linux-kernel - Re: [RFC PATCH 04/26] hugetlb: make huge_pte_lockptr take an explicit shift argument.

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]

Date:   Thu, 30 Jun 2022 17:35:14 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     James Houghton <jthoughton@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Jue Wang <juew@...gle.com>,
        Manish Mishra <manish.mishra@...anix.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 04/26] hugetlb: make huge_pte_lockptr take an
 explicit shift argument.

On Wed, Jun 29, 2022 at 03:24:45PM -0700, Mike Kravetz wrote:
> On 06/29/22 14:39, James Houghton wrote:
> > On Wed, Jun 29, 2022 at 2:04 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
> > >
> > > On 06/29/22 14:09, Muchun Song wrote:
> > > > On Mon, Jun 27, 2022 at 01:51:53PM -0700, Mike Kravetz wrote:
> > > > > On 06/24/22 17:36, James Houghton wrote:
> > > > > > This is needed to handle PTL locking with high-granularity mapping. We
> > > > > > won't always be using the PMD-level PTL even if we're using the 2M
> > > > > > hugepage hstate. It's possible that we're dealing with 4K PTEs, in which
> > > > > > case, we need to lock the PTL for the 4K PTE.
> > > > >
> > > > > I'm not really sure why this would be required.
> > > > > Why not use the PMD level lock for 4K PTEs?  Seems that would scale better
> > > > > with less contention than using the more coarse mm lock.
> > > > >
> > > >
> > > > Your words make me thing of another question unrelated to this patch.
> > > > We __know__ that arm64 supports continues PTE HugeTLB. huge_pte_lockptr()
> > > > did not consider this case, in this case, those HugeTLB pages are contended
> > > > with mm lock. Seems we should optimize this case. Something like:
> > > >
> > > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > > > index 0d790fa3f297..68a1e071bfc0 100644
> > > > --- a/include/linux/hugetlb.h
> > > > +++ b/include/linux/hugetlb.h
> > > > @@ -893,7 +893,7 @@ static inline gfp_t htlb_modify_alloc_mask(struct hstate *h, gfp_t gfp_mask)
> > > >  static inline spinlock_t *huge_pte_lockptr(struct hstate *h,
> > > >                                            struct mm_struct *mm, pte_t *pte)
> > > >  {
> > > > -       if (huge_page_size(h) == PMD_SIZE)
> > > > +       if (huge_page_size(h) <= PMD_SIZE)
> > > >                 return pmd_lockptr(mm, (pmd_t *) pte);
> > > >         VM_BUG_ON(huge_page_size(h) == PAGE_SIZE);
> > > >         return &mm->page_table_lock;
> > > >
> > > > I did not check if elsewhere needs to be changed as well. Just a primary
> > > > thought.
> > 
> > I'm not sure if this works. If hugetlb_pte_size(hpte) is PAGE_SIZE,
> > then `hpte.ptep` will be a pte_t, not a pmd_t -- I assume that breaks
> > things. So I think, when doing a HugeTLB PT walk down to PAGE_SIZE, we
> > need to separately keep track of the location of the PMD so that we
> > can use it to get the PMD lock.
> 
> I assume Muchun was talking about changing this in current code (before
> your changes) where huge_page_size(h) can not be PAGE_SIZE.
>

Yes, that's what I meant.

Thanks.