Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

Jan Kara
2020-08-21T10:13:37Z

On Thu 20-08-20 15:01:00, Linus Torvalds wrote:
> On Thu, Aug 20, 2020 at 2:54 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > I kind of prefer the new suggestion to remove code rather than adding new
> > codes. I definitely don't know enough on the side effect of it, especially
> > performance-wise on either ksm or swap, but... IIUC the worst case is we'll get
> > some perf report later on, and it seems also not hard to revert the patch later
> > if we want.
>
> Well, would you be willing to try this patch out?
>
> After you apply that patch, you should be able to remove the
> should_force_cow_break() games entirely, because a write to the page
> should always break cow towards the writer if there are any GUP users
> around (put another way: away from the GUP).
>
> However, to make the test meaningful, it really should do some swap testing too.
>
> Linus

> From f41082844ea82ad1278e167fe6e973fa4efc974a Mon Sep 17 00:00:00 2001
> From: Linus Torvalds <torvalds@linux-foundation.org>
> Date: Tue, 11 Aug 2020 14:23:04 -0700
> Subject: [PATCH] Trial do_wp_page() simplification
>
> How about we just make sure we're the only possible valid user fo the
> page before we bother to reuse it?
>
> Simplify, simplify, simplify.
>
> And get rid of the nasty serialization on the page lock at the same time.
>
> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> ---
> mm/memory.c | 58 +++++++++++++++--------------------------------------
> 1 file changed, 16 insertions(+), 42 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 602f4283122f..a43004dd2ff6 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2927,50 +2927,24 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> * not dirty accountable.
> */
> if (PageAnon(vmf->page)) {
> - int total_map_swapcount;
> - if (PageKsm(vmf->page) && (PageSwapCache(vmf->page) ||
> - page_count(vmf->page) != 1))
> + struct page *page = vmf->page;
> +
> + if (page_count(page) != 1)
> + goto copy;
> + if (!trylock_page(page))
> + goto copy;
> + if (page_mapcount(page) != 1 && page_count(page) != 1) {

So this condition looks strange to me... Did you mean:

if (page_mapcount(page) != 1 || page_count(page) != 1)

? Because page mapcount is >= 1 here, and if mapcount is > 1, then page_count
is certainly greater than 1 as well...

> + unlock_page(page);
> goto copy;
> - if (!trylock_page(vmf->page)) {
> - get_page(vmf->page);
> - pte_unmap_unlock(vmf->pte, vmf->ptl);
> - lock_page(vmf->page);
> - vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
> - vmf->address, &vmf->ptl);
> - if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> - update_mmu_tlb(vma, vmf->address, vmf->pte);
> - unlock_page(vmf->page);
> - pte_unmap_unlock(vmf->pte, vmf->ptl);
> - put_page(vmf->page);
> - return 0;
> - }
> - put_page(vmf->page);
> - }
> - if (PageKsm(vmf->page)) {

Also I know nothing about KSM but looking at reuse_ksm_page() I can see it
plays some tricks with page index & mapping even for pages with page_count
== 1 so you cannot just drop those bits AFAICT.

> - bool reused = reuse_ksm_page(vmf->page, vmf->vma,
> - vmf->address);
> - unlock_page(vmf->page);
> - if (!reused)
> - goto copy;
> - wp_page_reuse(vmf);
> - return VM_FAULT_WRITE;
> - }
> - if (reuse_swap_page(vmf->page, &total_map_swapcount)) {

Also I'm not sure if dropping this is safe for THP - reuse_swap_page()
seems to be a misnomer and seems to do also some THP handling. In
particular a comment before page_trans_huge_mapcount() states that
page_mapcount() isn't fully accurate for THP and page_trans_huge_mapcount()
should be used instead for checking whether a copy is needed on write fault.

Honza
> - if (total_map_swapcount == 1) {
> - /*
> - * The page is all ours. Move it to
> - * our anon_vma so the rmap code will
> - * not search our parent or siblings.
> - * Protected against the rmap code by
> - * the page lock.
> - */
> - page_move_anon_rmap(vmf->page, vma);
> - }
> - unlock_page(vmf->page);
> - wp_page_reuse(vmf);
> - return VM_FAULT_WRITE;
> }
> - unlock_page(vmf->page);
> + /*
> + * Ok, we've got the only map reference, and the only
> + * page count reference, and the page is locked,
> + * it's dark out, and we're wearing sunglasses. Hit it.
> + */
> + wp_page_reuse(vmf);
> + unlock_page(page);
> + return VM_FAULT_WRITE;
> } else if (unlikely((vma->vm_flags & (VM_WRITE|VM_SHARED)) ==
> (VM_WRITE|VM_SHARED))) {
> return wp_page_shared(vmf);
> --
> 2.28.0.218.gc12ef3d349
>

--
Jan Kara <jack@suse.com>
SUSE Labs, CR