linux-kernel - Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

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Date:   Thu, 18 Oct 2018 19:18:25 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Arun KS <arunks@...eaurora.org>, Vlastimil Babka <vbabka@...e.cz>,
        kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        boris.ostrovsky@...cle.com, jgross@...e.com,
        dan.j.williams@...el.com, iamjoonsoo.kim@....com,
        gregkh@...uxfoundation.org, osalvador@...e.de, malat@...ian.org,
        kirill.shutemov@...ux.intel.com, jrdr.linux@...il.com,
        yasu.isimatu@...il.com, mgorman@...hsingularity.net,
        aaron.lu@...el.com, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, vatsa@...eaurora.org,
        vinmenon@...eaurora.org, getarunks@...il.com
Subject: Re: [PATCH v5 1/2] memory_hotplug: Free pages as higher order

On Thu, 11 Oct 2018 09:55:03 +0200 Michal Hocko <mhocko@...nel.org> wrote:

> > > > > This is now not called anymore, although the xen/hv variants still do
> > > > > it. The function seems empty these days, maybe remove it as a followup
> > > > > cleanup?
> > > > >
> > > > > > -	__online_page_increment_counters(page);
> > > > > > -	__online_page_free(page);
> > > > > > +	__free_pages_core(page, order);
> > > > > > +	totalram_pages += (1UL << order);
> > > > > > +#ifdef CONFIG_HIGHMEM
> > > > > > +	if (PageHighMem(page))
> > > > > > +		totalhigh_pages += (1UL << order);
> > > > > > +#endif
> > > > >
> > > > > __online_page_increment_counters() would have used
> > > > > adjust_managed_page_count() which would do the changes under
> > > > > managed_page_count_lock. Are we safe without the lock? If yes, there
> > > > > should perhaps be a comment explaining why.
> > > > 
> > > > Looks unsafe without managed_page_count_lock.
> > > 
> > > Why does it matter actually? We cannot online/offline memory in
> > > parallel. This is not the case for the boot where we initialize memory
> > > in parallel on multiple nodes. So this seems to be safe currently unless
> > > I am missing something. A comment explaining that would be helpful
> > > though.
> > 
> > Other main callers of adjust_manage_page_count(),
> > 
> > static inline void free_reserved_page(struct page *page)
> > {
> >         __free_reserved_page(page);
> >         adjust_managed_page_count(page, 1);
> > }
> > 
> > static inline void mark_page_reserved(struct page *page)
> > {
> >         SetPageReserved(page);
> >         adjust_managed_page_count(page, -1);
> > }
> > 
> > Won't they race with memory hotplug?
> > 
> > Few more,
> > ./drivers/xen/balloon.c:519:            adjust_managed_page_count(page, -1);
> > ./drivers/virtio/virtio_balloon.c:175:  adjust_managed_page_count(page, -1);
> > ./drivers/virtio/virtio_balloon.c:196:  adjust_managed_page_count(page, 1);
> > ./mm/hugetlb.c:2158:                    adjust_managed_page_count(page, 1 <<
> > h->order);
> 
> They can, and I have missed those.

So this patch needs more work, yes?