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    SubjectRe: [PATCH] mm: reuse the unshared swapcache page in do_wp_page
    On Thu, Jan 13, 2022 at 03:46:54PM +0100, David Hildenbrand wrote:
    > On 13.01.22 15:39, Matthew Wilcox wrote:
    > > On Thu, Jan 13, 2022 at 10:03:18PM +0800, Liang Zhang wrote:
    > >> In current implementation, process's read requestions will fault in pages
    > >> with WP flags in PTEs. Next, if process emit a write requestion will go
    > >> into do_wp_page() and copy data to a new allocated page from the old one
    > >> due to refcount > 1 (page table mapped and swapcache), which could be
    > >> result in performance degradation. In fact, this page is exclusively owned
    > >> by this process and the duplication from old to a new allocated page is
    > >> really unnecessary.
    > >>
    > >> So In this situation, these unshared pages can be reused by its process.
    > >
    > > Let's bring Linus in on this, but I think this reintroduces all of the
    > > mapcount problems that we've been discussing recently.
    > >
    > > How about this as an alternative?
    > >
    > > +++ b/mm/memory.c
    > > @@ -3291,11 +3291,11 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
    > > struct page *page = vmf->page;
    > >
    > > /* PageKsm() doesn't necessarily raise the page refcount */
    > > - if (PageKsm(page) || page_count(page) != 1)
    > > + if (PageKsm(page) || page_count(page) != 1 + PageSwapCache(page))
    > > goto copy;
    > > if (!trylock_page(page))
    > > goto copy;
    > > - if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
    > > + if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1 + PageSwapCache(page)) {
    > > unlock_page(page);
    > > goto copy;
    > > }
    >
    > Funny, I was staring at swap reuse code as I received this mail ...
    > because if we're not using reuse_swap_page() here anymore, we shouldn't
    > really be reusing it anywhere for consistency, most prominently in
    > do_swap_page() when we handle vmf->flags & FAULT_FLAG_WRITE just
    > similarly as we do here ...
    >
    > And that's where things get hairy and I am still trying to figure out
    > all of the details.
    >
    > Regarding above: If the page is swapped out in multiple processes but
    > was only faulted into the current process R/O, and then we try to write:
    >
    > 1. Still in the swapcache: PageSwapCache()
    > 2. Mapped only by one process: page_mapcount(page) == 1
    > 3. Reference from one page table and the swap cache: page_count(page) ==
    >
    > But other processes could read-fault on the swapcache page, no?
    >
    > I think we'd really have to check against the swapcount as well ...
    > essentially reuse_swap_page(), no?

    Unfortunately the last digit is missing from your "3.", but I
    think you're absolutely right; we need to check swapcount. So
    once reuse_swap_page() checks page_count instead of mapcount, we'll
    be good?

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