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

[David Hildenbrand]

On 30.11.22 17:09, Peter Xu wrote:
> 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@xxxxxxxxxx>
> > > ---
> > >    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.

IMHO it makes coding a lot more consistent and less error-prone when not 
care about whether to the the lock or not (as an optimization) and just 
having this handled "automatically".

Optimizing a jump out would rather smell like a micro-optimization.

--
Thanks,

David / dhildenb