__GFP_LOW

[Matthew Wilcox <willy@infradead.org>]

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?