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

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Message-ID: <ac06dcfe-c370-4a37-9d1a-2b2c2ef076aa@nvidia.com>
Date: Wed, 30 Oct 2024 10:29:06 -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 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.

thanks,
-- 
John Hubbard