[PATCH 1/2] io_uring: use kvmalloc for fixed files

[Pavel Begunkov <asml.silence@gmail.com>]

Currenlty we hand code two-level tables for storing registered files,
pointers that requires more memory loads and a bunch of bit logic in the
hot path.

I expect for most cases it would be enough for applications to allocate
a small enough amount of fixed files, that would fit into a couple of
contiguous pages (512 entries per page of x64, 1024 for 2 pages). So, in
most cases I'd expect kvmalloc to be able to allocate contiguous memory
with no drawbacks but performance improvement.

If it can't (depends, around >=8 pages?), it will do vmalloc, so the
outcome is not clear, whether it's better to have 2 memory loads plus a
bunch of instructions or a TLB assisted virtual memory lookup.

Considering that we limit it to 64 pages max, and it should benefit
without disadvantages most of the users, it's the right thing to have.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 fs/io_uring.c | 33 ++++++++++-----------------------
 1 file changed, 10 insertions(+), 23 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 3dfd52813bb6..2fd54a21ed8b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -90,13 +90,8 @@
 #define IORING_MAX_ENTRIES	32768
 #define IORING_MAX_CQ_ENTRIES	(2 * IORING_MAX_ENTRIES)
 
-/*
- * Shift of 9 is 512 entries, or exactly one page on 64-bit archs
- */
-#define IORING_FILE_TABLE_SHIFT	9
-#define IORING_MAX_FILES_TABLE	(1U << IORING_FILE_TABLE_SHIFT)
-#define IORING_FILE_TABLE_MASK	(IORING_MAX_FILES_TABLE - 1)
-#define IORING_MAX_FIXED_FILES	(64 * IORING_MAX_FILES_TABLE)
+/* 512 entries per page on 64-bit archs, 64 pages max */
+#define IORING_MAX_FIXED_FILES	(1U << 15)
 #define IORING_MAX_RESTRICTIONS	(IORING_RESTRICTION_LAST + \
 				 IORING_REGISTER_LAST + IORING_OP_LAST)
 
@@ -233,8 +228,7 @@ struct io_rsrc_put {
 };
 
 struct io_file_table {
-	/* two level table */
-	struct io_fixed_file **files;
+	struct io_fixed_file *files;
 };
 
 struct io_rsrc_node {
@@ -6320,12 +6314,9 @@ static void io_wq_submit_work(struct io_wq_work *work)
 #define FFS_MASK		~(FFS_ASYNC_READ|FFS_ASYNC_WRITE|FFS_ISREG)
 
 static inline struct io_fixed_file *io_fixed_file_slot(struct io_file_table *table,
-						      unsigned i)
+						       unsigned i)
 {
-	struct io_fixed_file *table_l2;
-
-	table_l2 = table->files[i >> IORING_FILE_TABLE_SHIFT];
-	return &table_l2[i & IORING_FILE_TABLE_MASK];
+	return &table->files[i];
 }
 
 static inline struct file *io_file_from_index(struct io_ring_ctx *ctx,
@@ -7255,17 +7246,13 @@ static int io_rsrc_data_alloc(struct io_ring_ctx *ctx, rsrc_put_fn *do_put,
 
 static bool io_alloc_file_tables(struct io_file_table *table, unsigned nr_files)
 {
-	size_t size = nr_files * sizeof(struct io_fixed_file);
-
-	table->files = (struct io_fixed_file **)io_alloc_page_table(size);
+	table->files = kvcalloc(nr_files, sizeof(table->files[0]), GFP_KERNEL);
 	return !!table->files;
 }
 
-static void io_free_file_tables(struct io_file_table *table, unsigned nr_files)
+static void io_free_file_tables(struct io_file_table *table)
 {
-	size_t size = nr_files * sizeof(struct io_fixed_file);
-
-	io_free_page_table((void **)table->files, size);
+	kvfree(table->files);
 	table->files = NULL;
 }
 
@@ -7290,7 +7277,7 @@ static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
 			fput(file);
 	}
 #endif
-	io_free_file_tables(&ctx->file_table, ctx->nr_user_files);
+	io_free_file_tables(&ctx->file_table);
 	io_rsrc_data_free(ctx->file_data);
 	ctx->file_data = NULL;
 	ctx->nr_user_files = 0;
@@ -7757,7 +7744,7 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
 		if (file)
 			fput(file);
 	}
-	io_free_file_tables(&ctx->file_table, nr_args);
+	io_free_file_tables(&ctx->file_table);
 	ctx->nr_user_files = 0;
 out_free:
 	io_rsrc_data_free(ctx->file_data);
-- 
2.31.1