On Wed, 30 Nov 2022, Johannes Weiner wrote:
>
> Hm, I think the below should work for swap pages. Do you see anything
> obviously wrong with it, or scenarios I haven't considered?
>
I think you're overcomplicating it, with the __swap_count(ent) business,
and consequent unnecessarily detailed comments on the serialization.
Page/folio lock prevents a !page_mapped(page) becoming a page_mapped(page),
whether it's in swap cache or in file cache; it does not stop the sharing
count going further up, or down even to 0, but we just don't need to worry
about that sharing count - the MC_TARGET_PAGE case does not reject pages
with mapcount > 1, so why complicate the swap or file case in that way?
(Yes, it can be argued that all such sharing should be rejected; but we
didn't come here to argue improvements to memcg charge moving semantics:
just to minimize its effect on rmap, before it is fully deprecated.)
Or am I missing the point of why you add that complication?
> @@ -5637,6 +5645,46 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
Don't forget to trylock the page in the device_private case before this.
> * we call find_get_page() with swapper_space directly.
> */
> page = find_get_page(swap_address_space(ent), swp_offset(ent));
> +
> + /*
> + * Don't move shared charges. This isn't just for saner move
> + * semantics, it also ensures that page_mapped() is stable for
> + * the accounting in mem_cgroup_mapcount().
mem_cgroup_mapcount()??
> + *
> + * We have to serialize against the following paths: fork
> + * (which may copy a page map or a swap pte), fault (which may
> + * change a swap pte into a page map), unmap (which may cause
> + * a page map or a swap pte to disappear), and reclaim (which
> + * may change a page map into a swap pte).
> + *
> + * - Without swapcache, we only want to move the charge if
> + * there are no other swap ptes. With the pte lock, the
> + * swapcount is stable against all of the above scenarios
> + * when it's 1 (our pte), which is the case we care about.
> + *
> + * - When there is a page in swapcache, we only want to move
> + * charges when neither the page nor the swap entry are
> + * mapped elsewhere. The pte lock prevents our pte from
> + * being forked or unmapped. The page lock will stop faults
> + * against, and reclaim of, the swapcache page. So if the
> + * page isn't mapped, and the swap count is 1 (our pte), the
> + * test results are stable and the charge is exclusive.
> + */
> + if (!page && __swap_count(ent) != 1)
> + return NULL;
> +
> + if (page) {
> + if (!trylock_page(page)) {
> + put_page(page);
> + return NULL;
> + }
> + if (page_mapped(page) || __swap_count(ent) != 1) {
> + unlock_page(page);
> + put_page(page);
> + return NULL;
> + }
> + }
> +
> entry->val = ent.val;
>
> return page;
Looks right, without the __swap_count() additions and swap count comments.
And similar code in mc_handle_file_pte() - or are you saying that only
swap should be handled this way? I would disagree.
And matching trylock in mc_handle_present_pte() (and get_mctgt_type_thp()),
instead of in mem_cgroup_move_account().
I haven't checked to see where the page then needs to be unlocked,
probably some new places.
And I don't know what will be best for the preliminary precharge pass:
doesn't really want the page lock at all, but it may be unnecessary
complication to avoid taking it then unlocking it in that pass.
Hugh
