On 12.12.24 09:46, Barry Song wrote:
On Thu, Dec 12, 2024 at 9:29 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
On Thu, Dec 12, 2024 at 08:37:11PM +1300, Barry Song wrote:
From: Barry Song <v-songbaohua@xxxxxxxx>
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)
--
Cheers,
David / dhildenb
