy@...el.com>,
 David Hildenbrand <david@...hat.com>, Dave Airlie <airlied@...hat.com>,
 Gerd Hoffmann <kraxel@...hat.com>, Matthew Wilcox <willy@...radead.org>,
 Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>,
 Arnd Bergmann <arnd@...db.de>, Daniel Vetter <daniel.vetter@...ll.ch>,
 Dongwon Kim <dongwon.kim@...el.com>, Hugh Dickins <hughd@...gle.com>,
 Junxiao Chang <junxiao.chang@...el.com>,
 Mike Kravetz <mike.kravetz@...cle.com>, Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH] mm/gup: restore the ability to pin more than 2GB at a
 time

On 10/30/24 10:42 AM, Vlastimil Babka wrote:
> On 10/30/24 18:29, John Hubbard wrote:
>> On 10/30/24 4:03 AM, Vlastimil Babka wrote:
>>> On 10/30/24 05:39, John Hubbard wrote:
>>>> On 10/29/24 9:33 PM, Christoph Hellwig wrote:
>>>>> On Tue, Oct 29, 2024 at 09:30:41PM -0700, John Hubbard wrote:
>> ...
>>> It might be a regression even if you don't try to pin over 2GB. high-order
>>> (>costly order) allocations can fail and/or cause disruptive
>>> reclaim/compaction cycles even below MAX_PAGE_ORDER and it's better to use
>>> kvmalloc if physical contiguity is not needed, it will attempt the physical
>>> kmalloc() allocation with __GFP_NORETRY (little disruption) and fallback to
>>> vmalloc() quickly.
>>>
>>> Of course if there's a way to avoid the allocation completely, even beter.
>>
>> Why not both? I'm going to ask our driver team to batch the pinning calls,
>> as recommended nearby, just to be sure that we are following best
>> practices.
>>
>> But it also seems good to use kvmalloc() here, and avoid any other
>> regressions. That's also a best practice.
> 
> By "avoid the allocation completely" I meant David's proof of concept
> elsewhere in this thread, that seems to replace that kmalloc_array() with no
> allocation :)
> 

aha, OK let me look into that then.

thanks,
-- 
John Hubbard