Re: [PATCH v2 03/10] mm/hugetlb: Document huge_pte_offset usage

[David Hildenbrand]

On 07.12.22 21:49, John Hubbard wrote:
> On 12/7/22 12:30, 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@xxxxxxxxxx>
> > ---
> >    include/linux/hugetlb.h | 32 ++++++++++++++++++++++++++++++++
> >    1 file changed, 32 insertions(+)
>
> Looks good, with a couple of minor wording tweaks below that you might
> consider folding in, but either way,
>
> Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>
>
> > 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:
>
>        "responsible for"
>
> > + *
> > + *  (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.
>
> mmap sem is sooo two years ago! :)
>
> > + *      This is normally always the case, IOW we don't need to do anything
>
> "normally always" hurts my sense of logic. And "IOW" is for typing very quickly
> in chats or email, not for long term documentation that is written rarely
> and read many times.
>
> > + *      special.
>
> So putting all that together, maybe:
>
>    *  (1) For private mappings: pmd unsharing is not possible, so holding the
>    *      mmap_lock for either read or write is sufficient. Most callers already
>    *      hold the mmap_lock, so normally, no special action is required.

With that,

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb