From: David Hildenbrand <david@redhat.com>
To: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: linux-mm <linux-mm@kvack.org>,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
LKML <linux-kernel@vger.kernel.org>,
Michal Hocko <mhocko@suse.com>,
David Hildenbrand <david@redhat.com>,
Oscar Salvador <osalvador@suse.de>,
Dan Williams <dan.j.williams@intel.com>,
Sasha Levin <sashal@kernel.org>,
Tyler Hicks <tyhicks@linux.microsoft.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
sthemmin@microsoft.com
Subject: Re: Pinning ZONE_MOVABLE pages
Date: Fri, 20 Nov 2020 21:59:24 +0100 [thread overview]
Message-ID: <9452B231-23F3-48F5-A0E2-D6C5603F87F1@redhat.com> (raw)
In-Reply-To: <CA+CK2bBffHBxjmb9jmSKacm0fJMinyt3Nhk8Nx6iudcQSj80_w@mail.gmail.com>
> Am 20.11.2020 um 21:28 schrieb Pavel Tatashin <pasha.tatashin@soleen.com>:
>
> Recently, I encountered a hang that is happening during memory hot
> remove operation. It turns out that the hang is caused by pinned user
> pages in ZONE_MOVABLE.
>
> Kernel expects that all pages in ZONE_MOVABLE can be migrated, but
> this is not the case if a user applications such as through dpdk
> libraries pinned them via vfio dma map. Kernel keeps trying to
> hot-remove them, but refcnt never gets to zero, so we are looping
> until the hardware watchdog kicks in.
>
> We cannot do dma unmaps before hot-remove, because hot-remove is a
> slow operation, and we have thousands for network flows handled by
> dpdk that we just cannot suspend for the duration of hot-remove
> operation.
>
Hi!
It‘s a known problem also for VMs using vfio. I thought about this some while ago an came to the same conclusion: before performing long-term pinnings, we have to migrate pages off the movable zone. After that, it‘s too late.
What happens when we can‘t migrate (OOM on !MOVABLE memory, short-term pinning)? TBD.
> The solution is for dpdk to allocate pages from a zone below
> ZONE_MOVAVLE, i.e. ZONE_NORMAL/ZONE_HIGHMEM, but this is not possible.
> There is no user interface that we have that allows applications to
> select what zone the memory should come from.
>
> I've spoken with Stephen Hemminger, and he said that DPDK is moving in
> the direction of using transparent huge pages instead of HugeTLBs,
> which means that we need to allow at least anonymous, and anonymous
> transparent huge pages to come from non-movable zones on demand.
>
> Here is what I am proposing:
> 1. Add a new flag that is passed through pin_user_pages_* down to
> fault handlers, and allow the fault handler to allocate from a
> non-movable zone.
>
> Sample function stacks through which this info needs to be passed is this:
>
> pin_user_pages_remote(gup_flags)
> __get_user_pages_remote(gup_flags)
> __gup_longterm_locked(gup_flags)
> __get_user_pages_locked(gup_flags)
> __get_user_pages(gup_flags)
> faultin_page(gup_flags)
> Convert gup_flags into fault_flags
> handle_mm_fault(fault_flags)
>
> From handle_mm_fault(), the stack diverges into various faults,
> examples include:
>
> Transparent Huge Page
> handle_mm_fault(fault_flags)
> __handle_mm_fault(fault_flags)
> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask
> create_huge_pmd(vmf);
> do_huge_pmd_anonymous_page(vmf);
> mm_get_huge_zero_page(vma->vm_mm); -> flag is lost, so flag from
> vmf.gfp_mask should be passed as well.
>
> There are several other similar paths in a transparent huge page, also
> there is a named path where allocation is based on filesystems, and
> the flag should be honored there as well, but it does not have to be
> added at the same time.
>
> Regular Pages
> handle_mm_fault(fault_flags)
> __handle_mm_fault(fault_flags)
> Create: struct vm_fault vmf, use fault_flags to specify correct gfp_mask
> handle_pte_fault(vmf)
> do_anonymous_page(vmf);
> page = alloc_zeroed_user_highpage_movable(vma, vmf->address); ->
> replace change this call according to gfp_mask.
>
> The above only take care of the case if user application faults on the
> page during pinning time, but there are also cases where pages already
> exist.
>
> 2. Add an internal move_pages_zone() similar to move_pages() syscall
> but instead of migrating to a different NUMA node, migrate pages from
> ZONE_MOVABLE to another zone.
> Call move_pages_zone() on demand prior to pinning pages from
> vfio_pin_map_dma() for instance.
>
> 3. Perhaps, it also makes sense to add madvise() flag, to allocate
> pages from non-movable zone. When a user application knows that it
> will do DMA mapping, and pin pages for a long time, the memory that it
> allocates should never be migrated or hot-removed, so make sure that
> it comes from the appropriate place.
> The benefit of adding madvise() flag is that we won't have to deal
> with slow page migration during pin time, but the disadvantage is that
> we would need to change the user interface.
>
Hm, I am not sure we want to expose these details. What would be the semantics? „Might pin“? Hm, not sure.
Assume you start a fresh VM via QEMU with vfio. When we start mapping guest memory via vfio, that‘s usually the time memory will get populated. Not really much has to be migrated. I think this is even true during live migration.
I think selective DMA pinning (e.g., vIOMMU in QEMU) is different, where we keep pinning/unpinning on demand. But I guess even here, we will often reuse some pages over and over again.
> Before I start working on the above approaches, I would like to get an
> opinion from the community on an appropriate path forward for this
> problem. If what I described sounds reasonable, or if there are other
> ideas on how to address the problem that I am seeing.
At least 1 and 2 sound sane. 3 is TBD - but it‘s a pure optimization, so it can wait.
Thanks!
>
> Thank you,
> Pasha
>
next prev parent reply other threads:[~2020-11-20 20:59 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-20 20:27 Pinning ZONE_MOVABLE pages Pavel Tatashin
2020-11-20 20:59 ` David Hildenbrand [this message]
2020-11-20 21:17 ` Matthew Wilcox
2020-11-20 21:34 ` David Hildenbrand
2020-11-20 21:53 ` Pavel Tatashin
2020-11-20 21:58 ` Pavel Tatashin
2020-11-20 22:06 ` David Hildenbrand
2020-11-22 21:06 ` David Rientjes
2020-11-23 15:31 ` Pavel Tatashin
2020-11-23 9:01 ` Michal Hocko
2020-11-23 16:06 ` Pavel Tatashin
2020-11-23 17:15 ` Jason Gunthorpe
2020-11-23 17:54 ` Pavel Tatashin
2020-11-23 18:34 ` Jason Gunthorpe
2020-11-24 8:20 ` Michal Hocko
2020-11-23 15:04 ` Vlastimil Babka
2020-11-23 16:31 ` Pavel Tatashin
2020-11-24 8:24 ` Michal Hocko
2020-11-24 8:43 ` Michal Hocko
2020-11-24 8:44 ` David Hildenbrand
2020-11-24 6:49 ` John Hubbard
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=9452B231-23F3-48F5-A0E2-D6C5603F87F1@redhat.com \
--to=david@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=dan.j.williams@intel.com \
--cc=iamjoonsoo.kim@lge.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@suse.com \
--cc=osalvador@suse.de \
--cc=pasha.tatashin@soleen.com \
--cc=sashal@kernel.org \
--cc=sthemmin@microsoft.com \
--cc=tyhicks@linux.microsoft.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).