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Date:   Thu, 5 Apr 2018 20:54:44 +0200
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>,
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        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
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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