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SubjectRe: [Xen-devel] [PATCH v3] xen/grant-table: Avoid m2p_override during mapping
On Mon, 20 Jan 2014, Zoltan Kiss wrote:
> The grant mapping API does m2p_override unnecessarily: only gntdev needs it,
> for blkback and future netback patches it just cause a lock contention, as
> those pages never go to userspace. Therefore this series does the following:
> - the original functions were renamed to __gnttab_[un]map_refs, with a new
> parameter m2p_override
> - based on m2p_override either they follow the original behaviour, or just set
> the private flag and call set_phys_to_machine
> - gnttab_[un]map_refs are now a wrapper to call __gnttab_[un]map_refs with
> m2p_override false
> - a new function gnttab_[un]map_refs_userspace provides the old behaviour
>
> v2:
> - move the storing of the old mfn in page->index to gnttab_map_refs
> - move the function header update to a separate patch
>
> v3:
> - a new approach to retain old behaviour where it needed
> - squash the patches into one
>
> Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
> Suggested-by: David Vrabel <david.vrabel@citrix.com>
> ---
> drivers/block/xen-blkback/blkback.c | 15 +++----
> drivers/xen/gntdev.c | 13 +++---
> drivers/xen/grant-table.c | 81 +++++++++++++++++++++++++++++------
> include/xen/grant_table.h | 8 +++-
> 4 files changed, 87 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index aa846a4..87ded60 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -880,15 +880,17 @@ void gnttab_batch_copy(struct gnttab_copy *batch, unsigned count)
> }
> EXPORT_SYMBOL_GPL(gnttab_batch_copy);
>
> -int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> +int __gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> struct gnttab_map_grant_ref *kmap_ops,
> - struct page **pages, unsigned int count)
> + struct page **pages, unsigned int count,
> + bool m2p_override)
> {
> int i, ret;
> bool lazy = false;
> pte_t *pte;
> unsigned long mfn;
>
> + BUG_ON(kmap_ops && !m2p_override);
> ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, map_ops, count);
> if (ret)
> return ret;
> @@ -907,10 +909,12 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> set_phys_to_machine(map_ops[i].host_addr >> PAGE_SHIFT,
> map_ops[i].dev_bus_addr >> PAGE_SHIFT);
> }
> - return ret;
> + return 0;
> }
>
> - if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> + if (m2p_override &&
> + !in_interrupt() &&
> + paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> arch_enter_lazy_mmu_mode();
> lazy = true;
> }
> @@ -927,8 +931,18 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> } else {
> mfn = PFN_DOWN(map_ops[i].dev_bus_addr);
> }
> - ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> - &kmap_ops[i] : NULL);
> + if (m2p_override)
> + ret = m2p_add_override(mfn, pages[i], kmap_ops ?
> + &kmap_ops[i] : NULL);
> + else {
> + unsigned long pfn = page_to_pfn(pages[i]);
> + WARN_ON(PagePrivate(pages[i]));
> + SetPagePrivate(pages[i]);
> + set_page_private(pages[i], mfn);
> + pages[i]->index = pfn_to_mfn(pfn);
> + if (unlikely(!set_phys_to_machine(pfn, FOREIGN_FRAME(mfn))))
> + return -ENOMEM;

What happens if the page is PageHighMem?

This looks like a subset of m2p_add_override, but it is missing some
relevant bits, like the PageHighMem check, or the p2m(m2p(mfn)) == mfn
check. Maybe we can find a way to avoid duplicating the code.
We could split m2p_add_override in two functions or add yet another
parameter to it.


> + }
> if (ret)
> goto out;
> }
> @@ -937,17 +951,33 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> if (lazy)
> arch_leave_lazy_mmu_mode();
>
> - return ret;
> + return 0;
> +}
> +
> +int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
> + struct page **pages, unsigned int count)
> +{
> + return __gnttab_map_refs(map_ops, NULL, pages, count, false);
> }
> EXPORT_SYMBOL_GPL(gnttab_map_refs);
>
> -int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> +int gnttab_map_refs_userspace(struct gnttab_map_grant_ref *map_ops,
> + struct gnttab_map_grant_ref *kmap_ops,
> + struct page **pages, unsigned int count)
> +{
> + return __gnttab_map_refs(map_ops, kmap_ops, pages, count, true);
> +}
> +EXPORT_SYMBOL_GPL(gnttab_map_refs_userspace);
> +
> +int __gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> struct gnttab_map_grant_ref *kmap_ops,
> - struct page **pages, unsigned int count)
> + struct page **pages, unsigned int count,
> + bool m2p_override)
> {
> int i, ret;
> bool lazy = false;
>
> + BUG_ON(kmap_ops && !m2p_override);
> ret = HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, unmap_ops, count);
> if (ret)
> return ret;
> @@ -958,17 +988,26 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> set_phys_to_machine(unmap_ops[i].host_addr >> PAGE_SHIFT,
> INVALID_P2M_ENTRY);
> }
> - return ret;
> + return 0;
> }
>
> - if (!in_interrupt() && paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> + if (m2p_override &&
> + !in_interrupt() &&
> + paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
> arch_enter_lazy_mmu_mode();
> lazy = true;
> }
>
> for (i = 0; i < count; i++) {
> - ret = m2p_remove_override(pages[i], kmap_ops ?
> - &kmap_ops[i] : NULL);
> + if (m2p_override)
> + ret = m2p_remove_override(pages[i], kmap_ops ?
> + &kmap_ops[i] : NULL);
> + else {
> + unsigned long pfn = page_to_pfn(pages[i]);
> + WARN_ON(!PagePrivate(pages[i]));
> + ClearPagePrivate(pages[i]);
> + set_phys_to_machine(pfn, pages[i]->index);

same here: let's try to avoid code duplication


> + }
> if (ret)
> goto out;
> }
> @@ -977,10 +1016,24 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
> if (lazy)
> arch_leave_lazy_mmu_mode();
>
> - return ret;
> + return 0;
> +}
> +
> +int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *map_ops,
> + struct page **pages, unsigned int count)
> +{
> + return __gnttab_unmap_refs(map_ops, NULL, pages, count, false);
> }
> EXPORT_SYMBOL_GPL(gnttab_unmap_refs);
>
> +int gnttab_unmap_refs_userspace(struct gnttab_unmap_grant_ref *map_ops,
> + struct gnttab_map_grant_ref *kmap_ops,
> + struct page **pages, unsigned int count)
> +{
> + return __gnttab_unmap_refs(map_ops, kmap_ops, pages, count, true);
> +}
> +EXPORT_SYMBOL_GPL(gnttab_unmap_refs_userspace);
> +
> static unsigned nr_status_frames(unsigned nr_grant_frames)
> {
> BUG_ON(grefs_per_grant_frame == 0);


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