Date:   Tue, 6 Oct 2020 12:05:43 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Oscar Salvador <osalvador@...e.de>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during
 page isolation

On Tue 06-10-20 10:40:23, David Hildenbrand wrote:
> On 06.10.20 10:34, Michal Hocko wrote:
> > On Tue 22-09-20 16:37:12, Vlastimil Babka wrote:
> >> Page isolation can race with process freeing pages to pcplists in a way that
> >> a page from isolated pageblock can end up on pcplist. This can be fixed by
> >> repeated draining of pcplists, as done by patch "mm/memory_hotplug: drain
> >> per-cpu pages again during memory offline" in [1].
> >>
> >> David and Michal would prefer that this race was closed in a way that callers
> >> of page isolation who need stronger guarantees don't need to repeatedly drain.
> >> David suggested disabling pcplists usage completely during page isolation,
> >> instead of repeatedly draining them.
> >>
> >> To achieve this without adding special cases in alloc/free fastpath, we can use
> >> the same approach as boot pagesets - when pcp->high is 0, any pcplist addition
> >> will be immediately flushed.
> >>
> >> The race can thus be closed by setting pcp->high to 0 and draining pcplists
> >> once, before calling start_isolate_page_range(). The draining will serialize
> >> after processes that already disabled interrupts and read the old value of
> >> pcp->high in free_unref_page_commit(), and processes that have not yet disabled
> >> interrupts, will observe pcp->high == 0 when they are rescheduled, and skip
> >> pcplists. This guarantees no stray pages on pcplists in zones where isolation
> >> happens.
> >>
> >> This patch thus adds zone_pcplist_disable() and zone_pcplist_enable() functions
> >> that page isolation users can call before start_isolate_page_range() and after
> >> unisolating (or offlining) the isolated pages. A new zone->pcplist_disabled
> >> atomic variable makes sure we disable only pcplists once and don't enable
> >> them prematurely in case there are multiple users in parallel.
> >>
> >> We however have to avoid external updates to high and batch by taking
> >> pcp_batch_high_lock. To allow multiple isolations in parallel, change this lock
> >> from mutex to rwsem.
> > 
> > The overall idea makes sense. I just suspect you are over overcomplicating 
> > the implementation a bit. Is there any reason that we cannot start with
> > a really dumb implementation first. The only user of this functionality
> > is the memory offlining and that is already strongly synchronized
> > (mem_hotplug_begin) so a lot of trickery can be dropped here. Should we
> > find a new user later on we can make the implementation finer grained
> > but now it will not serve any purpose. So can we simply update pcp->high
> > and drain all pcp in the given zone and wait for all remote pcp draining
> > in zone_pcplist_enable and updte revert all that in zone_pcplist_enable.
> > We can stick to the existing pcp_batch_high_lock.
> > 
> > What do you think?
> > 
> 
> My two cents, we might want to make use of this in some cases of
> alloc_contig_range() soon ("try hard mode"). So I'd love to see a
> synchronized mechanism. However, that can be factored out into a
> separate patch, so this patch gets significantly simpler.

Exactly. And the incremental patch can be added along with the a-c-r try
harder mode.
-- 
Michal Hocko
SUSE Labs

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