On Tue, 6 Jun 2023, Yin, Fengwei wrote: > On 6/6/2023 10:41 AM, Hugh Dickins wrote: > > On Mon, 5 Jun 2023, Liam R. Howlett wrote: > >> > >> You mean "mm: update validate_mm() to use vma iterator" here I guess. I > >> have it as a different commit id in my branch. > >> > >> I 'restored' some of the checking because I was able to work around not > >> having the mt_dump() definition with the vma iterator. I'm now > >> wondering how wide spread CONFIG_DEBUG_VM is used and if I should not > >> have added these extra checks. > > > > Most CONFIG_DEBUG_VM checks are quite cheap, mostly VM_BUG_ONs for > Indeed. I had CONFIG_DEBUG_VM enabled and didn't see surprise perf report. > > > > easily checked conditions. If validate_mm() is still the kind of thing > > it used to be, checking through every vma on every mmap operation, please > > don't bring that into CONFIG_DEBUG_VM - it distorts performance too much, > > so always used to be under a separate CONFIG_DEBUG_VM_RB instead. > So does this mean CONFIG_DEBUG_VM is allowed to be enabled for performance > testing? Thanks. I was going to say: No, I did not mean that: I just meant that even developers not doing strict performance testing still like to keep a rough eye on performance changes; and historically CONFIG_DEBUG_VM has not distorted very much. But then I wonder about certain distros which (wrongly or rightly) turn CONFIG_DEBUG_VM on: I expect they do performance testing on their kernels. Hugh
