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

Stefano Stabellini
2024-10-23T01:25:36Z

+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?

> > /* 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)
>