linux-kernel - Re: [RFC PATCH v4 7/8] hugetlb: create hugetlb_unmap_file

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Date:   Fri, 29 Jul 2022 11:11:30 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...e.com>, Peter Xu <peterx@...hat.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Prakash Sangappa <prakash.sangappa@...cle.com>,
        James Houghton <jthoughton@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Ray Fucillo <Ray.Fucillo@...ersystems.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH v4 7/8] hugetlb: create hugetlb_unmap_file_folio to
 unmap single file folio

On 07/29/22 10:02, Miaohe Lin wrote:
> On 2022/7/7 4:23, Mike Kravetz wrote:
> > Create the new routine hugetlb_unmap_file_folio that will unmap a single
> > file folio.  This is refactored code from hugetlb_vmdelete_list.  It is
> > modified to do locking within the routine itself and check whether the
> > page is mapped within a specific vma before unmapping.
> > 
> > This refactoring will be put to use and expanded upon in a subsequent
> > patch adding vma specific locking.
> > 
> > Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
> > ---
> >  fs/hugetlbfs/inode.c | 124 +++++++++++++++++++++++++++++++++----------
> >  1 file changed, 95 insertions(+), 29 deletions(-)
> > 
> > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> > index 31bd4325fce5..0eac0ea2a245 100644
> > --- a/fs/hugetlbfs/inode.c
> > +++ b/fs/hugetlbfs/inode.c
> > @@ -396,6 +396,94 @@ static int hugetlbfs_write_end(struct file *file, struct address_space *mapping,
> >  	return -EINVAL;
> >  }
> >  
> > +/*
> > + * Called with i_mmap_rwsem held for inode based vma maps.  This makes
> > + * sure vma (and vm_mm) will not go away.  We also hold the hugetlb fault
> > + * mutex for the page in the mapping.  So, we can not race with page being
> > + * faulted into the vma.
> > + */
> > +static bool hugetlb_vma_maps_page(struct vm_area_struct *vma,
> > +				unsigned long addr, struct page *page)
> > +{
> > +	pte_t *ptep, pte;
> > +
> > +	ptep = huge_pte_offset(vma->vm_mm, addr,
> > +			huge_page_size(hstate_vma(vma)));
> > +
> > +	if (!ptep)
> > +		return false;
> > +
> > +	pte = huge_ptep_get(ptep);
> > +	if (huge_pte_none(pte) || !pte_present(pte))
> > +		return false;
> > +
> > +	if (pte_page(pte) == page)
> > +		return true;
> > +
> > +	return false;	/* WTH??? */
> 
> I'm sorry but what does WTH means? IIUC, this could happen if pte_page is a COW-ed private page?
> vma_interval_tree_foreach doesn't exclude the private mapping even after cow?

My apologies, I left that comment in during development and should have removed
it.  WTH is an acronym for 'What the Heck?".  I added it because I did not
think we should ever get to this return statement.

I am not sure if your COW of a private page would get us to this return
statement.  In any case, if we get there we need to return false.

Thank you for your analysis and comments!
-- 
Mike Kravetz