lkml.org 
[lkml]   [2020]   [Dec]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 0/6] prohibit pinning pages in ZONE_MOVABLE
From
Date
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

\
 
 \ /
  Last update: 2020-12-04 19:06    [W:0.312 / U:0.008 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site