[RFC PATCH v2 15/18] mm/filemap: don't store f_ra.prev_pos for FMODE_RANDOM files

["Kiryl Shutsemau (Meta)" <kas@kernel.org>]

filemap_read() writes the end of every read to file->f_ra.prev_pos, in
the cache line that fdget() and fdput() write when threads share the
file table.

Applications that read a file at random offsets from many threads, such
as RocksDB, set POSIX_FADV_RANDOM on it.  Readahead then never reads
prev_pos: page_cache_sync_ra() forces the read first.

Skip the store for such files after a read that filemap_read_rcu() did
in full: without folio references, the file's line is what the threads
contend on.  Keep it on the slow path, which takes folio references:
skipping it there made reads slower in my testing.

filemap_read() compares the start of a read with prev_pos, to tell a
read that goes on in the same folio from another access to it.  On an
FMODE_RANDOM file it can now be stale; with many threads on one file it
was another thread's read already.

Signed-off-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
---
 mm/filemap.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 182ae7d5db56..5998de3d6697 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3078,8 +3078,13 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 	if (fast_read) {
 		already_read += fast_read;
 		last_pos = iocb->ki_pos;
-		if (!iov_iter_count(iter))
-			goto out;
+		if (!iov_iter_count(iter)) {
+			file_accessed(filp);
+			/* Readahead of a random file ignores it: skip the shared store */
+			if (!(filp->f_mode & FMODE_RANDOM))
+				ra->prev_pos = last_pos;
+			return already_read;
+		}
 	}
 
 	do {
@@ -3161,7 +3166,6 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
 		}
 	} while (iov_iter_count(iter) && iocb->ki_pos < isize && !error);
 
-out:
 	file_accessed(filp);
 	ra->prev_pos = last_pos;
 	return already_read ? already_read : error;
-- 
2.54.0