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 9:29 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).

>

Thanks
Barry