Re: [RFC PATCH v1 36/57] xen: Remove PAGE_SIZE compile-time constant assumption

Stefano Stabellini
2024-10-25T01:26:58Z

On Thu, 24 Oct 2024, Ryan Roberts wrote:
> On 23/10/2024 02:23, Stefano Stabellini wrote:
> > +Julien
> >
> > On Wed, 16 Oct 2024, Ryan Roberts wrote:
> >> + Juergen Gross, Stefano Stabellini
> >>
> >> This was a rather tricky series to get the recipients correct for and my script
> >> did not realize that "supporter" was a pseudonym for "maintainer" so you were
> >> missed off the original post. Appologies!
> >>
> >> More context in cover letter:
> >> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> >>
> >>
> >> On 14/10/2024 11:58, Ryan Roberts wrote:
> >>> To prepare for supporting boot-time page size selection, refactor code
> >>> to remove assumptions about PAGE_SIZE being compile-time constant. Code
> >>> intended to be equivalent when compile-time page size is active.
> >>>
> >>> Allocate enough "frame_list" static storage in the balloon driver for
> >>> the maximum supported page size. Although continue to use only the first
> >>> PAGE_SIZE of the buffer at run-time to maintain existing behaviour.
> >>>
> >>> Refactor xen_biovec_phys_mergeable() to convert ifdeffery to c if/else.
> >>> For compile-time page size, the compiler will choose one branch and
> >>> strip the dead one. For boot-time, it can be evaluated at run time.
> >>>
> >>> Refactor a BUILD_BUG_ON to evaluate the limit (when the minimum
> >>> supported page size is selected at boot-time).
> >>>
> >>> Reserve enough storage for max page size in "struct remap_data" and
> >>> "struct xenbus_map_node".
> >>>
> >>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> >>> ---
> >>>
> >>> ***NOTE***
> >>> Any confused maintainers may want to read the cover note here for context:
> >>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
> >>>
> >>> drivers/xen/balloon.c | 11 ++++++-----
> >>> drivers/xen/biomerge.c | 12 ++++++------
> >>> drivers/xen/privcmd.c | 2 +-
> >>> drivers/xen/xenbus/xenbus_client.c | 5 +++--
> >>> drivers/xen/xlate_mmu.c | 6 +++---
> >>> include/xen/page.h | 2 ++
> >>> 6 files changed, 21 insertions(+), 17 deletions(-)
> >>>
> >>> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> >>> index 528395133b4f8..0ed5f6453af0e 100644
> >>> --- a/drivers/xen/balloon.c
> >>> +++ b/drivers/xen/balloon.c
> >>> @@ -131,7 +131,8 @@ struct balloon_stats balloon_stats;
> >>> EXPORT_SYMBOL_GPL(balloon_stats);
> >>>
> >>> /* We increase/decrease in batches which fit in a page */
> >>> -static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];
> >>> +static xen_pfn_t frame_list[PAGE_SIZE_MAX / sizeof(xen_pfn_t)];
> >>> +#define FRAME_LIST_NR_ENTRIES (PAGE_SIZE / sizeof(xen_pfn_t))
> >>>
> >>>
> >>> /* List of ballooned pages, threaded through the mem_map array. */
> >>> @@ -389,8 +390,8 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
> >>> unsigned long i;
> >>> struct page *page;
> >>>
> >>> - if (nr_pages > ARRAY_SIZE(frame_list))
> >>> - nr_pages = ARRAY_SIZE(frame_list);
> >>> + if (nr_pages > FRAME_LIST_NR_ENTRIES)
> >>> + nr_pages = FRAME_LIST_NR_ENTRIES;
> >>>
> >>> page = list_first_entry_or_null(&ballooned_pages, struct page, lru);
> >>> for (i = 0; i < nr_pages; i++) {
> >>> @@ -434,8 +435,8 @@ static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
> >>> int ret;
> >>> LIST_HEAD(pages);
> >>>
> >>> - if (nr_pages > ARRAY_SIZE(frame_list))
> >>> - nr_pages = ARRAY_SIZE(frame_list);
> >>> + if (nr_pages > FRAME_LIST_NR_ENTRIES)
> >>> + nr_pages = FRAME_LIST_NR_ENTRIES;
> >>>
> >>> for (i = 0; i < nr_pages; i++) {
> >>> page = alloc_page(gfp);
> >>> diff --git a/drivers/xen/biomerge.c b/drivers/xen/biomerge.c
> >>> index 05a286d24f148..28f0887e40026 100644
> >>> --- a/drivers/xen/biomerge.c
> >>> +++ b/drivers/xen/biomerge.c
> >>> @@ -8,16 +8,16 @@
> >>> bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
> >>> const struct page *page)
> >>> {
> >>> -#if XEN_PAGE_SIZE == PAGE_SIZE
> >>> - unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
> >>> - unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
> >>> + if (XEN_PAGE_SIZE == PAGE_SIZE) {
> >>> + unsigned long bfn1 = pfn_to_bfn(page_to_pfn(vec1->bv_page));
> >>> + unsigned long bfn2 = pfn_to_bfn(page_to_pfn(page));
> >>> +
> >>> + return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
> >>> + }
> >>>
> >>> - return bfn1 + PFN_DOWN(vec1->bv_offset + vec1->bv_len) == bfn2;
> >>> -#else
> >>> /*
> >>> * XXX: Add support for merging bio_vec when using different page
> >>> * size in Xen and Linux.
> >>> */
> >>> return false;
> >>> -#endif
> >>> }
> >>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> >>> index 9563650dfbafc..847f7b806caf7 100644
> >>> --- a/drivers/xen/privcmd.c
> >>> +++ b/drivers/xen/privcmd.c
> >>> @@ -557,7 +557,7 @@ static long privcmd_ioctl_mmap_batch(
> >>> state.global_error = 0;
> >>> state.version = version;
> >>>
> >>> - BUILD_BUG_ON(((PAGE_SIZE / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE) != 0);
> >>> + BUILD_BUG_ON(((PAGE_SIZE_MIN / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE_MAX) != 0);
> >
> > Is there any value in keep this test? And if so, what should it look
> > like? I think we should turn it into a WARN_ON:
> >
> > WARN_ON(((PAGE_SIZE / sizeof(xen_pfn_t)) % XEN_PFN_PER_PAGE) != 0);
> >
> > It doesn't make much sense having a BUILD_BUG_ON on a variable that can
> > change?
>
> I believe that as long as we assume sizeof(xen_pfn_t), PAGE_SIZE and
> XEN_PAGE_SIZE are all power-of-two sizes, then this single build-time test
> should cover all possible boot-time PAGE_SIZEs.
>
> Logic:
>
> If PAGE_SIZE and XEN_PAGE_SIZE are power-of-two, then XEN_PFN_PER_PAGE must also
> be power-of-two. XEN_PFN_PER_PAGE_MAX is just the worst case limit.
>
> (PAGE_SIZE_MIN / sizeof(xen_pfn_t)) is the number of xen_pfn_t that fit on
> smallest page.
>
> If you can get an integer multiple number of XEN_PFN_PER_PAGE_MAX on the
> smallest page, then it remains an integer multiple as PAGE_SIZE gets bigger,
> assuming it is restricted to power-of-two sizes.
>
> Perhaps there is a floor in my logic?
>
> I'd prefer to keep BUILD_BUG_ON where possible to avoid the additional image
> size bloat and runtime costs.

You are right. It would be nice to add a in-code comment to explain
this.

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

> >>> /* mmap_batch_fn guarantees ret == 0 */
> >>> BUG_ON(traverse_pages_block(m.num, sizeof(xen_pfn_t),
> >>> &pagelist, mmap_batch_fn, &state));
> >>> diff --git a/drivers/xen/xenbus/xenbus_client.c b/drivers/xen/xenbus/xenbus_client.c
> >>> index 51b3124b0d56c..99bde836c10c4 100644
> >>> --- a/drivers/xen/xenbus/xenbus_client.c
> >>> +++ b/drivers/xen/xenbus/xenbus_client.c
> >>> @@ -49,9 +49,10 @@
> >>>
> >>> #include "xenbus.h"
> >>>
> >>> -#define XENBUS_PAGES(_grants) (DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE))
> >>> +#define XENBUS_PAGES(_grants) (DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE))
> >>> +#define XENBUS_PAGES_MAX(_grants) (DIV_ROUND_UP(_grants, XEN_PFN_PER_PAGE_MIN))
> >>>
> >>> -#define XENBUS_MAX_RING_PAGES (XENBUS_PAGES(XENBUS_MAX_RING_GRANTS))
> >>> +#define XENBUS_MAX_RING_PAGES (XENBUS_PAGES_MAX(XENBUS_MAX_RING_GRANTS))
> >>>
> >>> struct xenbus_map_node {
> >>> struct list_head next;
> >>> diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
> >>> index f17c4c03db30c..a757c801a7542 100644
> >>> --- a/drivers/xen/xlate_mmu.c
> >>> +++ b/drivers/xen/xlate_mmu.c
> >>> @@ -74,9 +74,9 @@ struct remap_data {
> >>> int mapped;
> >>>
> >>> /* Hypercall parameters */
> >>> - int h_errs[XEN_PFN_PER_PAGE];
> >>> - xen_ulong_t h_idxs[XEN_PFN_PER_PAGE];
> >>> - xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE];
> >>> + int h_errs[XEN_PFN_PER_PAGE_MAX];
> >>> + xen_ulong_t h_idxs[XEN_PFN_PER_PAGE_MAX];
> >>> + xen_pfn_t h_gpfns[XEN_PFN_PER_PAGE_MAX];
> >>>
> >>> int h_iter; /* Iterator */
> >>> };
> >>> diff --git a/include/xen/page.h b/include/xen/page.h
> >>> index 285677b42943a..86683a30038a3 100644
> >>> --- a/include/xen/page.h
> >>> +++ b/include/xen/page.h
> >>> @@ -21,6 +21,8 @@
> >>> ((page_to_pfn(page)) << (PAGE_SHIFT - XEN_PAGE_SHIFT))
> >>>
> >>> #define XEN_PFN_PER_PAGE (PAGE_SIZE / XEN_PAGE_SIZE)
> >>> +#define XEN_PFN_PER_PAGE_MIN (PAGE_SIZE_MIN / XEN_PAGE_SIZE)
> >>> +#define XEN_PFN_PER_PAGE_MAX (PAGE_SIZE_MAX / XEN_PAGE_SIZE)
> >>>
> >>> #define XEN_PFN_DOWN(x) ((x) >> XEN_PAGE_SHIFT)
> >>> #define XEN_PFN_UP(x) (((x) + XEN_PAGE_SIZE-1) >> XEN_PAGE_SHIFT)
> >>
>