On Tue, Sep 06, 2022 at 09:35:41AM +0200, Michal Hocko wrote: > > From: "NeilBrown" <neilb@xxxxxxx> > > Subject: mm: discard __GFP_ATOMIC > > > > __GFP_ATOMIC serves little purpose. Its main effect is to set > > ALLOC_HARDER which adds a few little boosts to increase the chance of an > > allocation succeeding, one of which is to lower the water-mark at which it > > will succeed. > > > > It is *always* paired with __GFP_HIGH which sets ALLOC_HIGH which also > > adjusts this watermark. It is probable that other users of __GFP_HIGH > > should benefit from the other little bonuses that __GFP_ATOMIC gets. > > > > __GFP_ATOMIC also gives a warning if used with __GFP_DIRECT_RECLAIM. > > There is little point to this. We already get a might_sleep() warning if > > __GFP_DIRECT_RECLAIM is set. > > > > __GFP_ATOMIC allows the "watermark_boost" to be side-stepped. It is > > probable that testing ALLOC_HARDER is a better fit here. > > > > __GFP_ATOMIC is used by tegra-smmu.c to check if the allocation might > > sleep. This should test __GFP_DIRECT_RECLAIM instead. > > > > This patch: > > - removes __GFP_ATOMIC > > - causes __GFP_HIGH to set ALLOC_HARDER unless __GFP_NOMEMALLOC is set > > (as well as ALLOC_HIGH). > > - makes other adjustments as suggested by the above. > > > > The net result is not change to GFP_ATOMIC allocations. Other > > allocations that use __GFP_HIGH will benefit from a few different extra > > privileges. This affects: > > xen, dm, md, ntfs3 > > the vermillion frame buffer > > hibernation > > ksm > > swap > > all of which likely produce more benefit than cost if these selected > > allocation are more likely to succeed quickly. > > This is a good summary of the current usage and existing issues. It also > shows that the naming is tricky and allows people to make wrong calls > (tegra-smmu.c). I also thing that it is wrong to couple memory reserves > access to the reclaim constrains/expectations of the caller. > I think it's worth trying to get rid of __GFP_ATOMIC although this patch needs to be rebased. Without rebasing it, I suspect there is a corner case for reserving high order atomic blocks. A high-order atomic allocation might get confused with a __GFP_HIGH high-order allocation that can sleep. It would not be completely irrational to have such a caller if it was in a path that can tolerate a stall but stalling might have visible consequences. I'm also worried that the patch might allow __GFP_HIGH to ignore cpusets which is probably not intended by direct users like ksm. > > Link: https://lkml.kernel.org/r/163712397076.13692.4727608274002939094@xxxxxxxxxxxxxxxxxxxxx > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > > Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > > Cc: Michal Hocko <mhocko@xxxxxxxx> > > Cc: Thierry Reding <thierry.reding@xxxxxxxxx> > > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > > Yes, I am all for dropping the gfp flag. One thing that is not really > entirely clear to me, though, is whether we still need 3 levels of > memory reserves access. Can we just drop ALLOC_HARDER? With this patch > applied it serves RT tasks and conflates it with __GFP_HIGH users > essentially. So why do we need that additional level of reserves? I think this would fall under the "naming is hard". If __GFP_ATOMIC was removed, the ALLOC_ flags might need renaming to detect differences in high priority allocations (RT + GFP_ATOMIC), critical allocations (OOM) and ones that can access special reserves (GFP_ATOMIC high-order). -- Mel Gorman SUSE Labs
