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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: __GFP_LOW
Date: Thu, 5 Apr 2018 13:15:57 -0700
Message-ID: <20180405201557.GA3666@bombadil.infradead.org> (raw)
In-Reply-To: <20180405185444.GQ6312@dhcp22.suse.cz>

On Thu, Apr 05, 2018 at 08:54:44PM +0200, Michal Hocko wrote:
> On Thu 05-04-18 09:15:01, Matthew Wilcox wrote:
> > > well, hardcoded GFP_KERNEL from vmalloc guts is yet another, ehm,
> > > herritage that you are not so proud of.
> > 
> > Certainly not, but that's not what I'm concerned about; I'm concerned
> > about the allocation of the pages, not the allocation of the array
> > containing the page pointers.
> 
> Those pages will use the gfp flag you give to vmalloc IIRC. It is page
> tables that are GFP_KERNEL unconditionally.

Right.  But if I call vmalloc(1UL << 40, GFP_KERNEL) on a machine with
only half a terabyte of RAM, it will OOM in the exact same way that
Steven is reporting.

> > > > We could also have a GFP flag that says to only succeed if we're further
> > > > above the existing watermark than normal.  __GFP_LOW (==ALLOC_LOW),
> > > > if you like.  That would give us the desired behaviour of trying all of
> > > > the reclaim methods that GFP_KERNEL would, but not being able to exhaust
> > > > all the memory that GFP_KERNEL allocations would take.
> > > 
> > > Well, I would be really careful with yet another gfp mask. They are so
> > > incredibly hard to define properly and then people kinda tend to screw
> > > your best intentions with their usecases ;)
> > > Failing on low wmark is very close to __GFP_NORETRY or even
> > > __GFP_NOWAIT, btw. So let's try to not overthink this...
> > 
> > Oh, indeed.  We must be able to clearly communicate to users when they
> > should use this flag.  I have in mind something like this:
> > 
> >  * __GFP_HIGH indicates that the caller is high-priority and that granting
> >  *   the request is necessary before the system can make forward progress.
> >  *   For example, creating an IO context to clean pages.
> >  *
> >  * __GFP_LOW indicates that the caller is low-priority and that it should
> >  *   not be allocated pages that would cause the system to get into an
> >  *   out-of-memory situation.  For example, allocating multiple individual
> >  *   pages in order to satisfy a larger request.
> 
> So how exactly the low fits into GFP_NOWAIT, GFP_NORETRY and
> GFP_RETRY_MAFAIL? We _do_have several levels of how hard to try and we
> have users relying on that. And do not forget about costly vs.
> non-costly sizes.
> 
> That being said, we should not hijack this thread more and further
> enhancements should be discussed separatelly. I am all for making the
> whole allocation api less obscure but keep in mind that we have really
> hard historical restrictions.

Dropping the non-mm participants ...

>From a "user guide" perspective:

When allocating memory, you can choose:

 - 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)
   - Kill other processes to get their memory (!RETRY_MAYFAIL)
   - All of the above, and wait forever (NOFAIL)
   - Take from emergency reserves (HIGH)
   - ... but not the last parts of the regular reserves (LOW)

How does that look as an overview?

  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                                           ` Matthew Wilcox [this message]
2018-04-06  6:09                                             ` __GFP_LOW Michal Hocko
2018-04-08  4:27                                               ` __GFP_LOW Matthew Wilcox
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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