Re: [PATCH v5 3/4] mm: support large folios swapin as a whole for zRAM-like swapfile

[Barry Song <21cnbao@gmail.com>]

On Tue, Jul 30, 2024 at 3:13 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Jul 30, 2024 at 01:11:31AM +1200, Barry Song wrote:
> > for this zRAM case, it is a new allocated large folio, only
> > while all conditions are met, we will allocate and map
> > the whole folio. you can check can_swapin_thp() and
> > thp_swap_suitable_orders().
>
> YOU ARE DOING THIS WRONGLY!
>
> All of you anonymous memory people are utterly fixated on TLBs AND THIS
> IS WRONG.  Yes, TLB performance is important, particularly with crappy
> ARM designs, which I know a lot of you are paid to work on.  But you
> seem to think this is the only consideration, and you're making bad
> design choices as a result.  It's overly complicated, and you're leaving
> performance on the table.
>
> Look back at the results Ryan showed in the early days of working on
> large anonymous folios.  Half of the performance win on his system came
> from using larger TLBs.  But the other half came from _reduced software
> overhead_.  The LRU lock is a huge problem, and using large folios cuts
> the length of the LRU list, hence LRU lock hold time.
>
> Your _own_ data on how hard it is to get hold of a large folio due to
> fragmentation should be enough to convince you that the more large folios
> in the system, the better the whole system runs.  We should not decline to
> allocate large folios just because they can't be mapped with a single TLB!

I am not convinced. for a new allocated large folio, even alloc_anon_folio()
of do_anonymous_page() does the exactly same thing

alloc_anon_folio()
{
        /*
         * Get a list of all the (large) orders below PMD_ORDER that are enabled
         * for this vma. Then filter out the orders that can't be allocated over
         * the faulting address and still be fully contained in the vma.
         */
        orders = thp_vma_allowable_orders(vma, vma->vm_flags,
                        TVA_IN_PF | TVA_ENFORCE_SYSFS, BIT(PMD_ORDER) - 1);
        orders = thp_vma_suitable_orders(vma, vmf->address, orders);

}

you are not going to allocate a mTHP for an unaligned address for a new
PF.
Please point out where it is wrong.

Thanks
Barry