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Subject: Re: [PATCH 2/8] xen/balloon: Move common memory reservation routines
 to a module
To:     Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        jgross@suse.com, konrad.wilk@oracle.com
Cc:     daniel.vetter@intel.com, dongwon.kim@intel.com,
        matthew.d.roper@intel.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <20180525153331.31188-1-andr2000@gmail.com>
 <20180525153331.31188-3-andr2000@gmail.com>
 <59ab73b0-967b-a82f-3b0d-95f1b0dc40a5@oracle.com>
 <89de7bdb-8759-419f-63bf-8ed0d57650f0@gmail.com>
 <edfa937b-3311-98db-2e6f-b4083598f796@oracle.com>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <6ca7f428-eede-2c14-85fe-da4a20bcea0d@gmail.com>
Date:   Wed, 30 May 2018 11:29:48 +0300
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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?
>>
>>>