Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE

From: David Hildenbrand <david@redhat.com>
To: Pavel Tatashin <pasha.tatashin@soleen.com>,
	Jason Gunthorpe <jgg@ziepe.ca>
Cc: Joonsoo Kim <js1304@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>, Michal Hocko <mhocko@suse.com>,
	Oscar Salvador <osalvador@suse.de>,
	Dan Williams <dan.j.williams@intel.com>,
	Sasha Levin <sashal@kernel.org>,
	Tyler Hicks <tyhicks@linux.microsoft.com>,
	mike.kravetz@oracle.com, Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Mel Gorman <mgorman@suse.de>,
	Matthew Wilcox <willy@infradead.org>,
	David Rientjes <rientjes@google.com>,
	John Hubbard <jhubbard@nvidia.com>
Subject: Re: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE
Date: Fri, 4 Dec 2020 19:01:58 +0100	[thread overview]
Message-ID: <593822e5-4e1a-fdca-5500-4138d0f2b728@redhat.com> (raw)
In-Reply-To: <CA+CK2bCGGoBXg7FbhGMDdWRnePKFgvtsM_PJmA2qtMNsvPMZbg@mail.gmail.com>

On 04.12.20 18:50, Pavel Tatashin wrote:
>>> Yes, this indeed could be a problem for some configurations. I will
>>> add your comment to the commit log of one of the patches.
>>
>> It sounds like there is some inherent tension here, breaking THP's
>> when doing pin_user_pages() is a really nasty thing to do. DMA
>> benefits greatly from THP.
>>
>> I know nothing about ZONE_MOVABLE, is this auto-setup or an admin
>> option? If the result of this patch is standard systems can no longer
>> pin > 80% of their memory I have some regression concerns..
> 
> ZONE_MOVABLE can be configured via kernel parameter, or when memory
> nodes are onlined after hot-add; so this is something that admins
> configure. ZONE_MOVABLE is designed to gurantee memory hot-plug
> functionality, and not availability of THP, however, I did not know
> about the use case where some admins might configure ZONE_MOVABLE to
> increase availability of THP because pages are always migratable in
> them. The thing is, if we fragment ZONE_MOVABLE by pinning pages in
> it, the availability of THP also suffers.  We can migrate pages in
> ZONE_NORMAL, just not guaranteed, so we can create THP in ZONE_NORMAL
> as well, which is the usual case.

Right, we should document this at some place to make admins aware of
this. Something like

"Techniques that rely on long-term pinnings of memory (especially, RDMA
and vfio) are fundamentally problematic with ZONE_MOVABLE and,
therefore, memory hotunplug. Pinned pages cannot reside on ZONE_MOVABLE,
to guarantee that memory can still get hotunplugged - be aware that
pinning can fail even if there is plenty of free memory in ZONE_MOVABLE.
In addition, using ZONE_MOVABLE might make page pinning more expensive,
because pages have to be migrated off that zone first."

BTW, you might also want to update the comment for ZONE_MOVABLE in
include/linux/mmzone.h at the end of this series, removing the special
case of pinned pages (1.) and maybe adding what happens when trying to
pin pages on ZONE_MOVABLE.

-- 
Thanks,

David / dhildenb