Re: [PATCH RFC] mm: map zero-filled pages to zero_pfn while doing swap-in

[Barry Song <21cnbao@gmail.com>]

On Thu, Dec 12, 2024 at 9:51 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 12.12.24 09:46, Barry Song wrote:
> > On Thu, Dec 12, 2024 at 9:29 PM Christoph Hellwig <hch@infradead.org> wrote:
> >>
> >> On Thu, Dec 12, 2024 at 08:37:11PM +1300, Barry Song wrote:
> >>> From: Barry Song <v-songbaohua@oppo.com>
> >>>
> >>> While developing the zeromap series, Usama observed that certain
> >>> workloads may contain over 10% zero-filled pages. This may present
> >>> an opportunity to save memory by mapping zero-filled pages to zero_pfn
> >>> in do_swap_page(). If a write occurs later, do_wp_page() can
> >>> allocate a new page using the Copy-on-Write mechanism.
> >>
> >> Shouldn't this be done during, or rather instead of swap out instead?
> >> Swapping all zero pages out just to optimize the in-memory
> >> representation on seems rather backwards.
> >
> > I’m having trouble understanding your point—it seems like you might
> > not have fully read the code. :-)
> >
> > The situation is as follows: for a zero-filled page, we are currently
> > allocating a new
> > page unconditionally. By mapping this zero-filled page to zero_pfn, we could
> > save the memory used by this page.
> >
> > We don't need to allocate the memory until the page is written(which may never
> > happen).
>
> I think what Christoph means is that you would determine that at PTE
> unmap time, and directly place the zero page in there. So there would be
> no need to have the page fault at all.
>
> I suspect at PTE unmap time might be problematic, because we might still
> have other (i.e., GUP) references modifying that page, and we can only
> rely on the page content being stable after we flushed the TLB as well.
> (I recall some deferred flushing optimizations)

Yes, we need to follow a strict sequence:

1. try_to_unmap - unmap PTEs in all processes;
2. try_to_unmap_flush_dirty - flush deferred TLB shootdown;
3. pageout - zeromap will set 1 in bitmap if page is zero-filled

At the moment of pageout(), we can be confident that the page is zero-filled.

mapping to zeropage during unmap seems quite risky.

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry