Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

[Jan Kara]

On Mon 24-08-20 11:36:22, Kirill Tkhai wrote:
> On 22.08.2020 02:49, Peter Xu wrote:
> > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxxxxxxxxxxxx>
> > [peterx: add subject prefix]
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> >  mm/memory.c | 59 +++++++++++++++--------------------------------------
> >  1 file changed, 17 insertions(+), 42 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 602f4283122f..cb9006189d22 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2927,50 +2927,25 @@ 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;
> > +
> > +		/* PageKsm() doesn't necessarily raise the page refcount */
> 
> No, this is wrong. PageKSM() always raises refcount.

OK, then I'm confused. The comment before get_ksm_page() states:

 * get_ksm_page: checks if the page indicated by the stable node
 * is still its ksm page, despite having held no reference to it.
 * In which case we can trust the content of the page, and it
 * returns the gotten page; but if the page has now been zapped,
 * remove the stale node from the stable tree and return NULL.
...
 * You would expect the stable_node to hold a reference to the ksm page.
 * But if it increments the page's count, swapping out has to wait for
 * ksmd to come around again before it can free the page, which may take
 * seconds or even minutes: much too unresponsive.  So instead we use a
 * "keyhole reference": access to the ksm page from the stable node peeps
 * out through its keyhole to see if that page still holds the right key,
 * pointing back to this stable node.

So this all seems to indicate that KSM doesn't hold a proper page reference
and relies on anyone making page writeable to change page->mapping so that
KSM notices this and doesn't use the page anymore... Am I missing
something?

> There was another
> problem: KSM may raise refcount without lock_page(), and only then it
> takes the lock.  See get_ksm_page(GET_KSM_PAGE_NOLOCK) for the details.
> 
> So, reliable protection against parallel access requires to freeze page
> counter, which is made in reuse_ksm_page().

OK, this as well.

								Honza

> 
> > +		if (PageKsm(page) || page_count(page) != 1)
> > +			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 (!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)) {
> > -			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)) {
> > -			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);
> > 
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR