On Mon, Oct 14, 2024 at 5:03 PM 'John Hubbard' via kernel-team <kernel-team@xxxxxxxxxxx> wrote: > > On 10/14/24 4:56 PM, Yosry Ahmed wrote: > > On Mon, Oct 14, 2024 at 4:53 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > >> > >> On 10/14/24 4:48 PM, Yosry Ahmed wrote: > >>> On Mon, Oct 14, 2024 at 1:37 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > >>>> > >>>> Add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to store allocation tag > >>>> references directly in the page flags. This eliminates memory > >>>> overhead caused by page_ext and results in better performance > >>>> for page allocations. > >>>> If the number of available page flag bits is insufficient to > >>>> address all kernel allocations, profiling falls back to using > >>>> page extensions with an appropriate warning to disable this > >>>> config. > >>>> If dynamically loaded modules add enough tags that they can't > >>>> be addressed anymore with available page flag bits, memory > >>>> profiling gets disabled and a warning is issued. > >>> > >>> Just curious, why do we need a config option? If there are enough bits > >>> in page flags, why not use them automatically or fallback to page_ext > >>> otherwise? > >> > >> Or better yet, *always* fall back to page_ext, thus leaving the > >> scarce and valuable page flags available for other features? > >> > >> Sorry Suren, to keep coming back to this suggestion, I know > >> I'm driving you crazy here! But I just keep thinking it through > >> and failing to see why this feature deserves to consume so > >> many page flags. > > > > I think we already always use page_ext today. My understanding is that > > the purpose of this series is to give the option to avoid using > > page_ext if there are enough unused page flags anyway, which reduces > > memory waste and improves performance. > > > > My question is just why not have that be the default behavior with a > > config option, use page flags if there are enough unused bits, > > otherwise use page_ext. > > I agree that if you're going to implement this feature at all, then > keying off of CONFIG_MEM_ALLOC_PROFILING seems sufficient, and no > need to add CONFIG_PGALLOC_TAG_USE_PAGEFLAGS on top of that. Yosry's original guess was correct. If not for loadable modules we could get away with having no CONFIG_PGALLOC_TAG_USE_PAGEFLAGS. We could try to fit codetag references into page flags and if they do not fit we could fall back to page_ext. That works fine when we have a predetermined number of tags. But loadable modules make this number variable at runtime and there is a possibility we run out of page flag bits at runtime. In that case, the current patchset disables memory profiling and issues a warning that the user should disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS to avoid this situation. I expect this to be a rare case but it can happen and we have to provide a way for a user to avoid running out of bits, which is where CONFIG_PGALLOC_TAG_USE_PAGEFLAGS would be used. Thanks, Suren. > > thanks, > -- > John Hubbard > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
