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    SubjectRe: [External] Re: [PATCH v9 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page
    On Thu, Dec 17, 2020 at 6:52 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
    >
    > On 12/16/20 2:25 PM, Oscar Salvador wrote:
    > > On Wed, Dec 16, 2020 at 02:08:30PM -0800, Mike Kravetz wrote:
    > >>> + * vmemmap_rmap_walk - walk vmemmap page table
    > >>> +
    > >>> +static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
    > >>> + unsigned long end, struct vmemmap_rmap_walk *walk)
    > >>> +{
    > >>> + pte_t *pte;
    > >>> +
    > >>> + pte = pte_offset_kernel(pmd, addr);
    > >>> + do {
    > >>> + BUG_ON(pte_none(*pte));
    > >>> +
    > >>> + if (!walk->reuse)
    > >>> + walk->reuse = pte_page(pte[VMEMMAP_TAIL_PAGE_REUSE]);
    > >>
    > >> It may be just me, but I don't like the pte[-1] here. It certainly does work
    > >> as designed because we want to remap all pages in the range to the page before
    > >> the range (at offset -1). But, we do not really validate this 'reuse' page.
    > >> There is the BUG_ON(pte_none(*pte)) as a sanity check, but we do nothing similar
    > >> for pte[-1]. Based on the usage for HugeTLB pages, we can be confident that
    > >> pte[-1] is actually a pte. In discussions with Oscar, you mentioned another
    > >> possible use for these routines.
    > >
    > > Without giving it much of a thought, I guess we could duplicate the
    > > BUG_ON for the pte outside the loop, and add a new one for pte[-1].
    > > Also, since walk->reuse seems to not change once it is set, we can take
    > > it outside the loop? e.g:
    > >
    > > pte *pte;
    > >
    > > pte = pte_offset_kernel(pmd, addr);
    > > BUG_ON(pte_none(*pte));
    > > BUG_ON(pte_none(pte[VMEMMAP_TAIL_PAGE_REUSE]));
    > > walk->reuse = pte_page(pte[VMEMMAP_TAIL_PAGE_REUSE]);
    > > do {
    > > ....
    > > } while...
    > >
    > > Or I am not sure whether we want to keep it inside the loop in case
    > > future cases change walk->reuse during the operation.
    > > But to be honest, I do not think it is realistic of all future possible
    > > uses of this, so I would rather keep it simple for now.
    >
    > I was thinking about possibly passing the 'reuse' address as another parameter
    > to vmemmap_remap_reuse(). We could add this addr to the vmemmap_rmap_walk
    > struct and set walk->reuse when we get to the pte for that address. Of
    > course this would imply that the addr would need to be part of the range.

    Maybe adding another one parameter is unnecessary. How about doing
    this in the vmemmap_remap_reuse?

    The 'reuse' address just is start + PAGE_SIZE.

    void vmemmap_remap_free(unsigned long start, unsigned long size)
    {
    unsigned long end = start + size;
    unsigned long reuse_addr = start + PAGE_SIZE;
    LIST_HEAD(vmemmap_pages);

    struct vmemmap_remap_walk walk = {
    .remap_pte = vmemmap_remap_pte,
    .vmemmap_pages = &vmemmap_pages,
    .reuse_addr = reuse_addr.
    };

    }

    >
    > Ideally, we would walk the page table to get to the reuse page. My concern
    > was not explicitly about adding the BUG_ON. In more general use, *pte could
    > be the first entry on a pte page. And, then pte[-1] may not even be a pte.
    >
    > Again, I don't think this matters for the current HugeTLB use case. Just a
    > little concerned if code is put to use for other purposes.
    > --
    > Mike Kravetz



    --
    Yours,
    Muchun

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