lkml.org 
[lkml]   [2020]   [Jul]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v3] mm/hugetlb: avoid hardcoding while checking if cma is enable
From
Date
On 7/7/20 12:56 PM, Andrew Morton wrote:
> On Tue, 7 Jul 2020 16:02:04 +1200 Barry Song <song.bao.hua@hisilicon.com> wrote:
>
>> hugetlb_cma[0] can be NULL due to various reasons, for example, node0 has
>> no memory. so NULL hugetlb_cma[0] doesn't necessarily mean cma is not
>> enabled. gigantic pages might have been reserved on other nodes.
>
> I'm trying to figure out whether this should be backported into 5.7.1,
> but the changelog doesn't describe any known user-visible effects of
> the bug. Are there any?

Barry must have missed this email. He reported the issue so I was hoping
he would reply.

Based on the code changes, I believe the following could happen:
- Someone uses 'hugetlb_cma=' kernel command line parameter to reserve
CMA for gigantic pages.
- The system topology is such that no memory is on node 0. Therefore,
no CMA can be reserved for gigantic pages on node 0. CMA is reserved
on other nodes.
- The user also specifies a number of gigantic pages to pre-allocate on
the command line with hugepagesz=<gigantic_page_size> hugepages=<N>
- The routine which allocates gigantic pages from the bootmem allocator
will not detect CMA has been reserved as there is no memory on node 0.
Therefore, pages will be pre-allocated from bootmem allocator as well
as reserved in CMA.

This double allocation (bootmem and CMA) is the worst case scenario. Not
sure if this is what Barry saw, and I suspect this would rarely happen.

After writing this, I started to think that perhaps command line parsing
should be changed. If hugetlb_cma= is specified, it makes no sense to
pre-allocate gigantic pages. Therefore, the hugepages=<N> paramemter
should be ignored and flagged with a warning if hugetlb_cma= is specified.
This could be checked at parsing time and there would be no need for such
a check in the allocation code (except for sanity cheching).

Thoughts? I just cleaned up the parsing code and could make such a change
quite easily.
--
Mike Kravetz

\
 
 \ /
  Last update: 2020-07-08 19:46    [W:0.109 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site