Re: [RFC-PROTOTYPE 1/1] mm: Add __GFP_FAST_TRY flag

[Matthew Wilcox]

On Tue, Aug 04, 2020 at 07:02:14PM +0200, Vlastimil Babka wrote:
> > 2) There was a proposal from Matthew Wilcox: https://lkml.org/lkml/2020/7/31/1015
> > 
> > <snip>
> > On non-RT, we could make that lock a raw spinlock.  On RT, we could
> > decline to take the lock.  We'd need to abstract the spin_lock() away
> > behind zone_lock(zone), but that should be OK.
> > <snip>
> > 
> > It would be great to use any existing flag, say GFP_NOWAIT. Suppose we
> > decline to take the lock across the page allocator for RT. But there is
> > at least one path that does it outside of the page allocator. GFP_NOWAIT
> > can wakeup the kswapd, whereas a "wake-up path" uses sleepable lock:
> > 
> > wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait).
> > 
> > Probably it can be fixed by the excluding of waking of the kswapd process
> > defining something like below:
> 
> Is something missing here?
> 
> > what is equal to zero and i am not sure if __get_free_page(0) handles
> > all that correctly, though it allocates and seems working on my test
> > machine! Please note it is related to "if we can reuse existing flags".
> > 
> > In the meantime, please see below for a patch that adds a __GFP_FAST_TRY,
> > which can at least serve as a baseline against which other proposals can
> > be compared. The patch is based on the 5.8.0-rc3.
> > 
> > Please RFC.
> 
> At first glance __GFP_FAST_TRY (more descriptive name? __GFP_NO_LOCKS?) seems
> better than doing weird things with GFP_NOWAIT, but depends on the real benefits
> (hence my first questions).

I think what Vlad is trying to say is that even GFP_NOWAIT will wake
kswapd, which involves taking a spinlock.  If you specify 0 in your GFP
flags, then we won't wake kswapd.  So a simple:

#define GFP_NOLOCKS	0

should do the trick (modulo various casting, blah blah blah)