linux-kernel - Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous multi-size THP

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Message-ID: <94806b4f-2370-4999-9586-2c936955cb87@redhat.com>
Date:   Thu, 7 Dec 2023 14:28:36 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yu Zhao <yuzhao@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, Zi Yan <ziy@...dia.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Itaru Kitayama <itaru.kitayama@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Hugh Dickins <hughd@...gle.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Barry Song <21cnbao@...il.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 04/10] mm: thp: Support allocation of anonymous
 multi-size THP

>>
>> Right, but you know from the first loop which order is applicable (and will be
>> fed to the second loop) and could just pte_unmap(pte) + tryalloc. If that fails,
>> remap and try with the next orders.
> 
> You mean something like this?
> 
> 	pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> 	if (!pte)
> 		return ERR_PTR(-EAGAIN);
> 
> 	order = highest_order(orders);
> 	while (orders) {
> 		addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << order);
> 		if (!pte_range_none(pte + pte_index(addr), 1 << order)) {
> 			order = next_order(&orders, order);
> 			continue;
> 		}
> 
> 		pte_unmap(pte);
> 		
> 		folio = vma_alloc_folio(gfp, order, vma, addr, true);
> 		if (folio) {
> 			clear_huge_page(&folio->page, vmf->address, 1 << order);
> 			return folio;
> 		}
> 
> 		pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
> 		if (!pte)
> 			return ERR_PTR(-EAGAIN);
> 
> 		order = next_order(&orders, order);
> 	}
> 
> 	pte_unmap(pte);
> 
> I don't really like that because if high order folio allocations fail, then you
> are calling pte_range_none() again for the next lower order; once that check has
> succeeded for an order it shouldn't be required for any lower orders. In this
> case you also have lots of pte map/unmap.

I see what you mean.

> 
> The original version feels more efficient to me.
Yes it is. Adding in some comments might help, like

/*
  * Find the largest order where the aligned range is completely prot_none(). Note
  * that all remaining orders will be completely prot_none().
  */
...

/* Try allocating the largest of the remaining orders. */

> 
>>
>> That would make the code certainly easier to understand. That "orders" magic of
>> constructing, filtering, walking is confusing :)
>>
>>
>> I might find some time today to see if there is an easy way to cleanup all what
>> I spelled out above. It really is a mess. But likely that cleanup could be
>> deferred (but you're touching it, so ... :) ).
> 
> I'm going to ignore the last 5 words. I heard the "that cleanup could be
> deferred" part loud and clear though :)

:)

If we could stop passing orders into thp_vma_allowable_orders(), that would probably
be the biggest win. It's just all a confusing mess.

-- 
Cheers,

David / dhildenb