Re: [PATCH v1 1/3] mm/gup: consistently name GUP-fast functions

From: David Hildenbrand <david@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Mike Rapoport <rppt@kernel.org>, Jason Gunthorpe <jgg@nvidia.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev,
	linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	linux-perf-users@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-riscv@lists.infradead.org, x86@kernel.org
Subject: Re: [PATCH v1 1/3] mm/gup: consistently name GUP-fast functions
Date: Sat, 27 Apr 2024 08:58:26 +0200	[thread overview]
Message-ID: <0a9ebdaa-9346-49f4-b568-d0dce72d3108@redhat.com> (raw)
In-Reply-To: <ZiwjhkTTRG1ZjyLf@x1n>

On 26.04.24 23:58, Peter Xu wrote:
> On Fri, Apr 26, 2024 at 11:33:08PM +0200, David Hildenbrand wrote:
>> I raised this topic in the past, and IMHO we either (a) never should have
>> added COW support; or (b) added COW support by using ordinary anonymous
>> memory (hey, partial mappings of hugetlb pages! ;) ).
>>
>> After all, COW is an optimization to speed up fork and defer copying. It
>> relies on memory overcommit, but that doesn't really apply to hugetlb, so we
>> fake it ...
> 
> Good summary.
> 
>>
>> One easy ABI break I had in mind was to simply *not* allow COW-sharing of
>> anon hugetlb folios; for example, simply don't copy the page into the child.
>> Chances are there are not really a lot of child processes that would fail
>> ... but likely we would break *something*. So there is no easy way out :(
> 
> Right, not easy.  The thing is this is one spot out of many of the
> specialties, it also may or may not be worthwhile to have dedicated time
> while nobody yet has a problem with it.  It might be easier to start with
> v2, even though that's also hard to nail everything properly - the
> challenge can come from different angles.
> 
> Thanks for the sharings, helpful.  I'll go ahead with the Power fix on
> hugepd putting this aside.

Yes, hopefully we already do have a test case for that. When writing 
gup_longterm.c I was more focusing on memfd vs. ordinary file systems 
("filesystem type") than how it's mapped into the page tables.

> 
> I hope that before the end of this year, whatever I'll fix can go away, by
> removing hugepd completely from Linux.  For now that may or may not be as
> smooth, so we'd better still fix it.

Crossing fingers, I'm annoyed whenever I stumble over it :)

-- 
Cheers,

David / dhildenb

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