[PATCH 1/5] fs: add read support for RWF_UNCACHED

[Jens Axboe <axboe@kernel.dk>]

If RWF_UNCACHED is set for io_uring (or preadv2(2)), we'll drop the
cache for buffered reads if we are the ones instantiating it. If the
data is already cached, we leave it cached.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 include/linux/fs.h      |  3 +++
 include/uapi/linux/fs.h |  5 ++++-
 mm/filemap.c            | 46 ++++++++++++++++++++++++++++++++++++-----
 3 files changed, 48 insertions(+), 6 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 98e0349adb52..092ea2a4319b 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -314,6 +314,7 @@ enum rw_hint {
 #define IOCB_SYNC		(1 << 5)
 #define IOCB_WRITE		(1 << 6)
 #define IOCB_NOWAIT		(1 << 7)
+#define IOCB_UNCACHED		(1 << 8)
 
 struct kiocb {
 	struct file		*ki_filp;
@@ -3418,6 +3419,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
 		ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC);
 	if (flags & RWF_APPEND)
 		ki->ki_flags |= IOCB_APPEND;
+	if (flags & RWF_UNCACHED)
+		ki->ki_flags |= IOCB_UNCACHED;
 	return 0;
 }
 
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 379a612f8f1d..357ebb0e0c5d 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -299,8 +299,11 @@ typedef int __bitwise __kernel_rwf_t;
 /* per-IO O_APPEND */
 #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
 
+/* drop cache after reading or writing data */
+#define RWF_UNCACHED	((__force __kernel_rwf_t)0x00000040)
+
 /* mask of flags supported by the kernel */
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
-			 RWF_APPEND)
+			 RWF_APPEND | RWF_UNCACHED)
 
 #endif /* _UAPI_LINUX_FS_H */
diff --git a/mm/filemap.c b/mm/filemap.c
index bf6aa30be58d..ed23a11b3e34 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -933,8 +933,8 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
 }
 EXPORT_SYMBOL(add_to_page_cache_locked);
 
-int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
-				pgoff_t offset, gfp_t gfp_mask)
+static int __add_to_page_cache(struct page *page, struct address_space *mapping,
+			       pgoff_t offset, gfp_t gfp_mask, bool lru)
 {
 	void *shadow = NULL;
 	int ret;
@@ -956,9 +956,17 @@ int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
 		WARN_ON_ONCE(PageActive(page));
 		if (!(gfp_mask & __GFP_WRITE) && shadow)
 			workingset_refault(page, shadow);
-		lru_cache_add(page);
+		if (lru)
+			lru_cache_add(page);
 	}
 	return ret;
+
+}
+
+int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
+				pgoff_t offset, gfp_t gfp_mask)
+{
+	return __add_to_page_cache(page, mapping, offset, gfp_mask, true);
 }
 EXPORT_SYMBOL_GPL(add_to_page_cache_lru);
 
@@ -2032,6 +2040,7 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	offset = *ppos & ~PAGE_MASK;
 
 	for (;;) {
+		bool drop_page = false;
 		struct page *page;
 		pgoff_t end_index;
 		loff_t isize;
@@ -2048,6 +2057,9 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		if (!page) {
 			if (iocb->ki_flags & IOCB_NOWAIT)
 				goto would_block;
+			/* UNCACHED implies no read-ahead */
+			if (iocb->ki_flags & IOCB_UNCACHED)
+				goto no_cached_page;
 			page_cache_sync_readahead(mapping,
 					ra, filp,
 					index, last_index - index);
@@ -2147,6 +2159,26 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 		offset &= ~PAGE_MASK;
 		prev_offset = offset;
 
+		/*
+		 * If we're dropping this page due to drop-behind, then
+		 * lock it first. Ignore errors here, we can just leave it
+		 * in the page cache. Note that we didn't add this page to
+		 * the LRU when we added it to the page cache. So if we
+		 * fail removing it, or lock it, add to the LRU.
+		 */
+		if (drop_page) {
+			bool addlru = true;
+
+			if (!lock_page_killable(page)) {
+				if (page->mapping == mapping)
+					addlru = !remove_mapping(mapping, page);
+				else
+					addlru = false;
+				unlock_page(page);
+			}
+			if (addlru)
+				lru_cache_add(page);
+		}
 		put_page(page);
 		written += ret;
 		if (!iov_iter_count(iter))
@@ -2234,8 +2266,12 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
 			error = -ENOMEM;
 			goto out;
 		}
-		error = add_to_page_cache_lru(page, mapping, index,
-				mapping_gfp_constraint(mapping, GFP_KERNEL));
+		if (iocb->ki_flags & IOCB_UNCACHED)
+			drop_page = true;
+
+		error = __add_to_page_cache(page, mapping, index,
+				mapping_gfp_constraint(mapping, GFP_KERNEL),
+				!drop_page);
 		if (error) {
 			put_page(page);
 			if (error == -EEXIST) {
-- 
2.24.0