virtiofsd: Add -o allow_direct_io|no_allow_direct_io options

Due to the commit 65da4539803373ec4eec97ffc49ee90083e56efd, the O_DIRECT
open flag of guest applications will be discarded by virtiofsd. While
this behavior makes it consistent with the virtio-9p scheme when guest
applications use direct I/O, we no longer have any chance to bypass the
host page cache.

Therefore, we add a flag 'allow_direct_io' to lo_data. If '-o
 no_allow_direct_io' option is added, or none of '-o allow_direct_io' or
 '-o no_allow_direct_io' is added, the 'allow_direct_io' will be set to
 0, and virtiofsd discards O_DIRECT as before. If '-o allow_direct_io'
is added to the starting command-line, 'allow_direct_io' will be set to
1, so that the O_DIRECT flags will be retained and host page cache can
be bypassed.

Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200824105957.61265-1-zhangjiachen.jaycee@bytedance.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

2 files changed, 18 insertions, 6 deletions

diff --git a/tools/virtiofsd/helper.c b/tools/virtiofsd/helper.c
index 7bc5d7dc5a..85770d63f1 100644
--- a/tools/virtiofsd/helper.c
+++ b/tools/virtiofsd/helper.c
@@ -178,6 +178,10 @@ void fuse_cmdline_help(void)
            "                               (0 leaves rlimit unchanged)\n"
            "                               default: min(1000000, fs.file-max - 16384)\n"
            "                                        if the current rlimit is lower\n"
+           "    -o allow_direct_io|no_allow_direct_io\n"
+           "                               retain/discard O_DIRECT flags passed down\n"
+           "                               to virtiofsd from guest applications.\n"
+           "                               default: no_allow_direct_io\n"
            );
 }
 
diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
index 784330e0e4..0b229ebd57 100644
--- a/tools/virtiofsd/passthrough_ll.c
+++ b/tools/virtiofsd/passthrough_ll.c
@@ -151,6 +151,7 @@ struct lo_data {
     int timeout_set;
     int readdirplus_set;
     int readdirplus_clear;
+    int allow_direct_io;
     struct lo_inode root;
     GHashTable *inodes; /* protected by lo->mutex */
     struct lo_map ino_map; /* protected by lo->mutex */
@@ -179,6 +180,8 @@ static const struct fuse_opt lo_opts[] = {
     { "cache=always", offsetof(struct lo_data, cache), CACHE_ALWAYS },
     { "readdirplus", offsetof(struct lo_data, readdirplus_set), 1 },
     { "no_readdirplus", offsetof(struct lo_data, readdirplus_clear), 1 },
+    { "allow_direct_io", offsetof(struct lo_data, allow_direct_io), 1 },
+    { "no_allow_direct_io", offsetof(struct lo_data, allow_direct_io), 0 },
     FUSE_OPT_END
 };
 static bool use_syslog = false;
@@ -1516,7 +1519,8 @@ static void lo_releasedir(fuse_req_t req, fuse_ino_t ino,
     fuse_reply_err(req, 0);
 }
 
-static void update_open_flags(int writeback, struct fuse_file_info *fi)
+static void update_open_flags(int writeback, int allow_direct_io,
+                              struct fuse_file_info *fi)
 {
     /*
      * With writeback cache, kernel may send read requests even
@@ -1541,10 +1545,13 @@ static void update_open_flags(int writeback, struct fuse_file_info *fi)
 
     /*
      * O_DIRECT in guest should not necessarily mean bypassing page
-     * cache on host as well. If somebody needs that behavior, it
-     * probably should be a configuration knob in daemon.
+     * cache on host as well. Therefore, we discard it by default
+     * ('-o no_allow_direct_io'). If somebody needs that behavior,
+     * the '-o allow_direct_io' option should be set.
      */
-    fi->flags &= ~O_DIRECT;
+    if (!allow_direct_io) {
+        fi->flags &= ~O_DIRECT;
+    }
 }
 
 static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
@@ -1576,7 +1583,7 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name,
         goto out;
     }
 
-    update_open_flags(lo->writeback, fi);
+    update_open_flags(lo->writeback, lo->allow_direct_io, fi);
 
     fd = openat(parent_inode->fd, name, (fi->flags | O_CREAT) & ~O_NOFOLLOW,
                 mode);
@@ -1786,7 +1793,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi)
     fuse_log(FUSE_LOG_DEBUG, "lo_open(ino=%" PRIu64 ", flags=%d)\n", ino,
              fi->flags);
 
-    update_open_flags(lo->writeback, fi);
+    update_open_flags(lo->writeback, lo->allow_direct_io, fi);
 
     sprintf(buf, "%i", lo_fd(req, ino));
     fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
@@ -2823,6 +2830,7 @@ int main(int argc, char *argv[])
         .debug = 0,
         .writeback = 0,
         .posix_lock = 0,
+        .allow_direct_io = 0,
         .proc_self_fd = -1,
     };
     struct lo_map_elem *root_elem;