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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>
 <6ca7f428-eede-2c14-85fe-da4a20bcea0d@gmail.com>
 <5dd3378d-ac32-691e-1f80-7218a5d07fd6@oracle.com>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <43c17501-8865-6e1f-1a92-d947755d8fa8@gmail.com>
Date:   Wed, 30 May 2018 20:46:00 +0300
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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
>
>