On Fri, Jan 06, 2023 at 09:35:24AM +0000, Mel Gorman wrote: > On Thu, Jan 05, 2023 at 03:49:44PM +0200, Mike Rapoport wrote: > > Hi Mel, > > > > On Tue, Nov 29, 2022 at 03:17:01PM +0000, Mel Gorman wrote: > > > From: NeilBrown <neilb@xxxxxxx> > > > > > > __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 > > > - allows __GFP_HIGH allocations to ignore watermark boosting as well > > > as GFP_ATOMIC requests. > > > - 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. > > > > > > [mgorman: Minor adjustments to rework on top of a series] > > > Link: https://lkml.kernel.org/r/163712397076.13692.4727608274002939094@xxxxxxxxxxxxxxxxxxxxx > > > Signed-off-by: NeilBrown <neilb@xxxxxxx> > > > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > > > --- > > > Documentation/mm/balance.rst | 2 +- > > > > Documentation/core-api/memory-allocation.rst needs an update as well, and > > there are other mentions of GFP_ATOMIC in Documentation/ > > > > What part do you think needs updating in that file? > > There are two references to GFP_ATOMIC in that file, one about accessing > reserves and another about non-sleeping allocations and the accuracy > does not change after the series. You are right, I got confused. > If anything, this statement should change because it invites GFP_ATOMIC > abuse for spurious reasons > > * If you think that accessing memory reserves is justified and the kernel > will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``. Care to send a patch? ;-) > There are other references to GFP_ATOMIC in Documentation/ that are are a > little inaccurate but not in a way that is harmful and is not changed by > the series. For example; > > GFP_ATOMIC requests are kernel internal allocations that must > be satisfied, immediately. The kernel may drop some request, > in rare cases even panic, if a GFP_ATOMIC alloc fails. > > This is a stronger statement than GFP_ATOMIC deserves but it's close enough > in the given context. > > -- > Mel Gorman > SUSE Labs > -- Sincerely yours, Mike.
