On Sat 30-04-22 11:30:28, Andrew Morton wrote: Sorry, this got lost in my inbox. Thanks Andrew for poking me. > 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. > 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? -- Michal Hocko SUSE Labs
