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-rw-r--r--arch/x86/xen/p2m.c36
1 files changed, 36 insertions, 0 deletions
diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
index ffd08c414e91..64effdc6da94 100644
--- a/arch/x86/xen/p2m.c
+++ b/arch/x86/xen/p2m.c
@@ -706,6 +706,7 @@ int m2p_add_override(unsigned long mfn, struct page *page,
unsigned long uninitialized_var(address);
unsigned level;
pte_t *ptep = NULL;
+ int ret = 0;
pfn = page_to_pfn(page);
if (!PageHighMem(page)) {
@@ -741,6 +742,24 @@ int m2p_add_override(unsigned long mfn, struct page *page,
list_add(&page->lru, &m2p_overrides[mfn_hash(mfn)]);
spin_unlock_irqrestore(&m2p_override_lock, flags);
+ /* p2m(m2p(mfn)) == mfn: the mfn is already present somewhere in
+ * this domain. Set the FOREIGN_FRAME_BIT in the p2m for the other
+ * pfn so that the following mfn_to_pfn(mfn) calls will return the
+ * pfn from the m2p_override (the backend pfn) instead.
+ * We need to do this because the pages shared by the frontend
+ * (xen-blkfront) can be already locked (lock_page, called by
+ * do_read_cache_page); when the userspace backend tries to use them
+ * with direct_IO, mfn_to_pfn returns the pfn of the frontend, so
+ * do_blockdev_direct_IO is going to try to lock the same pages
+ * again resulting in a deadlock.
+ * As a side effect get_user_pages_fast might not be safe on the
+ * frontend pages while they are being shared with the backend,
+ * because mfn_to_pfn (that ends up being called by GUPF) will
+ * return the backend pfn rather than the frontend pfn. */
+ ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
+ if (ret == 0 && get_phys_to_machine(pfn) == mfn)
+ set_phys_to_machine(pfn, FOREIGN_FRAME(mfn));
+
return 0;
}
EXPORT_SYMBOL_GPL(m2p_add_override);
@@ -752,6 +771,7 @@ int m2p_remove_override(struct page *page, bool clear_pte)
unsigned long uninitialized_var(address);
unsigned level;
pte_t *ptep = NULL;
+ int ret = 0;
pfn = page_to_pfn(page);
mfn = get_phys_to_machine(pfn);
@@ -821,6 +841,22 @@ int m2p_remove_override(struct page *page, bool clear_pte)
} else
set_phys_to_machine(pfn, page->index);
+ /* p2m(m2p(mfn)) == FOREIGN_FRAME(mfn): the mfn is already present
+ * somewhere in this domain, even before being added to the
+ * m2p_override (see comment above in m2p_add_override).
+ * If there are no other entries in the m2p_override corresponding
+ * to this mfn, then remove the FOREIGN_FRAME_BIT from the p2m for
+ * the original pfn (the one shared by the frontend): the backend
+ * cannot do any IO on this page anymore because it has been
+ * unshared. Removing the FOREIGN_FRAME_BIT from the p2m entry of
+ * the original pfn causes mfn_to_pfn(mfn) to return the frontend
+ * pfn again. */
+ mfn &= ~FOREIGN_FRAME_BIT;
+ ret = __get_user(pfn, &machine_to_phys_mapping[mfn]);
+ if (ret == 0 && get_phys_to_machine(pfn) == FOREIGN_FRAME(mfn) &&
+ m2p_find_override(mfn) == NULL)
+ set_phys_to_machine(pfn, mfn);
+
return 0;
}
EXPORT_SYMBOL_GPL(m2p_remove_override);