On Thu, Dec 12, 2024 at 9:51 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > 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) 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
