Re: [PATCH 6/7] mm/page_alloc: Give GFP_ATOMIC and non-blocking allocations access to reserves

[Mel Gorman <mgorman@techsingularity.net>]

On Sun, Jan 15, 2023 at 09:10:13AM +1100, NeilBrown wrote:
> On Thu, 12 Jan 2023, Mel Gorman wrote:
> > On Wed, Jan 11, 2023 at 04:58:02PM +0100, Michal Hocko wrote:
> > > On Mon 09-01-23 15:16:30, Mel Gorman wrote:
> > > > Explicit GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a bit
> > > > vague. In preparation for removing __GFP_ATOMIC, give GFP_ATOMIC and
> > > > other non-blocking allocation requests equal access to reserve.  Rename
> > > > ALLOC_HARDER to ALLOC_NON_BLOCK to make it more clear what the flag
> > > > means.
> > > 
> > > GFP_NOWAIT can be also used for opportunistic allocations which can and
> > > should fail quickly if the memory is tight and more elaborate path
> > > should be taken (e.g. try higher order allocation first but fall back to
> > > smaller request if the memory is fragmented). Do we really want to give
> > > those access to memory reserves as well?
> > 
> > Good question. Without __GFP_ATOMIC, GFP_NOWAIT only differs from GFP_ATOMIC
> > by __GFP_HIGH but that is not enough to distinguish between a caller that
> > cannot sleep versus one that is speculatively attempting an allocation but
> > has other options.
> 
> Isn't that a distinction without a difference?

Ideally yes but it's not always clear what the consequences of failure are.

> A caller than cannot sleep MUST have other options, because failure is
> always possible.
> The "other option" might be failure (error to user space, dropped packets
> etc), but sometimes failure IS an option.
> 

True, but it varies how gracefully it's handled and there is some cut&paste
involved and other cases where the GFP_ATOMIC usage predated the existance
or awareness of NOWAIT.

> So the difference between ATOMIC and NOWAIT boils down to the perceived
> cost of the "other options".  If that cost is high, then include
> __GFP_HIGH to get GFP_ATOMIC.  If that cost is low, then don't include
> __GFP_HIGH and get GFP_NOWAIT.
> 

Again, ideally yes but not necessary true. It depends on how careful
the caller was. The core appears to get it right in the cases I checked,
I'm less sure about drivers.

> I don't think there is any useful third option that is worth supporting.
> 

That's what we'll find out over time once the series hits a released
kernel.

-- 
Mel Gorman
SUSE Labs