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. 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``. 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
