[pbs-devel] [PATCH proxmox-backup 2/2] fix #5982: garbage collection: check atime updates are honored

[Christian Ebner]

Check if the filesystem the chunk store is located on actually
updates the atime when performing the marking of the chunks in
phase 1 of the garbage collection. Since it is not enough to check if
a single/first chunks atime is updated, since the filesystem can be
mounted via the `relatime` option, find the first chunk which is'
outside the relatime's 24 hour cutoff window and check the update on
that chunk only.

Allow to disable the atime update checks via the optional datastores
tuning parameter, but enable them by default.

Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5982
Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
---
 pbs-datastore/src/datastore.rs | 50 +++++++++++++++++++++++++++++++---
 1 file changed, 46 insertions(+), 4 deletions(-)

diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
index 75c0c16ab..9aa509e27 100644
--- a/pbs-datastore/src/datastore.rs
+++ b/pbs-datastore/src/datastore.rs
@@ -4,6 +4,7 @@ use std::os::unix::ffi::OsStrExt;
 use std::os::unix::io::AsRawFd;
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, LazyLock, Mutex};
+use std::time::UNIX_EPOCH;
 
 use anyhow::{bail, format_err, Error};
 use nix::unistd::{unlinkat, UnlinkatFlags};
@@ -1029,13 +1030,15 @@ impl DataStore {
         Ok(list)
     }
 
-    // mark chunks  used by ``index`` as used
+    // mark chunks  used by ``index`` as used,
+    // fail if the chunks atime is not updated as expected by the filesystem.
     fn index_mark_used_chunks<I: IndexFile>(
         &self,
         index: I,
         file_name: &Path, // only used for error reporting
         status: &mut GarbageCollectionStatus,
         worker: &dyn WorkerTaskContext,
+        min_atime: &mut Option<i64>,
     ) -> Result<(), Error> {
         status.index_file_count += 1;
         status.index_data_bytes += index.index_bytes();
@@ -1044,6 +1047,20 @@ impl DataStore {
             worker.check_abort()?;
             worker.fail_on_shutdown()?;
             let digest = index.index_digest(pos).unwrap();
+
+            // Check if the chunk store actually honors `atime` updates by checking the first chunk
+            // outside of the cutoff range. If atime is not honored garbage collection must fail,
+            // as otherwise chunks still in use can be lost.
+            let mut old_atime = None;
+            if let Some(min_atime) = min_atime {
+                let metadata = self.stat_chunk(digest)?;
+                let atime = metadata.accessed()?;
+                let atime_epoch: i64 = atime.duration_since(UNIX_EPOCH)?.as_secs().try_into()?;
+                if atime_epoch < *min_atime {
+                    old_atime = Some(atime)
+                }
+            }
+
             if !self.inner.chunk_store.cond_touch_chunk(digest, false)? {
                 let hex = hex::encode(digest);
                 warn!(
@@ -1061,6 +1078,19 @@ impl DataStore {
                     self.inner.chunk_store.cond_touch_path(&bad_path, false)?;
                 }
             }
+
+            // `atime` was outside range for this chunk, check that it has been updated.
+            if let Some(old_atime) = old_atime {
+                let metadata = self.stat_chunk(digest)?;
+                let atime = metadata.accessed()?;
+                if old_atime < atime {
+                    // atime was updated, do not check further chunks
+                    *min_atime = None;
+                } else {
+                    let hex = hex::encode(digest);
+                    bail!("atime not updated for chunk {hex}, is atime support enabled?");
+                }
+            }
         }
         Ok(())
     }
@@ -1069,6 +1099,7 @@ impl DataStore {
         &self,
         status: &mut GarbageCollectionStatus,
         worker: &dyn WorkerTaskContext,
+        min_atime: &mut Option<i64>,
     ) -> Result<(), Error> {
         let image_list = self.list_images()?;
         let image_count = image_list.len();
@@ -1097,12 +1128,12 @@ impl DataStore {
                             let index = FixedIndexReader::new(file).map_err(|e| {
                                 format_err!("can't read index '{}' - {}", img.to_string_lossy(), e)
                             })?;
-                            self.index_mark_used_chunks(index, &img, status, worker)?;
+                            self.index_mark_used_chunks(index, &img, status, worker, min_atime)?;
                         } else if archive_type == ArchiveType::DynamicIndex {
                             let index = DynamicIndexReader::new(file).map_err(|e| {
                                 format_err!("can't read index '{}' - {}", img.to_string_lossy(), e)
                             })?;
-                            self.index_mark_used_chunks(index, &img, status, worker)?;
+                            self.index_mark_used_chunks(index, &img, status, worker, min_atime)?;
                         }
                     }
                 }
@@ -1170,10 +1201,21 @@ impl DataStore {
                 upid: Some(upid.to_string()),
                 ..Default::default()
             };
+            let tuning: DatastoreTuning = serde_json::from_value(
+                DatastoreTuning::API_SCHEMA
+                    .parse_property_string(gc_store_config.tuning.as_deref().unwrap_or(""))?,
+            )?;
+            let mut atime_check_ref = if tuning.check_gc_atime.unwrap_or(true) {
+                // Cutoff atime including a 24h grace period for filesystems mounted with
+                // `relatime` option.
+                Some(phase1_start_time - 3600 * 24)
+            } else {
+                None
+            };
 
             info!("Start GC phase1 (mark used chunks)");
 
-            self.mark_used_chunks(&mut gc_status, worker)?;
+            self.mark_used_chunks(&mut gc_status, worker, &mut atime_check_ref)?;
 
             info!("Start GC phase2 (sweep unused chunks)");
             self.inner.chunk_store.sweep_unused_chunks(
-- 
2.39.5