[PATCH mm-unstable v2 2/2] mm: support POSIX_FADV_NOREUSE

[Yu Zhao]

This patch adds POSIX_FADV_NOREUSE to vma_has_recency() so that the
LRU algorithm can ignore access to mapped files marked by this flag.

The advantages of POSIX_FADV_NOREUSE are:
1. Unlike MADV_SEQUENTIAL and MADV_RANDOM, it does not alter the
   default readahead behavior.
2. Unlike MADV_SEQUENTIAL and MADV_RANDOM, it does not split VMAs and
   therefore does not take mmap_lock.
3. Unlike MADV_COLD, setting it has a negligible cost, regardless of
   how many pages it affects.

Its limitations are:
1. Like POSIX_FADV_RANDOM and POSIX_FADV_SEQUENTIAL, it currently does
   not support range. IOW, its scope is the entire file.
2. It currently does not ignore access through file descriptors.
   Specifically, for the active/inactive LRU, given a file page shared
   by two users and one of them having set POSIX_FADV_NOREUSE on the
   file, this page will be activated upon the second user accessing
   it. This corner case can be covered by checking POSIX_FADV_NOREUSE
   before calling folio_mark_accessed() on the read path. But it is
   considered not worth the effort.

There have been a few attempts to support POSIX_FADV_NOREUSE, e.g.,
[1]. This time the goal is to fill a niche: a few desktop
applications, e.g., large file transferring and video
encoding/decoding, want fast file streaming with mmap() rather than
direct IO. Among those applications, an SVT-AV1 regression was
reported when running with MGLRU [2]. The following test can reproduce
that regression.

  kb=$(awk '/MemTotal/ { print $2 }' /proc/meminfo)
  kb=$((kb - 8*1024*1024))

  modprobe brd rd_nr=1 rd_size=$kb
  dd if=/dev/zero of=/dev/ram0 bs=1M

  mkfs.ext4 /dev/ram0
  mount /dev/ram0 /mnt/
  swapoff -a

  fallocate -l 8G /mnt/swapfile
  mkswap /mnt/swapfile
  swapon /mnt/swapfile

  wget http://ultravideo.cs.tut.fi/video/Bosphorus_3840x2160_120fps_420_8bit_YUV_Y4M.7z
  7z e -o/mnt/ Bosphorus_3840x2160_120fps_420_8bit_YUV_Y4M.7z
  SvtAv1EncApp --preset 12 -w 3840 -h 2160 \
               -i /mnt/Bosphorus_3840x2160.y4m

For MGLRU, the following change showed a [9-11]% increase in FPS,
which makes it on par with the active/inactive LRU.

  patch Source/App/EncApp/EbAppMain.c <<EOF
  31a32
  > #include <fcntl.h>
  35d35
  < #include <fcntl.h> /* _O_BINARY */
  117a118
  >             posix_fadvise(config->mmap.fd, 0, 0, POSIX_FADV_NOREUSE);
  EOF

[1] https://lore.kernel.org/r/1308923350-7932-1-git-send-email-andrea@xxxxxxxxxxxxxxx/
[2] https://openbenchmarking.org/result/2209259-PTS-MGLRU8GB57

Signed-off-by: Yu Zhao <yuzhao@xxxxxxxxxx>
---
 include/linux/fs.h        | 2 ++
 include/linux/mm_inline.h | 3 +++
 mm/fadvise.c              | 5 ++++-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 066555ad1bf8..5660ed0edf1a 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -166,6 +166,8 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File supports DIRECT IO */
 #define	FMODE_CAN_ODIRECT	((__force fmode_t)0x400000)
 
+#define	FMODE_NOREUSE		((__force fmode_t)0x800000)
+
 /* File was opened by fanotify and shouldn't generate fanotify events */
 #define FMODE_NONOTIFY		((__force fmode_t)0x4000000)
 
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index fe5b8449e14a..064f92c78bfa 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -600,6 +600,9 @@ static inline bool vma_has_recency(struct vm_area_struct *vma)
 	if (vma->vm_flags & (VM_SEQ_READ | VM_RAND_READ))
 		return false;
 
+	if (vma->vm_file && (vma->vm_file->f_mode & FMODE_NOREUSE))
+		return false;
+
 	return true;
 }
 
diff --git a/mm/fadvise.c b/mm/fadvise.c
index bf04fec87f35..fb7c5f43fd2a 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -80,7 +80,7 @@ int generic_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 	case POSIX_FADV_NORMAL:
 		file->f_ra.ra_pages = bdi->ra_pages;
 		spin_lock(&file->f_lock);
-		file->f_mode &= ~FMODE_RANDOM;
+		file->f_mode &= ~(FMODE_RANDOM | FMODE_NOREUSE);
 		spin_unlock(&file->f_lock);
 		break;
 	case POSIX_FADV_RANDOM:
@@ -107,6 +107,9 @@ int generic_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
 		force_page_cache_readahead(mapping, file, start_index, nrpages);
 		break;
 	case POSIX_FADV_NOREUSE:
+		spin_lock(&file->f_lock);
+		file->f_mode |= FMODE_NOREUSE;
+		spin_unlock(&file->f_lock);
 		break;
 	case POSIX_FADV_DONTNEED:
 		__filemap_fdatawrite_range(mapping, offset, endbyte,
-- 
2.39.0.314.g84b9a713c41-goog