Messages in this thread | | | Subject | Re: [PATCH 2/8] xen/balloon: Move common memory reservation routines to a module | From | Oleksandr Andrushchenko <> | 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? >> >>>
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