Re: __GFP_LOW

[Michal Hocko]

On Sat 07-04-18 21:27:09, Matthew Wilcox wrote:
> 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.

Well, I was probably not clear here. KSWAPD_RECLAIM is not something you
want to drop because this is a cooperative flag. If you do not use it
then you are effectivelly pushing others to the direct reclaim because
the kswapd won't be woken up and won't do the background work. Your
working make it sound as a good thing to drop.

> > >    - 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.

So what are we going to do if the LOW watermark cannot succeed?

> 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.

So who would be a user of __GFP_LOW?

> ---
> 
> 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:

I am not sure I follow here. Is the RECLAIM_ an internal thing to the
allocator?

> 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.

Well, it would be more understandable if this was written as
(GFP_KERNEL | __GFP_HIGH) & ~(__GFP_FS|__GFP_IO)
 
> I also want to add __GFP_KILL (to be part of the GFP_KERNEL definition).

What does __GFP_KILL means?

> 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.

Well, it is not all that great because some flags add capabilities while
some remove them but, well, life is hard when you try to extend an
interface which was not all that great from the very beginning.

> For example,
> this test in kvmalloc_node:
> 
>         WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL);
> 
> doesn't catch RETRY_MAYFAIL being set.

It doesn't really have to. We want to catch obviously broken gfp flags
here. That means mostly GFP_NO{FS,IO} because those might simply
deadlock. RETRY_MAYFAIL is even supported to some extend.
-- 
Michal Hocko
SUSE Labs