linux-kernel - Re: [PATCH] mm: initialize deferred pages with interrupts enabled

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Date:   Wed, 1 Apr 2020 16:27:33 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        David Hildenbrand <david@...hat.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH] mm: initialize deferred pages with interrupts enabled

On Wed, Apr 1, 2020 at 3:57 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Wed 01-04-20 15:32:38, Pavel Tatashin wrote:
> > Initializing struct pages is a long task and keeping interrupts disabled
> > for the duration of this operation introduces a number of problems.
> >
> > 1. jiffies are not updated for long period of time, and thus incorrect time
> >    is reported. See proposed solution and discussion here:
> >    lkml/20200311123848.118638-1-shile.zhang@...ux.alibaba.com
>
> http://lkml.kernel.org/r/20200311123848.118638-1-shile.zhang@linux.alibaba.com
>
> > 2. It prevents farther improving deferred page initialization by allowing
> >    inter-node multi-threading.
> >
> > We are keeping interrupts disabled to solve a rather theoretical problem
> > that was never observed in real world (See 3a2d7fa8a3d5).
> >
> > Lets keep interrupts enabled. In case we ever encounter a scenario where
> > an interrupt thread wants to allocate large amount of memory this early in
> > boot we can deal with that by growing zone (see deferred_grow_zone()) by
> > the needed amount before starting deferred_init_memmap() threads.
> >
> > Before:
> > [    1.232459] node 0 initialised, 12058412 pages in 1ms
> >
> > After:
> > [    1.632580] node 0 initialised, 12051227 pages in 436ms
> >
>
> Fixes: 3a2d7fa8a3d5 ("mm: disable interrupts while initializing deferred pages")
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
>
> I would much rather see pgdat_resize_lock completely out of both the
> allocator and deferred init path altogether but this can be done in a
> separate patch. This one looks slightly safer for stable backports.

This is what I wanted to do, but after studying deferred_grow_zone(),
I do not see a simple way to solve this. It is one thing to fail an
allocation, and it is another thing to have a corruption because of
race.

> To be completely honest I would love to see the resize lock go away
> completely. That might need a deeper thought but I believe it is
> something that has never been done properly.
>
> Acked-by: Michal Hocko <mhocko@...e.com>

Thank you,
Pasha


>
> Thanks!
>
> > ---
> >  mm/page_alloc.c | 21 +++++++--------------
> >  1 file changed, 7 insertions(+), 14 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3c4eb750a199..4498a13b372d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1792,6 +1792,13 @@ static int __init deferred_init_memmap(void *data)
> >       BUG_ON(pgdat->first_deferred_pfn > pgdat_end_pfn(pgdat));
> >       pgdat->first_deferred_pfn = ULONG_MAX;
> >
> > +     /*
> > +      * Once we unlock here, the zone cannot be grown anymore, thus if an
> > +      * interrupt thread must allocate this early in boot, zone must be
> > +      * pre-grown prior to start of deferred page initialization.
> > +      */
> > +     pgdat_resize_unlock(pgdat, &flags);
> > +
> >       /* Only the highest zone is deferred so find it */
> >       for (zid = 0; zid < MAX_NR_ZONES; zid++) {
> >               zone = pgdat->node_zones + zid;
> > @@ -1812,8 +1819,6 @@ static int __init deferred_init_memmap(void *data)
> >       while (spfn < epfn)
> >               nr_pages += deferred_init_maxorder(&i, zone, &spfn, &epfn);
> >  zone_empty:
> > -     pgdat_resize_unlock(pgdat, &flags);
> > -
> >       /* Sanity check that the next zone really is unpopulated */
> >       WARN_ON(++zid < MAX_NR_ZONES && populated_zone(++zone));
> >
> > @@ -1854,18 +1859,6 @@ deferred_grow_zone(struct zone *zone, unsigned int order)
> >               return false;
> >
> >       pgdat_resize_lock(pgdat, &flags);
> > -
> > -     /*
> > -      * If deferred pages have been initialized while we were waiting for
> > -      * the lock, return true, as the zone was grown.  The caller will retry
> > -      * this zone.  We won't return to this function since the caller also
> > -      * has this static branch.
> > -      */
> > -     if (!static_branch_unlikely(&deferred_pages)) {
> > -             pgdat_resize_unlock(pgdat, &flags);
> > -             return true;
> > -     }
> > -
> >       /*
> >        * If someone grew this zone while we were waiting for spinlock, return
> >        * true, as there might be enough pages already.
> > --
> > 2.17.1
> >
>
> --
> Michal Hocko
> SUSE Labs