linux-kernel - Re: [PATCH v9 0/8] stg mail -e --version=v9 \

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Date:   Wed, 11 Sep 2019 14:19:41 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        virtio-dev@...ts.oasis-open.org, kvm list <kvm@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oscar Salvador <osalvador@...e.de>,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Pankaj Gupta <pagupta@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Nitesh Narayan Lal <nitesh@...hat.com>,
        Rik van Riel <riel@...riel.com>, lcapitulino@...hat.com,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>, ying.huang@...el.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \

On Wed 11-09-19 08:08:38, Michael S. Tsirkin wrote:
> On Wed, Sep 11, 2019 at 01:36:19PM +0200, Michal Hocko wrote:
> > On Tue 10-09-19 14:23:40, Alexander Duyck wrote:
> > [...]
> > > We don't put any limitations on the allocator other then that it needs to
> > > clean up the metadata on allocation, and that it cannot allocate a page
> > > that is in the process of being reported since we pulled it from the
> > > free_list. If the page is a "Reported" page then it decrements the
> > > reported_pages count for the free_area and makes sure the page doesn't
> > > exist in the "Boundary" array pointer value, if it does it moves the
> > > "Boundary" since it is pulling the page.
> > 
> > This is still a non-trivial limitation on the page allocation from an
> > external code IMHO. I cannot give any explicit reason why an ordering on
> > the free list might matter (well except for page shuffling which uses it
> > to make physical memory pattern allocation more random) but the
> > architecture seems hacky and dubious to be honest. It shoulds like the
> > whole interface has been developed around a very particular and single
> > purpose optimization.
> > 
> > I remember that there was an attempt to report free memory that provided
> > a callback mechanism [1], which was much less intrusive to the internals
> > of the allocator yet it should provide a similar functionality. Did you
> > see that approach? How does this compares to it? Or am I completely off
> > when comparing them?
> > 
> > [1] mostly likely not the latest version of the patchset
> > http://lkml.kernel.org/r/1502940416-42944-5-git-send-email-wei.w.wang@intel.com
> 
> Linus nacked that one. He thinks invoking callbacks with lots of
> internal mm locks is too fragile.

I would be really curious how much he would be happy about injecting
other restrictions on the allocator like this patch proposes. This is
more intrusive as it has a higher maintenance cost longterm IMHO.
-- 
Michal Hocko
SUSE Labs