linux-kernel - Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <70844AE7-794D-475D-A660-F02EED4BA301@linux.dev>
Date:   Thu, 31 Aug 2023 11:54:28 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        David Rientjes <rientjes@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Xiongchun Duan <duanxiongchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/12] hugetlb: batch PMD split for bulk vmemmap dedup



> On Aug 31, 2023, at 00:03, Joao Martins <joao.m.martins@...cle.com> wrote:
> 
> On 30/08/2023 12:13, Joao Martins wrote:
>> On 30/08/2023 09:09, Muchun Song wrote:
>>> On 2023/8/26 03:04, Mike Kravetz wrote:
>>>> +
>>>> +        /*
>>>> +         * We are only splitting, not remapping the hugetlb vmemmap
>>>> +         * pages.
>>>> +         */
>>>> +        if (bulk)
>>>> +            continue;
>>> 
>>> Actually, we don not need a flag to detect this situation, you could
>>> use "!@...k->remap_pte" to determine whether we should go into the
>>> next level traversal of the page table. ->remap_pte is used to traverse
>>> the pte entry, so it make senses to continue to the next pmd entry if
>>> it is NULL.
>>> 
>> 
>> Yeap, great suggestion.
>> 
>>>> +
>>>>           vmemmap_pte_range(pmd, addr, next, walk);
>>>>       } while (pmd++, addr = next, addr != end);
>>>>   @@ -197,7 +211,8 @@ static int vmemmap_remap_range(unsigned long start,
>>>> unsigned long end,
>>>>               return ret;
>>>>       } while (pgd++, addr = next, addr != end);
>>>>   -    flush_tlb_kernel_range(start, end);
>>>> +    if (!(walk->flags & VMEMMAP_REMAP_ONLY_SPLIT))
>>>> +        flush_tlb_kernel_range(start, end);
>>> 
>>> This could be:
>>> 
>>>     if (walk->remap_pte)
>>>         flush_tlb_kernel_range(start, end);
>>> 
>> Yeap.
>> 
> 
> Quite correction: This stays as is, except with a flag rename. That is because
> this is actual flush that we intend to batch in the next patch. And while the
> PMD split could just use !walk->remap_pte, the next patch would just need to
> test NO_TLB_FLUSH flag. Meaning we endup anyways just testing for this
> to-be-consolidated flag

I think this really should be "if (walk->remap_pte && !(flag & VMEMMAP_NO_TLB_FLUSH))"
in your next patch. This TLB flushing only make sense for the case of existing of
@walk->remap_pte. I know "if (!(flag & VMEMMAP_NO_TLB_FLUSH))" check is suitable for your
use case, but what if a user (even if it does not exist now, but it may in the future)
passing a NULL @walk->remap_pte and not specifying VMEMMAP_NO_TLB_FLUSH? Then we will
do a useless TLB flushing. This is why I suggest you change this to "if (walk->remap_pte)"
in this patch and change it to "if (walk->remap_pte && !(flag & VMEMMAP_NO_TLB_FLUSH))"
in the next patch.

Thanks.