Add max_background setting to S3FilesystemConfig (#1746)

Add `max_background` setting to `S3FilesystemConfig`.

### Does this change impact existing behavior?

No.

### Does this change need a changelog entry? Does it require a version
change?

Yes, new version of `mountpoint-s3-fs`.

---

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and I agree to the terms of
the [Developer Certificate of Origin
(DCO)](https://developercertificate.org/).

---------

Signed-off-by: Vlad Volodkin <vlaad@amazon.co.uk>
Co-authored-by: Vlad Volodkin <vlaad@amazon.co.uk>

Author: vladem

mountpoint-s3-fs/examples/mount_from_config.rs

@@ -85,6 +85,9 @@ struct ConfigOptions {
 
     /// Disk cache configuration
     disk_cache: Option<DiskCacheConfig>,
+
+    /// Limits the number of concurrent FUSE requests that the kernel may send, default: 64
+    max_background_fuse_requests: Option<u16>,
 }
 
 impl ConfigOptions {
@@ -103,6 +106,7 @@ impl ConfigOptions {
     fn build_filesystem_config(&self) -> Result<S3FilesystemConfig> {
         let mut fs_config = S3FilesystemConfig {
             cache_config: CacheConfig::new(mountpoint_s3_fs::fs::TimeToLive::Indefinite),
+            max_background_fuse_requests: self.max_background_fuse_requests,
             ..Default::default()
         };
 

mountpoint-s3-fs/src/fs.rs

@@ -177,27 +177,15 @@ where
         self.next_handle.fetch_add(1, Ordering::SeqCst)
     }
 
-    /// Helper to return the u16 value in an environment variable, or panic.  Useful for unstable overrides.
-    fn parse_env_var_to_u16(var_name: &str, var_value: std::ffi::OsString) -> u16 {
-        var_value
-            .to_string_lossy()
-            .parse::<u16>()
-            .unwrap_or_else(|_| panic!("Invalid value for environment variable {var_name}. Must be positive integer."))
-    }
-
     pub async fn init(&self, config: &mut KernelConfig) -> Result<(), libc::c_int> {
-        const ENV_VAR_KEY_MAX_BACKGROUND: &str = "UNSTABLE_MOUNTPOINT_MAX_BACKGROUND";
-        const ENV_VAR_KEY_CONGESTION_THRESHOLD: &str = "UNSTABLE_MOUNTPOINT_CONGESTION_THRESHOLD";
         let _ = config.add_capabilities(fuser::consts::FUSE_DO_READDIRPLUS);
-        // Set max_background FUSE parameter to 64 by default, or override with environment variable.
-        // NOTE: Support for this environment variable may be removed in future without notice.
-        if let Some(user_max_background) = std::env::var_os(ENV_VAR_KEY_MAX_BACKGROUND) {
-            let max_background = Self::parse_env_var_to_u16(ENV_VAR_KEY_MAX_BACKGROUND, user_max_background);
+        // Set max_background FUSE parameter to 64 by default, may be overriden with config setting or by an environment variable.
+        if let Some(max_background) = self.config.max_background_fuse_requests() {
             let old = config
                 .set_max_background(max_background)
                 .unwrap_or_else(|_| panic!("Unable to set FUSE max_background configuration to {max_background}"));
-            tracing::warn!(
-                "Successfully overridden FUSE max_background configuration to {} (was {}) from unstable environment variable.",
+            tracing::info!(
+                "Successfully set FUSE max_background configuration to {} (was {}).",
                 max_background,
                 old
             );
@@ -213,14 +201,11 @@ where
         }
 
         // Override FUSE congestion threshold if environment variable is present.
-        // NOTE: Support for this environment variable may be removed in future without notice.
-        if let Some(user_congestion_threshold) = std::env::var_os(ENV_VAR_KEY_CONGESTION_THRESHOLD) {
-            let congestion_threshold =
-                Self::parse_env_var_to_u16(ENV_VAR_KEY_CONGESTION_THRESHOLD, user_congestion_threshold);
+        if let Some(congestion_threshold) = self.config.fuse_congestion_threshold() {
             let old = config
                 .set_congestion_threshold(congestion_threshold)
                 .unwrap_or_else(|_| panic!("unable to set FUSE congestion_threshold to {congestion_threshold}"));
-            tracing::warn!(
+            tracing::info!(
                 "Successfully overridden FUSE congestion_threshold configuration to {} (was {}) from unstable environment variable.",
                 congestion_threshold,
                 old

mountpoint-s3-fs/src/fs/config.rs

@@ -43,6 +43,10 @@ pub struct S3FilesystemConfig {
     pub mem_limit: u64,
     /// Prefetcher configuration
     pub prefetcher_config: PrefetcherConfig,
+    /// Limits the number of concurrent FUSE requests that the kernel may send. Default is 64.
+    /// This option may also be configured by `UNSTABLE_MOUNTPOINT_MAX_BACKGROUND` environment variable,
+    /// but the value specified in the config takes priority.
+    pub max_background_fuse_requests: Option<u16>,
 }
 
 impl Default for S3FilesystemConfig {
@@ -67,6 +71,7 @@ impl Default for S3FilesystemConfig {
             use_upload_checksums: true,
             mem_limit: MINIMUM_MEM_LIMIT,
             prefetcher_config: Default::default(),
+            max_background_fuse_requests: None,
         }
     }
 }
@@ -78,6 +83,35 @@ impl S3FilesystemConfig {
             incremental_upload: self.incremental_upload,
         }
     }
+
+    pub fn max_background_fuse_requests(&self) -> Option<u16> {
+        // NOTE: Support for this environment variable may be removed in future without notice.
+        const ENV_VAR_KEY_MAX_BACKGROUND: &str = "UNSTABLE_MOUNTPOINT_MAX_BACKGROUND";
+        if self.max_background_fuse_requests.is_some() {
+            self.max_background_fuse_requests
+        } else if let Some(user_max_background) = std::env::var_os(ENV_VAR_KEY_MAX_BACKGROUND) {
+            let max_background = Self::parse_env_var_to_u16(ENV_VAR_KEY_MAX_BACKGROUND, user_max_background);
+            Some(max_background)
+        } else {
+            None
+        }
+    }
+
+    pub fn fuse_congestion_threshold(&self) -> Option<u16> {
+        // NOTE: Support for this environment variable may be removed in future without notice.
+        const ENV_VAR_KEY_CONGESTION_THRESHOLD: &str = "UNSTABLE_MOUNTPOINT_CONGESTION_THRESHOLD";
+        std::env::var_os(ENV_VAR_KEY_CONGESTION_THRESHOLD).map(|user_congestion_threshold| {
+            Self::parse_env_var_to_u16(ENV_VAR_KEY_CONGESTION_THRESHOLD, user_congestion_threshold)
+        })
+    }
+
+    /// Helper to return the u16 value in an environment variable, or panic.  Useful for unstable overrides.
+    fn parse_env_var_to_u16(var_name: &str, var_value: std::ffi::OsString) -> u16 {
+        var_value
+            .to_string_lossy()
+            .parse::<u16>()
+            .unwrap_or_else(|_| panic!("Invalid value for environment variable {var_name}. Must be positive integer."))
+    }
 }
 
 #[derive(Debug, Clone)]