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Date:   Wed, 5 Jul 2023 18:20:21 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>
cc:     Hugh Dickins <hughd@google.com>, Jason Gunthorpe <jgg@ziepe.ca>,
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Subject: Re: [PATCH v2 07/12] s390: add pte_free_defer() for pgtables sharing
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On Wed, 5 Jul 2023, Gerald Schaefer wrote:
> On Tue, 4 Jul 2023 10:03:57 -0700 (PDT)
> Hugh Dickins <hughd@google.com> wrote:
> > On Tue, 4 Jul 2023, Gerald Schaefer wrote:
> > > On Sat, 1 Jul 2023 21:32:38 -0700 (PDT)
> > > Hugh Dickins <hughd@google.com> wrote:  
> > > > On Thu, 29 Jun 2023, Hugh Dickins wrote:  
> > ...
> > > > --- a/arch/s390/mm/pgalloc.c
> > > > +++ b/arch/s390/mm/pgalloc.c
> > > > @@ -229,6 +229,15 @@ void page_table_free_pgste(struct page *page)
> > > >   * logic described above. Both AA bits are set to 1 to denote a 4KB-pgtable
> > > >   * while the PP bits are never used, nor such a page is added to or removed
> > > >   * from mm_context_t::pgtable_list.
> > > > + *
> > > > + * pte_free_defer() overrides those rules: it takes the page off pgtable_list,
> > > > + * and prevents both 2K fragments from being reused. pte_free_defer() has to
> > > > + * guarantee that its pgtable cannot be reused before the RCU grace period
> > > > + * has elapsed (which page_table_free_rcu() does not actually guarantee).  
> > > 
> > > Hmm, I think page_table_free_rcu() has to guarantee the same, i.e. not
> > > allow reuse before grace period elapsed. And I hope that it does so, by
> > > setting the PP bits, which would be noticed in page_table_alloc(), in
> > > case the page would be seen there.
> > > 
> > > Unlike pte_free_defer(), page_table_free_rcu() would add pages back to the
> > > end of the list, and so they could be seen in page_table_alloc(), but they
> > > should not be reused before grace period elapsed and __tlb_remove_table()
> > > cleared the PP bits, as far as I understand.
> > > 
> > > So what exactly do you mean with "which page_table_free_rcu() does not actually
> > > guarantee"?  
> > 
> > I'll answer without locating and re-reading what Jason explained earlier,
> > perhaps in a separate thread, about pseudo-RCU-ness in tlb_remove_table():
> > he may have explained it better.  And without working out again all the
> > MMU_GATHER #defines, and which of them do and do not apply to s390 here.
> > 
> > The detail that sticks in my mind is the fallback in tlb_remove_table()
> 
> Ah ok, I was aware of that "semi-RCU" fallback logic in tlb_remove_table(),
> but that is rather a generic issue, and not s390-specific.

Yes.

> I thought you
> meant some s390-oddity here, of which we have a lot, unfortunately...
> Of course, we call tlb_remove_table() from our page_table_free_rcu(), so
> I guess you could say that page_table_free_rcu() cannot guarantee what
> tlb_remove_table() cannot guarantee.
> 
> Maybe change to "which page_table_free_rcu() does not actually guarantee,
> by calling tlb_remove_table()", to make it clear that this is not a problem
> of page_table_free_rcu() itself.

Okay - I'll rephrase slightly to avoid being sued by s390's lawyers :-)

> 
> > in mm/mmu_gather.c: if its __get_free_page(GFP_NOWAIT) fails, it cannot
> > batch the tables for freeing by RCU, and resorts instead to an immediate 
> > TLB flush (I think: that again involves chasing definitions) followed by
> > tlb_remove_table_sync_one() - which just delivers an interrupt to each CPU,
> > and is commented: 
> > /*
> >  * This isn't an RCU grace period and hence the page-tables cannot be
> >  * assumed to be actually RCU-freed.
> >  *
> >  * It is however sufficient for software page-table walkers that rely on
> >  * IRQ disabling.
> >  */
> > 
> > Whether that's good for your PP pages or not, I've given no thought:
> > I've just taken it on trust that what s390 has working today is good.
> 
> Yes, we should be fine with that, current code can be trusted :-)

Glad to hear it :-)  Yes, I think it's not actually relying on the "rcu"
implied by the function name.

> 
> > 
> > If that __get_free_page(GFP_NOWAIT) fallback instead used call_rcu(),
> > then I would not have written "(which page_table_free_rcu() does not
> > actually guarantee)".  But it cannot use call_rcu() because it does
> > not have an rcu_head to work with - it's in some generic code, and
> > there is no MMU_GATHER_CAN_USE_PAGE_RCU_HEAD for architectures to set.
> > 
> > And Jason would have much preferred us to address the issue from that
> > angle; but not only would doing so destroy my sanity, I'd also destroy
> > 20 architectures TLB-flushing, unbuilt and untested, in the attempt.
> 
> Oh yes, if your changes would have allowed to get rid of that "semi RCU"
> logic, that would really be a major boost in popularity, I guess. But
> it probably is as it is, because it is not so easily fixed...

I'm hoping that this series might help stir someone else to get into that.

> 
> > 
> > ...
> > > > @@ -325,10 +346,17 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
> > > >  		 */
> > > >  		mask = atomic_xor_bits(&page->_refcount, 0x11U << (bit + 24));
> > > >  		mask >>= 24;
> > > > -		if (mask & 0x03U)
> > > > +		if ((mask & 0x03U) && !PageActive(page)) {
> > > > +			/*
> > > > +			 * Other half is allocated, and neither half has had
> > > > +			 * its free deferred: add page to head of list, to make
> > > > +			 * this freed half available for immediate reuse.
> > > > +			 */
> > > >  			list_add(&page->lru, &mm->context.pgtable_list);
> > > > -		else
> > > > -			list_del(&page->lru);
> > > > +		} else {
> > > > +			/* If page is on list, now remove it. */
> > > > +			list_del_init(&page->lru);
> > > > +		}  
> > > 
> > > Ok, we might end up with some unnecessary list_del_init() here, e.g. if
> > > other half is still allocated, when called from pte_free_defer() on a
> > > fully allocated page, which was not on the list (and with PageActive, and
> > > (mask & 0x03U) true).
> > > Not sure if adding an additional mask check to the else path would be
> > > needed, but it seems that list_del_init() should also be able to handle
> > > this.  
> > 
> > list_del_init() is very cheap in the unnecessary case: the cachelines
> > required are already there.  You don't want a flag to say whether to
> > call it or not, it is already the efficient approach.
> 
> Yes, I also see no functional issue here. Just thought that the extra
> write could be avoided, e.g. by checking for list_empty() or mask first.
> But I guess that is simply the benefit of list_del_init(), that you
> don't have to check, at least if it is guaranteed that rcu_head is
> never in use here.
> 
> Then maybe adjust the comment, because now it makes you wonder, when
> you read (and understand) the code, you see that this list_del_init()
> might also be called for pages not on the list.

Sorry, I don't understand what clarification you're asking for there.
I thought
			/* If page is on list, now remove it. */
			list_del_init(&page->lru);
was good enough comment.

(I certainly don't want to enumerate the cases when it is or is not
already on the list there, that would be misery; but I don't think
that's the adjustment you were asking for either.)

> 
> > 
> > (But you were right not to use it in your pt_frag_refcount version,
> > because there we were still trying to do the call_rcu() per fragment
> > rather than per page, so page->lru could have been on the RCU queue.)
> 
> That is actually the one thing I still try to figure out, by drawing
> pictures, i.e. if we really really never end up here on list_del_init(),
> while using rcu_head, e.g. by racing PageActive.

There is no race with PageActive being seen when the table page is
finally to be freed (by RCU or not).  But there is definitely a harmless
race with pte_free_defer()er of other half setting PageActive an instant
after page_table_free() checked PageActive here.  So maybe this
page_table_free() does a list_add(), which the racer then list_del_init()s
when it gets the mm->context.lock; or maybe they both list_del_init().

> 
> > 
> > > 
> > > Same thought applies to the similar logic in page_table_free_rcu()
> > > below.
> > >   
> > > >  		spin_unlock_bh(&mm->context.lock);
> > > >  		mask = atomic_xor_bits(&page->_refcount, 0x10U << (bit + 24));
> > > >  		mask >>= 24;
> > > > @@ -342,8 +370,10 @@ void page_table_free(struct mm_struct *mm, unsigned long *table)
> > > >  	}
> > > >  
> > > >  	page_table_release_check(page, table, half, mask);
> > > > -	pgtable_pte_page_dtor(page);
> > > > -	__free_page(page);
> > > > +	if (TestClearPageActive(page))
> > > > +		call_rcu(&page->rcu_head, pte_free_now);
> > > > +	else
> > > > +		pte_free_now(&page->rcu_head);  
> > > 
> > > This ClearPageActive, and the similar thing in __tlb_remove_table() below,
> > > worries me a bit, because it is done outside the spin_lock. It "feels" like
> > > there could be some race with the PageActive checks inside the spin_lock,
> > > but when drawing some pictures, I could not find any such scenario yet.
> > > Also, our existing spin_lock is probably not supposed to protect against
> > > PageActive changes anyway, right?  
> > 
> > Here (and similarly in __tlb_remove_table()) is where we are about to free
> > the page table page: both of the fragments have already been released,
> > there is nobody left who could be racing against us to set PageActive.
> 
> Yes, that is what makes this approach so nice, i.e. no more checking
> for HH bits or worry about double call_rcu(), simply do the the freeing
> whenever the page is ready. At least in theory, still drawing pictures :-)

Please do keep drawing: and perhaps you can sell them afterwards :-)

> 
> But this really looks very good to me, and also works with LTP not worse
> than the other approaches.

Great, thanks for all your help Gerald.

Hugh