On Thu, Jan 13, 2022 at 09:44:04AM -0800, Linus Torvalds wrote:
> IOW, the COW path would do
>
> trylock - copy if fails
> try to remove from swap cache
> if page_count() is now 1, we can reuse it
>
> Note how the "try to remove from swap cache" is entirely independent
> of whether we then reuse it or not.
>
> And yes, we can have optimistic other tests - like not even bothering
> to trylock if we can see that the page-count is so elevated that it
> makes no difference and trying to remove from swap cache would be just
> pointless extra work (both the removal itself, and then potentially
> later re-additions).
>
> But those should be seen for what they are - not important for
> semantics, only a "don't bother, this page has so many users that we
> already know that removing the swapcache one doesn't make any
> difference at all".
I think what you mean is something like ...
+++ b/mm/memory.c
@@ -3290,15 +3290,13 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
if (PageAnon(vmf->page)) {
struct page *page = vmf->page;
- /* PageKsm() doesn't necessarily raise the page refcount */
- if (PageKsm(page) || page_count(page) != 1)
+ /* Lots of people are using this page, just copy */
+ if (page_count(page) > 5)
goto copy;
if (!trylock_page(page))
goto copy;
- if (PageKsm(page) || page_mapcount(page) != 1 || page_count(page) != 1) {
- unlock_page(page);
- goto copy;
- }
+ if (!reuse_swap_page(page, NULL))
+ goto unlock;
/*
* Ok, we've got the only map reference, and the only
* page count reference, and the page is locked,
@@ -3311,6 +3309,8 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
(VM_WRITE|VM_SHARED))) {
return wp_page_shared(vmf);
}
+unlock:
+ unlock_page(page);
copy:
/*
* Ok, we need to copy. Oh, well..
... with a suitably sensible reuse_swap_page().
