Re: [External] Re: [PATCH v9 03/11] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

[Muchun Song]

On Thu, Dec 17, 2020 at 6:52 AM Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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