efi: Reduce the amount of memblock reservations for persistent allocations

The current implementation of efi_mem_reserve_persistent() is rather
naive, in the sense that for each invocation, it creates a separate
linked list entry to describe the reservation. Since the linked list
entries themselves need to persist across subsequent kexec reboots,
every reservation created this way results in two memblock_reserve()
calls at the next boot.

On arm64 systems with 100s of CPUs, this may result in a excessive
number of memblock reservations, and needless fragmentation.

So instead, make use of the newly updated struct linux_efi_memreserve
layout to put multiple reservations into a single linked list entry.
This should get rid of the numerous tiny memblock reservations, and
effectively cut the total number of reservations in half on arm64
systems with many CPUs.

 [ mingo: build warning fix. ]
Tested-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arend van Spriel <arend.vanspriel@broadcom.com>
Cc: Bhupesh Sharma <bhsharma@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Joe Perches <joe@perches.com>
Cc: Jon Hunter <jonathanh@nvidia.com>
Cc: Julien Thierry <julien.thierry@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Nathan Chancellor <natechancellor@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: YiFei Zhu <zhuyifei1999@gmail.com>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20181129171230.18699-11-ard.biesheuvel@linaro.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

drivers/firmware/efi/efi.c

@@ -997,8 +997,8 @@ static int __init efi_memreserve_map_root(void)
 int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 {
 	struct linux_efi_memreserve *rsv;
-	int rsvsize = EFI_MEMRESERVE_SIZE(1);
-	int rc;
+	unsigned long prsv;
+	int rc, index;
 
 	if (efi_memreserve_root == (void *)ULONG_MAX)
 		return -ENODEV;
@@ -1009,11 +1009,24 @@ int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 			return rc;
 	}
 
-	rsv = kmalloc(rsvsize, GFP_ATOMIC);
+	/* first try to find a slot in an existing linked list entry */
+	for (prsv = efi_memreserve_root->next; prsv; prsv = rsv->next) {
+		rsv = __va(prsv);
+		index = atomic_fetch_add_unless(&rsv->count, 1, rsv->size);
+		if (index < rsv->size) {
+			rsv->entry[index].base = addr;
+			rsv->entry[index].size = size;
+
+			return 0;
+		}
+	}
+
+	/* no slot found - allocate a new linked list entry */
+	rsv = (struct linux_efi_memreserve *)__get_free_page(GFP_ATOMIC);
 	if (!rsv)
 		return -ENOMEM;
 
-	rsv->size = 1;
+	rsv->size = EFI_MEMRESERVE_COUNT(PAGE_SIZE);
 	atomic_set(&rsv->count, 1);
 	rsv->entry[0].base = addr;
 	rsv->entry[0].size = size;

include/linux/efi.h

@@ -1724,4 +1724,7 @@ struct linux_efi_memreserve {
 #define EFI_MEMRESERVE_SIZE(count) (sizeof(struct linux_efi_memreserve) + \
 	(count) * sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
 
+#define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \
+	/ sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
+
 #endif /* _LINUX_EFI_H */