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

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

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.
> -boris
Thank you,
Oleksandr
> > Could you please tell which of the above will fit better?
> >
> > >

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