Re: [Xen-devel] [PATCH 2/8] xen/balloon: Move common memory reservation routines to a module

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 05/30/2018 06:54 PM, Boris Ostrovsky wrote:
> On 05/30/2018 04:29 AM, Oleksandr Andrushchenko wrote:
> > On 05/29/2018 11:03 PM, Boris Ostrovsky wrote:
> > > On 05/29/2018 02:22 PM, Oleksandr Andrushchenko wrote:
> > > > On 05/29/2018 09:04 PM, Boris Ostrovsky wrote:
> > > > > On 05/25/2018 11:33 AM, Oleksandr Andrushchenko wrote:
> > > > > @@ -463,11 +457,6 @@ static enum bp_state
> > > > > increase_reservation(unsigned long nr_pages)
> > > > >         int rc;
> > > > >         unsigned long i;
> > > > >         struct page   *page;
> > > > > -    struct xen_memory_reservation reservation = {
> > > > > -        .address_bits = 0,
> > > > > -        .extent_order = EXTENT_ORDER,
> > > > > -        .domid        = DOMID_SELF
> > > > > -    };
> > > > >           if (nr_pages > ARRAY_SIZE(frame_list))
> > > > >             nr_pages = ARRAY_SIZE(frame_list);
> > > > > @@ -486,9 +475,7 @@ static enum bp_state
> > > > > increase_reservation(unsigned long nr_pages)
> > > > >             page = balloon_next_page(page);
> > > > >         }
> > > > >     -    set_xen_guest_handle(reservation.extent_start, frame_list);
> > > > > -    reservation.nr_extents = nr_pages;
> > > > > -    rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
> > > > > +    rc = xenmem_reservation_increase(nr_pages, frame_list);
> > > > >         if (rc <= 0)
> > > > >             return BP_EAGAIN;
> > > > >     @@ -496,29 +483,7 @@ static enum bp_state
> > > > > increase_reservation(unsigned long nr_pages)
> > > > >             page = balloon_retrieve(false);
> > > > >             BUG_ON(page == NULL);
> > > > >     -#ifdef CONFIG_XEN_HAVE_PVMMU
> > > > > -        /*
> > > > > -         * We don't support PV MMU when Linux and Xen is using
> > > > > -         * different page granularity.
> > > > > -         */
> > > > > -        BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);
> > > > > -
> > > > > -        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> > > > > -            unsigned long pfn = page_to_pfn(page);
> > > > > -
> > > > > -            set_phys_to_machine(pfn, frame_list[i]);
> > > > > -
> > > > > -            /* Link back into the page tables if not highmem. */
> > > > > -            if (!PageHighMem(page)) {
> > > > > -                int ret;
> > > > > -                ret = HYPERVISOR_update_va_mapping(
> > > > > -                        (unsigned long)__va(pfn << PAGE_SHIFT),
> > > > > -                        mfn_pte(frame_list[i], PAGE_KERNEL),
> > > > > -                        0);
> > > > > -                BUG_ON(ret);
> > > > > -            }
> > > > > -        }
> > > > > -#endif
> > > > > +        xenmem_reservation_va_mapping_update(1, &page,
> > > > > &frame_list[i]);
> > > > >
> > > > > Can you make a single call to xenmem_reservation_va_mapping_update(rc,
> > > > > ...)? You need to keep track of pages but presumable they can be put
> > > > > into an array (or a list). In fact, perhaps we can have
> > > > > balloon_retrieve() return a set of pages.
> > > > This is actually how it is used later on for dma-buf, but I just
> > > > didn't want
> > > > to alter original balloon code too much, but this can be done, in
> > > > order of simplicity:
> > > >
> > > > 1. Similar to frame_list, e.g. static array of struct page* of size
> > > > ARRAY_SIZE(frame_list):
> > > > more static memory is used, but no allocations
> > > >
> > > > 2. Allocated at run-time with kcalloc: allocation can fail
> > > If this is called in freeing DMA buffer code path or in error path then
> > > we shouldn't do it.
> > >
> > >
> > > > 3. Make balloon_retrieve() return a set of pages: will require
> > > > list/array allocation
> > > > and handling, allocation may fail, balloon_retrieve prototype change
> > > balloon pages are strung on the lru list. Can we keep have
> > > balloon_retrieve return a list of pages on that list?
> > First of all, before we go deep in details, I will highlight
> > the goal of the requested change: for balloon driver we call
> > xenmem_reservation_va_mapping_update(*1*, &page, &frame_list[i]);
> > from increase_reservation
> > and
> > xenmem_reservation_va_mapping_reset(*1*, &page);
> > from decrease_reservation and it seems to be not elegant because of
> > that one page/frame passed while we might have multiple pages/frames
> > passed at once.
> >
> > In the balloon driver the producer of pages for increase_reservation
> > is balloon_retrieve(false) and for decrease_reservation it is
> > alloc_page(gfp).
> > In case of decrease_reservation the page is added on the list:
> > LIST_HEAD(pages);
> > [...]
> > list_add(&page->lru, &pages);
> >
> > and in case of increase_reservation it is retrieved page by page
> > and can be put on a list as well with the same code from
> > decrease_reservation, e.g.
> > LIST_HEAD(pages);
> > [...]
> > list_add(&page->lru, &pages);
> >
> > Thus, both decrease_reservation and increase_reservation may hold
> > their pages on a list before calling
> > xenmem_reservation_va_mapping_{update|reset}.
> >
> > For that we need a prototype change:
> > xenmem_reservation_va_mapping_reset(<nr_pages>, <list of pages>);
> > But for xenmem_reservation_va_mapping_update it will look like:
> > xenmem_reservation_va_mapping_update(<nr_pages>, <list of pages>,
> > <array of frames>)
> > which seems to be inconsistent. Converting entries of the static
> > frame_list array
> > into corresponding list doesn't seem to be cute as well.
> >
> > For dma-buf use-case arrays are more preferable as dma-buf constructs
> > scatter-gather
> > tables from array of pages etc. and if page list is passed then it
> > needs to be
> > converted into page array anyways.
> >
> > So, we can:
> > 1. Keep the prototypes as is, e.g. accept array of pages and use
> > nr_pages == 1 in
> > case of balloon driver (existing code)
> > 2. Statically allocate struct page* array in the balloon driver and
> > fill it with pages
> > when those pages are retrieved:
> > static struct page *page_list[ARRAY_SIZE(frame_list)];
> > which will take additional 8KiB of space on 64-bit platform, but
> > simplify things a lot.
> > 3. Allocate struct page *page_list[ARRAY_SIZE(frame_list)] dynamically
> >
> > As to Boris' suggestion "balloon pages are strung on the lru list. Can
> > we keep have
> > balloon_retrieve return a list of pages on that list?"
> > Because of alloc_xenballooned_pages' retry logic for page retireval, e.g.
> >      while (pgno < nr_pages) {
> >          page = balloon_retrieve(true);
> >          if (page) {
> > [...]
> >          } else {
> >              ret = add_ballooned_pages(nr_pages - pgno);
> > [...]
> >      }
> > I wouldn't change things that much.
> >
> > IMO, we can keep 1 page based API with the only overhead for balloon
> > driver of
> > function calls to xenmem_reservation_va_mapping_{update|reset} for
> > each page.
>
> I still think what I suggested is doable but we can come back to it
> later and keep your per-page implementation for now.
>
> BTW, I also think you can further simplify
> xenmem_reservation_va_mapping_* routines by bailing out right away if
> xen_feature(XENFEAT_auto_translated_physmap). In fact, you might even
> make them inlines, along the lines of
>
> inline void xenmem_reservation_va_mapping_reset(unsigned long count,
>                                          struct page **pages)
> {
> #ifdef CONFIG_XEN_HAVE_PVMMU
>         if (!xen_feature(XENFEAT_auto_translated_physmap))
>                 __xenmem_reservation_va_mapping_reset(...)
> #endif
> }
How about:

#ifdef CONFIG_XEN_HAVE_PVMMU
static inline __xenmem_reservation_va_mapping_reset(struct page *page)
{
[...]
}
#endif

and

void xenmem_reservation_va_mapping_reset(unsigned long count,
                     struct page **pages)
{
#ifdef CONFIG_XEN_HAVE_PVMMU
    if (!xen_feature(XENFEAT_auto_translated_physmap)) {
        int i;

        for (i = 0; i < count; i++)
            __xenmem_reservation_va_mapping_reset(pages[i]);
    }
#endif
}

This way I can use __xenmem_reservation_va_mapping_reset(page);
instead of xenmem_reservation_va_mapping_reset(1, &page);
> Or some such.
>
> -boris
>
> -boris

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