[PATCH v2 1/4] fs/pipe: move the prealloc pool to per-pipe infrastructure

[Breno Leitao <leitao@debian.org>]

Move struct anon_pipe_prealloc and PIPE_PREALLOC_MAX to pipe_fs_i.h and
embed the pool in each pipe via a new prealloc field, next to the existing
tmp_page[2] cache (which will be removed by the end of this patchset).
Add PIPE_PREALLOC_KEEP for the post-write trim target.

PIPE_PREALLOC_KEEP (2) is the number of pages that will be buffered in
the pipe (similar to struct page *tmp_page[2]), which will be removed
soon.

The on-stack prealloc pool used by anon_pipe_write() is unchanged; this
only adds the per-pipe storage that later patches switch the read/write
paths over to.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 fs/pipe.c                 |  7 -------
 include/linux/pipe_fs_i.h | 19 +++++++++++++++++++
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 429b0714ec575..325fd9757dbdd 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -111,13 +111,6 @@ void pipe_double_lock(struct pipe_inode_info *pipe1,
 	pipe_lock(pipe2);
 }
 
-#define PIPE_PREALLOC_MAX 8
-
-struct anon_pipe_prealloc {
-	struct page *pages[PIPE_PREALLOC_MAX];
-	unsigned int count;
-};
-
 /*
  * Pre-allocate pages outside pipe->mutex for multi-page writes.
  * alloc_page() with GFP_HIGHUSER can sleep in reclaim and runs memcg
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index a1eeed8006694..796860cbddf30 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -13,6 +13,9 @@
 #define PIPE_BUF_FLAG_LOSS	0x40	/* Message loss happened after this buffer */
 #endif
 
+#define PIPE_PREALLOC_MAX	8	/* max pages in prealloc pool */
+#define PIPE_PREALLOC_KEEP	2	/* keep at least this many after trim */
+
 /**
  *	struct pipe_buffer - a linux kernel pipe buffer
  *	@page: the page containing the data for the pipe buffer
@@ -57,6 +60,20 @@ union pipe_index {
 	};
 };
 
+/**
+ *	struct anon_pipe_prealloc - per-pipe page preallocation pool
+ *	@pages: array of cached pages (pool)
+ *	@count: number of pages currently in the pool
+ *
+ * Each pipe keeps a small bounded pool of preallocated pages to reduce
+ * allocation overhead during writes. The pool is bounded at PIPE_PREALLOC_MAX
+ * and trimmed down to PIPE_PREALLOC_KEEP after a write completes.
+ */
+struct anon_pipe_prealloc {
+	struct page *pages[PIPE_PREALLOC_MAX];
+	unsigned int count;
+};
+
 /**
  *	struct pipe_inode_info - a linux kernel pipe
  *	@mutex: mutex protecting the whole thing
@@ -68,6 +85,7 @@ union pipe_index {
  *	@ring_size: total number of buffers (should be a power of 2)
  *	@nr_accounted: The amount this pipe accounts for in user->pipe_bufs
  *	@tmp_page: cached released page
+ *	@prealloc: per-pipe page preallocation pool
  *	@readers: number of current readers of this pipe
  *	@writers: number of current writers of this pipe
  *	@files: number of struct file referring this pipe (protected by ->i_lock)
@@ -99,6 +117,7 @@ struct pipe_inode_info {
 	bool note_loss;
 #endif
 	struct page *tmp_page[2];
+	struct anon_pipe_prealloc prealloc;
 	struct fasync_struct *fasync_readers;
 	struct fasync_struct *fasync_writers;
 	struct pipe_buffer *bufs;

-- 
2.53.0-Meta