[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <Y4eAJuZRG0CLP7PW@x1n>
Date: Wed, 30 Nov 2022 11:09:10 -0500
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
James Houghton <jthoughton@...gle.com>,
Jann Horn <jannh@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Andrea Arcangeli <aarcange@...hat.com>,
Rik van Riel <riel@...riel.com>,
Nadav Amit <nadav.amit@...il.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Muchun Song <songmuchun@...edance.com>,
Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 03/10] mm/hugetlb: Document huge_pte_offset usage
On Wed, Nov 30, 2022 at 11:24:34AM +0100, David Hildenbrand wrote:
> On 29.11.22 20:35, Peter Xu wrote:
> > huge_pte_offset() is potentially a pgtable walker, looking up pte_t* for a
> > hugetlb address.
> >
> > Normally, it's always safe to walk a generic pgtable as long as we're with
> > the mmap lock held for either read or write, because that guarantees the
> > pgtable pages will always be valid during the process.
> >
> > But it's not true for hugetlbfs, especially shared: hugetlbfs can have its
> > pgtable freed by pmd unsharing, it means that even with mmap lock held for
> > current mm, the PMD pgtable page can still go away from under us if pmd
> > unsharing is possible during the walk.
> >
> > So we have two ways to make it safe even for a shared mapping:
> >
> > (1) If we're with the hugetlb vma lock held for either read/write, it's
> > okay because pmd unshare cannot happen at all.
> >
> > (2) If we're with the i_mmap_rwsem lock held for either read/write, it's
> > okay because even if pmd unshare can happen, the pgtable page cannot
> > be freed from under us.
> >
> > Document it.
> >
> > Signed-off-by: Peter Xu <peterx@...hat.com>
> > ---
> > include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++
> > 1 file changed, 32 insertions(+)
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index 551834cd5299..81efd9b9baa2 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -192,6 +192,38 @@ extern struct list_head huge_boot_pages;
> > pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> > unsigned long addr, unsigned long sz);
> > +/*
> > + * huge_pte_offset(): Walk the hugetlb pgtable until the last level PTE.
> > + * Returns the pte_t* if found, or NULL if the address is not mapped.
> > + *
> > + * Since this function will walk all the pgtable pages (including not only
> > + * high-level pgtable page, but also PUD entry that can be unshared
> > + * concurrently for VM_SHARED), the caller of this function should be
> > + * responsible of its thread safety. One can follow this rule:
> > + *
> > + * (1) For private mappings: pmd unsharing is not possible, so it'll
> > + * always be safe if we're with the mmap sem for either read or write.
> > + * This is normally always the case, IOW we don't need to do anything
> > + * special.
>
> Maybe worth mentioning that hugetlb_vma_lock_read() and friends already
> optimize for private mappings, to not take the VMA lock if not required.
Yes we can. I assume this is not super urgent so I'll hold a while to see
whether there's anything else that needs amending for the documents.
Btw, even with hugetlb_vma_lock_read() checking SHARED for a private only
code path it's still better to not take the lock at all, because that still
contains a function jump which will be unnecesary.
Thanks,
--
Peter Xu
Powered by blists - more mailing lists