From: Uladzislau Rezki <urezki@gmail.com>
To: Michal Hocko <mhocko@suse.com>
Cc: Mel Gorman <mgorman@techsingularity.net>,
Uladzislau Rezki <urezki@gmail.com>,
Vlastimil Babka <vbabka@suse.cz>,
LKML <linux-kernel@vger.kernel.org>, RCU <rcu@vger.kernel.org>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
"Theodore Y . Ts'o" <tytso@mit.edu>,
Joel Fernandes <joel@joelfernandes.org>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Subject: Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
Date: Mon, 5 Oct 2020 17:08:01 +0200 [thread overview]
Message-ID: <20201005150801.GC17959@pc636> (raw)
In-Reply-To: <20201002090507.GB4555@dhcp22.suse.cz>
On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote:
> On Fri 02-10-20 09:50:14, Mel Gorman wrote:
> > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote:
> > > On Thu 01-10-20 21:26:26, Uladzislau Rezki wrote:
> > > > >
> > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in
> > > > > the allocator (different from what you posted in previous version) so that it
> > > > > only looks at the flag after it tries to allocate from pcplist and finds out
> > > > > it's empty. So, no inventing of new page allocator entry points or checks such
> > > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't
> > > > > affect existing fast paths.
> > > > >
> > > > OK. Now i see. Please have a look below at the patch, so we fully understand
> > > > each other. If that is something that is close to your view or not:
> > > >
> > > > <snip>
> > > > t a/include/linux/gfp.h b/include/linux/gfp.h
> > > > index c603237e006c..7e613560a502 100644
> > > > --- a/include/linux/gfp.h
> > > > +++ b/include/linux/gfp.h
> > > > @@ -39,8 +39,9 @@ struct vm_area_struct;
> > > > #define ___GFP_HARDWALL 0x100000u
> > > > #define ___GFP_THISNODE 0x200000u
> > > > #define ___GFP_ACCOUNT 0x400000u
> > > > +#define ___GFP_NO_LOCKS 0x800000u
> > >
> > > Even if a new gfp flag gains a sufficient traction and support I am
> > > _strongly_ opposed against consuming another flag for that. Bit space is
> > > limited.
> >
> > That is definitely true. I'm not happy with the GFP flag at all, the
> > comment is at best a damage limiting move. It still would be better for
> > a memory pool to be reserved and sized for critical allocations.
>
> Completely agreed. The only existing usecase is so special cased that a
> dedicated pool is not only easier to maintain but it should be also much
> better tuned for the specific workload. Something not really feasible
> with the allocator.
>
> > > Besides that we certainly do not want to allow craziness like
> > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we?
> >
> > That would deserve to be taken to a dumpster and set on fire. The flag
> > combination could be checked in the allocator but the allocator path fast
> > paths are bad enough already.
>
> If a new allocation/gfp mode is absolutely necessary then I believe that
> the most reasoanble way forward would be
> #define GFP_NO_LOCK ((__force gfp_t)0)
>
Agree. Even though i see that some code should be adjusted for it. There are
a few users of the __get_free_page(0); So, need to double check it:
<snip>
[ 0.650351] BUG: kernel NULL pointer dereference, address: 0000000000000010
[ 0.651083] #PF: supervisor read access in kernel mode
[ 0.651639] #PF: error_code(0x0000) - not-present page
[ 0.652200] PGD 0 P4D 0
[ 0.652523] Oops: 0000 [#1] SMP NOPTI
[ 0.652668] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.9.0-rc7-next-20200930+ #140
[ 0.652668] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
[ 0.652668] RIP: 0010:__find_event_file+0x21/0x80
<snip>
Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page().
If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages().
__kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks
what can be easily converted to raw variant.
--
Vlad Rezki
next prev parent reply other threads:[~2020-10-05 15:08 UTC|newest]
Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-18 19:48 [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 1/4] rcu/tree: Add a work to allocate pages from regular context Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func Uladzislau Rezki (Sony)
2020-09-21 7:47 ` Michal Hocko
2020-09-21 15:45 ` Paul E. McKenney
2020-09-21 16:03 ` Michal Hocko
2020-09-21 19:48 ` Uladzislau Rezki
2020-09-22 7:50 ` Michal Hocko
2020-09-22 13:12 ` Uladzislau Rezki
2020-09-22 15:35 ` Michal Hocko
2020-09-23 10:37 ` Mel Gorman
2020-09-23 15:41 ` Paul E. McKenney
2020-09-23 23:22 ` Mel Gorman
2020-09-24 8:16 ` Uladzislau Rezki
2020-09-24 11:16 ` Peter Zijlstra
2020-09-24 15:16 ` Uladzislau Rezki
2020-09-24 11:19 ` Peter Zijlstra
2020-09-24 15:21 ` Uladzislau Rezki
2020-09-25 8:15 ` Peter Zijlstra
2020-09-25 10:25 ` Uladzislau Rezki
2020-09-24 15:38 ` Paul E. McKenney
2020-09-25 8:26 ` Peter Zijlstra
2020-09-26 14:37 ` Paul E. McKenney
2020-09-25 8:05 ` Michal Hocko
2020-09-25 15:31 ` Uladzislau Rezki
2020-09-25 15:47 ` Michal Hocko
2020-09-29 16:25 ` Uladzislau Rezki
2020-09-30 9:27 ` Michal Hocko
2020-09-30 12:35 ` Uladzislau Rezki
2020-09-30 12:44 ` Michal Hocko
2020-09-30 13:39 ` Uladzislau Rezki
2020-09-30 16:46 ` Michal Hocko
2020-09-30 20:36 ` Uladzislau Rezki
2020-09-30 15:25 ` Joel Fernandes
2020-09-30 16:48 ` Michal Hocko
2020-09-30 17:03 ` Joel Fernandes
2020-09-30 17:22 ` Michal Hocko
2020-09-30 17:48 ` Joel Fernandes
2020-09-25 16:17 ` Mel Gorman
2020-09-25 17:57 ` Uladzislau Rezki
2020-09-22 15:49 ` Paul E. McKenney
2020-09-22 3:35 ` Paul E. McKenney
2020-09-22 8:03 ` Michal Hocko
2020-09-22 15:46 ` Paul E. McKenney
2020-09-23 11:27 ` Uladzislau Rezki
2020-09-29 10:15 ` Vlastimil Babka
2020-09-29 22:07 ` Uladzislau Rezki
2020-09-30 10:35 ` Michal Hocko
2020-10-01 19:32 ` Uladzislau Rezki
2020-09-30 14:39 ` Vlastimil Babka
2020-09-30 15:37 ` Joel Fernandes
2020-10-01 19:26 ` Uladzislau Rezki
2020-10-02 7:11 ` Michal Hocko
2020-10-02 8:50 ` Mel Gorman
2020-10-02 9:05 ` Michal Hocko
2020-10-05 15:08 ` Uladzislau Rezki [this message]
2020-10-05 15:41 ` Michal Hocko
2020-10-06 22:25 ` Uladzislau Rezki
2020-10-07 10:02 ` Michal Hocko
2020-10-07 11:02 ` Uladzislau Rezki
2020-10-02 9:07 ` Peter Zijlstra
2020-10-02 9:45 ` Mel Gorman
2020-10-02 9:58 ` Peter Zijlstra
2020-10-02 10:19 ` Mel Gorman
2020-10-02 14:41 ` Paul E. McKenney
2020-10-06 10:03 ` Mel Gorman
2020-10-06 15:41 ` Paul E. McKenney
2020-10-05 13:58 ` Uladzislau Rezki
2020-10-02 8:06 ` Mel Gorman
2020-10-05 14:12 ` Uladzislau Rezki
2020-09-18 19:48 ` [PATCH 3/4] rcu/tree: use " Uladzislau Rezki (Sony)
2020-09-18 19:48 ` [PATCH 4/4] rcu/tree: Use schedule_delayed_work() instead of WQ_HIGHPRI queue Uladzislau Rezki (Sony)
2020-09-20 15:06 ` Paul E. McKenney
2020-09-21 13:27 ` Uladzislau Rezki
2020-09-18 22:15 ` [PATCH 0/4] kvfree_rcu() and _LOCK_NESTING/_PREEMPT_RT Paul E. McKenney
2020-09-30 15:52 ` Joel Fernandes
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