linux-kernel - Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP

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Date: Wed, 31 Jan 2024 12:13:05 +0100
From: David Hildenbrand <david@...hat.com>
To: Ryan Roberts <ryan.roberts@....com>, linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
 Nick Piggin <npiggin@...il.com>, Peter Zijlstra <peterz@...radead.org>,
 Michael Ellerman <mpe@...erman.id.au>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
 Alexander Gordeev <agordeev@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Arnd Bergmann <arnd@...db.de>,
 linux-arch@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH v1 9/9] mm/memory: optimize unmap/zap with PTE-mapped THP

>>>> -        folio_remove_rmap_pte(folio, page, vma);
>>>> +        folio_remove_rmap_ptes(folio, page, nr, vma);
>>>> +
>>>> +        /* Only sanity-check the first page in a batch. */
>>>>            if (unlikely(page_mapcount(page) < 0))
>>>>                print_bad_pte(vma, addr, ptent, page);
>>>
>>> Is there a case for either removing this all together or moving it into
>>> folio_remove_rmap_ptes()? It seems odd to only check some pages.
>>>
>>
>> I really wanted to avoid another nasty loop here.
>>
>> In my thinking, for 4k folios, or when zapping subpages of large folios, we
>> still perform the exact same checks. Only when batching we don't. So if there is
>> some problem, there are ways to get it triggered. And these problems are barely
>> ever seen.
>>
>> folio_remove_rmap_ptes() feels like the better place -- especially because the
>> delayed-rmap handling is effectively unchecked. But in there, we cannot
>> "print_bad_pte()".
>>
>> [background: if we had a total mapcount -- iow cheap folio_mapcount(), I'd check
>> here that the total mapcount does not underflow, instead of checking per-subpage]
> 
> All good points... perhaps extend the comment to describe how this could be
> solved in future with cheap total_mapcount()? Or in the commit log if you prefer?

I'll add more meat to the cover letter, thanks!

-- 
Cheers,

David / dhildenb