linux-kernel - Re: [patch 0/2 for-4.20] mm, thp: fix remote access and allocation regressions

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Date:   Wed, 5 Dec 2018 13:14:37 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        s.priebe@...fihost.ag, mgorman@...hsingularity.net,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        alex.williamson@...hat.com, lkp@...org, kirill@...temov.name,
        Andrew Morton <akpm@...ux-foundation.org>,
        zi.yan@...rutgers.edu
Subject: Re: [patch 0/2 for-4.20] mm, thp: fix remote access and allocation
 regressions

On Wed, 5 Dec 2018, Michal Hocko wrote:

> > As we've been over countless times, this is the desired effect for 
> > workloads that fit on a single node.  We want local pages of the native 
> > page size because they (1) are accessed faster than remote hugepages and 
> > (2) are candidates for collapse by khugepaged.
> > 
> > For applications that do not fit in a single node, we have discussed 
> > possible ways to extend the API to allow remote faulting of hugepages, 
> > absent remote fragmentation as well, then the long-standing behavior is 
> > preserved and large applications can use the API to increase their thp 
> > success rate.
> 
> OK, I just give up. This doesn't lead anywhere. You keep repeating the
> same stuff over and over, neglect other usecases and actually force them
> to do something special just to keep your very specific usecase which
> you clearly refuse to abstract into a form other people can experiment
> with or at least provide more detailed broken down numbers for a more
> serious analyses. Fault latency is only a part of the picture which is
> much more complex. Look at Mel's report to get an impression of what
> might be really useful for a _productive_ discussion.

The other usecases is part of patch 2/2 in this series that is 
functionally similar to the __GFP_COMPACT_ONLY patch that Andrea proposed.  
We can also work to extend the API to allow remote thp allocations.

Patch 1/2 reverts the behavior of commit ac5b2c18911f ("mm: thp: relax 
__GFP_THISNODE for MADV_HUGEPAGE mappings") which added NUMA locality on 
top of an already conflated madvise mode.  Prior to this commit that was 
merged for 4.20, *all* thp faults were constrained to the local node; this 
has been the case for three years and even prior to that in other kernels.  
It turns out that allowing remote allocations introduces access latency in 
the presence of local fragmentation.

The solution is not to conflate MADV_HUGEPAGE with any sematic that 
suggests it allows remote thp allocations, especially when that changes 
long-standing behavior, regresses my usecase, and regresses the kernel 
test robot.

I'll change patch 1/2 to not touch new_page() so that we are only 
addressing thp faults and post a v2.