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From: Matthew Wilcox <willy@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, Vlastimil Babka <vbabka@suse.cz>
Subject: Re: __GFP_LOW
Date: Sat, 7 Apr 2018 21:27:09 -0700
Message-ID: <20180408042709.GC32632@bombadil.infradead.org> (raw)
In-Reply-To: <20180406060953.GA8286@dhcp22.suse.cz>

On Fri, Apr 06, 2018 at 08:09:53AM +0200, Michal Hocko wrote:
> OK, we already split the documentation into these categories. So we got
> at least the structure right ;)

Yes, this part of the documentation makes sense to me :-)

> >  - What kind of memory to allocate (DMA, NORMAL, HIGHMEM)
> >  - Where to get the pages from
> >    - Local node only (THISNODE)
> >    - Only in compliance with cpuset policy (HARDWALL)
> >    - Spread the pages between zones (WRITE)
> >    - The movable zone (MOVABLE)
> >    - The reclaimable zone (RECLAIMABLE)
> >  - What you are willing to do if no free memory is available:
> >    - Nothing at all (NOWAIT)
> >    - Use my own time to free memory (DIRECT_RECLAIM)
> >      - But only try once (NORETRY)
> >      - Can call into filesystems (FS)
> >      - Can start I/O (IO)
> >      - Can sleep (!ATOMIC)
> >    - Steal time from other processes to free memory (KSWAPD_RECLAIM)
> 
> What does that mean? If I drop the flag, do not steal? Well I do because
> they will hit direct reclaim sooner...

If they allocate memory, sure.  A process which stays in its working
set won't, unless it's preempted by kswapd.

> >    - Kill other processes to get their memory (!RETRY_MAYFAIL)
> 
> Not really for costly orders.

Yes, need to be more precise there.

> >    - All of the above, and wait forever (NOFAIL)
> >    - Take from emergency reserves (HIGH)
> >    - ... but not the last parts of the regular reserves (LOW)
> 
> What does that mean and how it is different from NOWAIT? Is this about
> the low watermark and if yes do we want to teach users about this and
> make the whole thing even more complicated?  Does it wake
> kswapd? What is the eagerness ordering? LOW, NOWAIT, NORETRY,
> RETRY_MAYFAIL, NOFAIL?

LOW doesn't quite fit into the eagerness scale with the other flags;
instead it's composable with them.  So you can specify NOWAIT | LOW,
NORETRY | LOW, NOFAIL | LOW, etc.  All I have in mind is something
like this:

        if (alloc_flags & ALLOC_HIGH)
                min -= min / 2;
+	if (alloc_flags & ALLOC_LOW)
+		min += min / 2;

The idea is that a GFP_KERNEL | __GFP_LOW allocation cannot force a
GFP_KERNEL allocation into an OOM situation because it cannot take
the last pages of memory before the watermark.  It can still make a
GFP_KERNEL allocation *more likely* to hit OOM (just like any other kind
of allocation can), but it can't do it by itself.

---

I've been wondering about combining the DIRECT_RECLAIM, NORETRY,
RETRY_MAYFAIL and NOFAIL flags together into a single field:
0 => RECLAIM_NEVER,	/* !DIRECT_RECLAIM */
1 => RECLAIM_ONCE,	/* NORETRY */
2 => RECLAIM_PROGRESS,	/* RETRY_MAYFAIL */
3 => RECLAIM_FOREVER,	/* NOFAIL */

The existance of __GFP_RECLAIM makes this a bit tricky.  I honestly don't
know what this code is asking for:

kernel/power/swap.c:                       __get_free_page(__GFP_RECLAIM | __GFP_HIGH);
but I suspect I'll have to find out.  There's about 60 places to look at.

I also want to add __GFP_KILL (to be part of the GFP_KERNEL definition).
That way, each bit that you set in the GFP mask increases the things the
page allocator can do to get memory for you.  At the moment, RETRY_MAYFAIL
subtracts the ability to kill other tasks, which is unusual.  For example,
this test in kvmalloc_node:

        WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);

doesn't catch RETRY_MAYFAIL being set.

  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-29 10:41 [PATCH v1] kernel/trace:check the val against the available mem Zhaoyang Huang
2018-03-29 16:05 ` Steven Rostedt
2018-03-30  3:32   ` Zhaoyang Huang
2018-03-30 14:07     ` Steven Rostedt
2018-03-30  6:53 ` [Kernel-patch-test] " kbuild test robot
2018-03-30  6:54 ` kbuild test robot
2018-03-30 14:20 ` Steven Rostedt
2018-03-30 16:37   ` Joel Fernandes
2018-03-30 19:10     ` Steven Rostedt
2018-03-30 20:37       ` Joel Fernandes
2018-03-30 20:53   ` Matthew Wilcox
2018-03-30 21:30     ` Steven Rostedt
2018-03-30 21:42       ` Steven Rostedt
2018-03-30 23:38         ` Joel Fernandes
2018-03-31  1:41           ` Steven Rostedt
2018-03-31  2:18             ` Matthew Wilcox
2018-03-31  3:07               ` Steven Rostedt
2018-03-31  5:44                 ` Joel Fernandes
2018-04-02  0:52         ` Zhaoyang Huang
2018-04-03 11:06   ` Michal Hocko
2018-04-03 11:51     ` Steven Rostedt
2018-04-03 12:16       ` Michal Hocko
2018-04-03 12:23         ` Steven Rostedt
2018-04-03 12:35           ` Michal Hocko
2018-04-03 13:32             ` Steven Rostedt
2018-04-03 13:56               ` Michal Hocko
2018-04-03 14:17                 ` Steven Rostedt
2018-04-03 16:11                   ` Michal Hocko
2018-04-03 16:59                     ` Steven Rostedt
2018-04-03 22:56                     ` Steven Rostedt
2018-04-04  6:20                       ` Michal Hocko
2018-04-04 12:21                         ` Joel Fernandes
2018-04-04 12:59                         ` Steven Rostedt
2018-04-04 14:10                           ` Michal Hocko
2018-04-04 14:25                             ` Steven Rostedt
2018-04-04 14:42                               ` Michal Hocko
2018-04-04 15:04                                 ` Steven Rostedt
2018-04-04 15:27                                   ` Michal Hocko
2018-04-04 15:38                                     ` Steven Rostedt
2018-04-04  2:58                 ` Zhaoyang Huang
2018-04-04  6:23                   ` Michal Hocko
2018-04-04  9:29                     ` Zhaoyang Huang
2018-04-04 14:11                     ` Steven Rostedt
2018-04-04 14:23                       ` Michal Hocko
2018-04-04 14:31                         ` Steven Rostedt
2018-04-04 14:47                           ` Michal Hocko
2018-04-04 15:47                         ` Steven Rostedt
2018-04-05  2:58                           ` Matthew Wilcox
2018-04-05  4:12                             ` Joel Fernandes
2018-04-05 14:22                               ` Matthew Wilcox
2018-04-05 14:27                                 ` Michal Hocko
2018-04-05 14:34                                   ` Steven Rostedt
2018-04-05 15:13                                   ` Matthew Wilcox
2018-04-05 15:32                                     ` Michal Hocko
2018-04-05 16:15                                       ` Matthew Wilcox
2018-04-05 18:54                                         ` Michal Hocko
2018-04-05 20:15                                           ` __GFP_LOW Matthew Wilcox
2018-04-06  6:09                                             ` __GFP_LOW Michal Hocko
2018-04-08  4:27                                               ` Matthew Wilcox [this message]
2018-04-09  7:34                                                 ` __GFP_LOW Michal Hocko
2018-04-09 15:51                                                   ` __GFP_LOW Matthew Wilcox
2018-04-09 18:14                                                     ` __GFP_LOW Michal Hocko
     [not found]                                                       ` <CA+JonM0HG9kWb6-0iyDQ8UMxTeR-f=+ZL89t5DvvDULDC8Sfyw@mail.gmail.com>
2018-04-10 12:19                                                         ` __GFP_LOW Matthew Wilcox
2018-04-05 14:30                                 ` [PATCH v1] kernel/trace:check the val against the available mem Steven Rostedt

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