On Thu, 29 Jun 2023 15:59:07 +0200
Alexander Gordeev <agordeev@xxxxxxxxxxxxx> wrote:
> On Wed, Jun 28, 2023 at 09:16:24PM +0200, Gerald Schaefer wrote:
> > On Tue, 20 Jun 2023 00:51:19 -0700 (PDT)
> > Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> Hi Gerald, Hugh!
>
> ...
> > @@ -407,6 +445,88 @@ void __tlb_remove_table(void *_table)
> > __free_page(page);
> > }
> >
> > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +static void pte_free_now0(struct rcu_head *head);
> > +static void pte_free_now1(struct rcu_head *head);
>
> What about pte_free_lower() / pte_free_upper()?
I actually like the 0/1 better, I always get confused what exactly we
mean with "lower / upper" in our code and comments. Is it the first
or second half? With 0/1 it is immediately clear to me.
>
> ...
> > +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> > +{
> > + unsigned int bit, mask;
> > + struct page *page;
> > +
> > + page = virt_to_page(pgtable);
> > + if (mm_alloc_pgste(mm)) {
> > + /*
> > + * TODO: Do we need gmap_unlink(mm, pgtable, addr), like in
> > + * page_table_free_rcu()?
> > + * If yes -> need addr parameter here, like in pte_free_tlb().
> > + */
> > + call_rcu(&page->rcu_head, pte_free_pgste);
> > + return;
> > +}
> > + bit = ((unsigned long)pgtable & ~PAGE_MASK) / (PTRS_PER_PTE * sizeof(pte_t));
> > +
> > + spin_lock_bh(&mm->context.lock);
> > + mask = atomic_xor_bits(&page->_refcount, 0x15U << (bit + 24));
>
> This makes the bit logic increasingly complicated to me.
I think it is well in line with existing code in page_table_free[_rcu].
Only instead of doing xor with 0x11U, it does xor with 0x15U to also
switch on the H bit while at it.
>
> What if instead we set the rule "one bit at a time only"?
> That means an atomic group bit flip is only allowed between
> pairs of bits, namely:
>
> bit flip initiated from
> ----------- ----------------------------------------
> P <- A page_table_free(), page_table_free_rcu()
> H <- A pte_free_defer()
> P <- H pte_free_half()
>
> In the current model P bit could be on together with H
> bit simultaneously. That actually brings in equation
> nothing.
P bit has to be set at the latest when __tlb_remove_table() gets
called, because there it is checked / cleared. It might be possible
to not set it in pte_free_defer() already, but only later in
pte_free_half() RCU callback, before calling __tlb_remove_table().
But that would not be in line any more with existing code, where it
is already set before scheduling the RCU callback.
Therefore, I would rather stick to the current approach, unless
you see some bug in it.
>
> Besides, this check in page_table_alloc() (while still
> correct) makes one (well, me) wonder "what about HH bits?":
>
> mask = (mask | (mask >> 4)) & 0x03U;
> if (mask != 0x03U) {
> ...
> }
Without adding fragments back to the list, it is not necessary
to check any H bits page_table_alloc(), or so I hope. Actually,
I like that aspect most, i.e. we have as little impact on current
code as possible.
And H bits are only relevant for preventing double use of rcu_head,
which is what they were designed for, and only the new code has
to care about them.
