Re: [PATCH] zram/zcomp: use GFP_NOIO to allocate streams

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 24 Nov 2015 09:30:27 +0900 Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> On (11/23/15 15:18), Andrew Morton wrote:
> [..]
> > > --- a/drivers/block/zram/zcomp_lz4.c
> > > +++ b/drivers/block/zram/zcomp_lz4.c
> > > @@ -20,10 +20,13 @@ static void *zcomp_lz4_create(void)
> > >  	void *ret;
> > >  
> > >  	ret = kzalloc(LZ4_MEM_COMPRESS,
> > > -			__GFP_NORETRY|__GFP_NOWARN|__GFP_NOMEMALLOC);
> > > -	if (!ret)
> > > -		ret = vzalloc(LZ4_MEM_COMPRESS);
> > > -	return ret;
> > > +			__GFP_NORETRY | __GFP_NOWARN | __GFP_NOMEMALLOC);
> > 
> > But here we've still lost __GFP_RECLAIM, unnecessarily.  And it's quite
> > unclear why __GFP_NORETRY and __GFP_NOMEMALLOC are being used.
> 
> __GFP_NORETRY
> 
> we are guaranteed to have at least one compression stream, so sooner or
> later every IO operation will be served. any IO that has failed in
> zcomp_lz4_create() or zcomp_lzo_create() will simply wait for already
> available compression stream to become idle. so this allocation is not
> so dramatically important - we just increase the level of parallelism
> (N idle streams let N IO operations to execute concurrently). apart from
> that we are in a low memory condition (or whatever was the reason the
> kernel failed to allocate LZ4_MEM_COMPRESS or LZO1X_MEM_COMPRESS) and
> we can avoid pressuring the kernel furher.
> 
> for the same reason __GFP_NOMEMALLOC is used -- we don't want to waste
> an emergency memory for compression streams.
> 

Doesn't make a lot of sense to me.  We use a weakened gfp for the
kmalloc and if that fails, fall into vmalloc() using the stronger gfp
anyway.

Perhaps it makes sense for higher-order allocations: we don't want to
thrash around trying to create an order-2 page - we'd prefer to give up
and fall into vmalloc to do a bunch of order-0 allocations.

But this argument holds for 1000 other kmalloc->vmalloc allocation
attempts - what's special about this one?

And whatever is the reason for this peculiar setup,

a) where's the proof that the change is actually beneficial?

b) let's get a good code comment in place so that future readers are not
   similarly puzzled.