[PATCH v4 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

Mike Rapoport
2020-08-18T14:17:34Z

From: Mike Rapoport <rppt@linux.ibm.com>

Removing a PAGE_SIZE page from the direct map every time such page is
allocated for a secret memory mapping will cause severe fragmentation of
the direct map. This fragmentation can be reduced by using PMD-size pages
as a pool for small pages for secret memory mappings.

Add a gen_pool per secretmem inode and lazily populate this pool with
PMD-size pages.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
mm/secretmem.c | 107 ++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 88 insertions(+), 19 deletions(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 3293f761076e..333eb18fb483 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c
@@ -12,6 +12,7 @@
#include <linux/bitops.h>
#include <linux/printk.h>
#include <linux/pagemap.h>
+#include <linux/genalloc.h>
#include <linux/syscalls.h>
#include <linux/pseudo_fs.h>
#include <linux/set_memory.h>
@@ -40,24 +41,66 @@
#define SECRETMEM_FLAGS_MASK SECRETMEM_MODE_MASK

struct secretmem_ctx {
+ struct gen_pool *pool;
unsigned int mode;
};

-static struct page *secretmem_alloc_page(gfp_t gfp)
+static int secretmem_pool_increase(struct secretmem_ctx *ctx, gfp_t gfp)
{
- /*
- * FIXME: use a cache of large pages to reduce the direct map
- * fragmentation
- */
- return alloc_page(gfp);
+ unsigned long nr_pages = (1 << PMD_PAGE_ORDER);
+ struct gen_pool *pool = ctx->pool;
+ unsigned long addr;
+ struct page *page;
+ int err;
+
+ page = alloc_pages(gfp, PMD_PAGE_ORDER);
+ if (!page)
+ return -ENOMEM;
+
+ addr = (unsigned long)page_address(page);
+ split_page(page, PMD_PAGE_ORDER);
+
+ err = gen_pool_add(pool, addr, PMD_SIZE, NUMA_NO_NODE);
+ if (err) {
+ __free_pages(page, PMD_PAGE_ORDER);
+ return err;
+ }
+
+ __kernel_map_pages(page, nr_pages, 0);
+
+ return 0;
+}
+
+static struct page *secretmem_alloc_page(struct secretmem_ctx *ctx,
+ gfp_t gfp)
+{
+ struct gen_pool *pool = ctx->pool;
+ unsigned long addr;
+ struct page *page;
+ int err;
+
+ if (gen_pool_avail(pool) < PAGE_SIZE) {
+ err = secretmem_pool_increase(ctx, gfp);
+ if (err)
+ return NULL;
+ }
+
+ addr = gen_pool_alloc(pool, PAGE_SIZE);
+ if (!addr)
+ return NULL;
+
+ page = virt_to_page(addr);
+ get_page(page);
+
+ return page;
}

static vm_fault_t secretmem_fault(struct vm_fault *vmf)
{
+ struct secretmem_ctx *ctx = vmf->vma->vm_file->private_data;
struct address_space *mapping = vmf->vma->vm_file->f_mapping;
struct inode *inode = file_inode(vmf->vma->vm_file);
pgoff_t offset = vmf->pgoff;
- unsigned long addr;
struct page *page;
int ret = 0;

@@ -66,7 +109,7 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)

page = find_get_entry(mapping, offset);
if (!page) {
- page = secretmem_alloc_page(vmf->gfp_mask);
+ page = secretmem_alloc_page(ctx, vmf->gfp_mask);
if (!page)
return vmf_error(-ENOMEM);

@@ -74,14 +117,8 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
if (unlikely(ret))
goto err_put_page;

- ret = set_direct_map_invalid_noflush(page);
- if (ret)
- goto err_del_page_cache;
-
- addr = (unsigned long)page_address(page);
- flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-
__SetPageUptodate(page);
+ set_page_private(page, (unsigned long)ctx);

ret = VM_FAULT_LOCKED;
}
@@ -89,8 +126,6 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
vmf->page = page;
return ret;

-err_del_page_cache:
- delete_from_page_cache(page);
err_put_page:
put_page(page);
return vmf_error(ret);
@@ -138,7 +173,11 @@ static int secretmem_migratepage(struct address_space *mapping,

static void secretmem_freepage(struct page *page)
{
- set_direct_map_default_noflush(page);
+ unsigned long addr = (unsigned long)page_address(page);
+ struct secretmem_ctx *ctx = (struct secretmem_ctx *)page_private(page);
+ struct gen_pool *pool = ctx->pool;
+
+ gen_pool_free(pool, addr, PAGE_SIZE);
}

static const struct address_space_operations secretmem_aops = {
@@ -163,13 +202,18 @@ static struct file *secretmem_file_create(unsigned long flags)
if (!ctx)
goto err_free_inode;

+ ctx->pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
+ if (!ctx->pool)
+ goto err_free_ctx;
+
file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
O_RDWR, &secretmem_fops);
if (IS_ERR(file))
- goto err_free_ctx;
+ goto err_free_pool;

mapping_set_unevictable(inode->i_mapping);

+ inode->i_private = ctx;
inode->i_mapping->private_data = ctx;
inode->i_mapping->a_ops = &secretmem_aops;

@@ -183,6 +227,8 @@ static struct file *secretmem_file_create(unsigned long flags)

return file;

+err_free_pool:
+ gen_pool_destroy(ctx->pool);
err_free_ctx:
kfree(ctx);
err_free_inode:
@@ -221,11 +267,34 @@ SYSCALL_DEFINE1(memfd_secret, unsigned long, flags)
return err;
}

+static void secretmem_cleanup_chunk(struct gen_pool *pool,
+ struct gen_pool_chunk *chunk, void *data)
+{
+ unsigned long start = chunk->start_addr;
+ unsigned long end = chunk->end_addr;
+ unsigned long nr_pages, addr;
+
+ nr_pages = (end - start + 1) / PAGE_SIZE;
+ __kernel_map_pages(virt_to_page(start), nr_pages, 1);
+
+ for (addr = start; addr < end; addr += PAGE_SIZE)
+ put_page(virt_to_page(addr));
+}
+
+static void secretmem_cleanup_pool(struct secretmem_ctx *ctx)
+{
+ struct gen_pool *pool = ctx->pool;
+
+ gen_pool_for_each_chunk(pool, secretmem_cleanup_chunk, ctx);
+ gen_pool_destroy(pool);
+}
+
static void secretmem_evict_inode(struct inode *inode)
{
struct secretmem_ctx *ctx = inode->i_private;

truncate_inode_pages_final(&inode->i_data);
+ secretmem_cleanup_pool(ctx);
clear_inode(inode);
kfree(ctx);
}
--
2.26.2