Re: [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO

[Dan Williams]

On Mon, Jan 4, 2021 at 12:11 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
>
> > Am 04.01.2021 um 20:52 schrieb Dave Hansen <dave.hansen@xxxxxxxxx>:
> >
> > On 1/4/21 11:27 AM, Matthew Wilcox wrote:
> >>> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
> >>> On 12/21/20 8:30 AM, Liang Li wrote:
> >>>> --- a/include/linux/page-flags.h
> >>>> +++ b/include/linux/page-flags.h
> >>>> @@ -137,6 +137,9 @@ enum pageflags {
> >>>> #endif
> >>>> #ifdef CONFIG_64BIT
> >>>>    PG_arch_2,
> >>>> +#endif
> >>>> +#ifdef CONFIG_PREZERO_PAGE
> >>>> +    PG_zero,
> >>>> #endif
> >>>>    __NR_PAGEFLAGS,
> >>>
> >>> I don't think this is worth a generic page->flags bit.
> >>>
> >>> There's a ton of space in 'struct page' for pages that are in the
> >>> allocator.  Can't we use some of that space?
> >>
> >> I was going to object to that too, but I think the entire approach is
> >> flawed and needs to be thrown out.  It just nukes the caches in extremely
> >> subtle and hard to measure ways, lowering overall system performance.
> >
> > Yeah, it certainly can't be the default, but it *is* useful for thing
> > where we know that there are no cache benefits to zeroing close to where
> > the memory is allocated.
> >
> > The trick is opting into it somehow, either in a process or a VMA.
> >
>
> The patch set is mostly trying to optimize starting a new process. So process/vma doesn‘t really work.
>
> I still wonder if using tmpfs/shmem cannot somehow be used to cover the original use case of starting a new vm fast (or rebooting an existing one involving restarting the process).

If it's rebooting a VM then file-backed should be able to skip the
zeroing because the stale data exposure is only to itself. If the
memory is being repurposed to a new VM then the file needs to be
reallocated / re-zeroed just like the anonymous case.