From: Michal Hocko <mhocko@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Joel Fernandes <joelaf@google.com>,
Steven Rostedt <rostedt@goodmis.org>,
Zhaoyang Huang <huangzhaoyang@gmail.com>,
Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
kernel-patch-test@lists.linaro.org,
Andrew Morton <akpm@linux-foundation.org>,
"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v1] kernel/trace:check the val against the available mem
Date: Thu, 5 Apr 2018 20:54:44 +0200 [thread overview]
Message-ID: <20180405185444.GQ6312@dhcp22.suse.cz> (raw)
In-Reply-To: <20180405161501.GD28128@bombadil.infradead.org>
On Thu 05-04-18 09:15:01, Matthew Wilcox wrote:
> On Thu, Apr 05, 2018 at 05:32:40PM +0200, Michal Hocko wrote:
> > On Thu 05-04-18 08:13:59, Matthew Wilcox wrote:
> > > Argh. The comment confused me. OK, now I've read the source and
> > > understand that GFP_KERNEL | __GFP_RETRY_MAYFAIL tries exactly as hard
> > > as GFP_KERNEL *except* that it won't cause OOM itself. But any other
> > > simultaneous GFP_KERNEL allocation without __GFP_RETRY_MAYFAIL will
> > > cause an OOM. (And that's why we're having a conversation)
> >
> > Well, I can udnerstand how this can be confusing. The all confusion
> > boils down to the small-never-fails semantic we have. So all reclaim
> > modificators (__GFP_NOFAIL, __GFP_RETRY_MAYFAIL, __GFP_NORETRY) only
> > modify the _default_ behavior.
>
> Now that I understand the flag, I'll try to write a more clear
> explanation.
Good luck with that. It took me several iterations to land with the
current state. It is quite hard to be not misleading yet understandable.
> > > That's a problem because we have places in the kernel that call
> > > kv[zm]alloc(very_large_size, GFP_KERNEL), and that will turn into vmalloc,
> > > which will do the exact same thing, only it will trigger OOM all by itself
> > > (assuming the largest free chunk of address space in the vmalloc area
> > > is larger than the amount of free memory).
> >
> > 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.
> > > 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.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-04-05 18:54 UTC|newest]
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 [this message]
2018-04-05 20:15 ` __GFP_LOW Matthew Wilcox
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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