[pbs-devel] [PATCH v5 proxmox-backup 3/8] fix #5982: garbage collection: check atime updates are honored

[Thomas Lamprecht]

Am 06.03.25 um 15:52 schrieb Christian Ebner:
> Check if the filesystem backing the chunk store actually updates the
> atime to avoid potential data loss in phase 2 of garbage collection,
> in case the atime update is not honored.
> 
> Perform the check before phase 1 of garbage collection, as well as
> on datastore creation. The latter to early detect and disallow
> datastore creation on filesystem configurations which otherwise most
> likely would lead to data losses.
> 
> Enable the atime update check by default, but allow to opt-out by
> setting a datastore tuning parameter flag for backwards compatibility.
> This is honored by both, garbage collection and datastore creation.
> 
> The check uses a 4 MiB fixed sized, unencypted and compressed chunk
> as test marker, inserted if not present. This all zero-chunk is very
> likely anyways for unencrypted backup contents with large all-zero
> regions using fixed size chunking (e.g. VMs).
> 
> To avoid cases were the timestamp will not be updated because of the
> Linux kernels timestamp granularity, sleep in-between stating and
> utimensat for 1 second.
> 
> Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5982
> Signed-off-by: Christian Ebner <c.ebner at proxmox.com>
> ---
> changes since version 4:
> - Improve logging of values if atime update fails.
> - fix incorrect comment
> 
>  pbs-datastore/src/chunk_store.rs | 101 ++++++++++++++++++++++++++++---
>  pbs-datastore/src/datastore.rs   |  13 ++++
>  src/api2/config/datastore.rs     |   1 +
>  3 files changed, 108 insertions(+), 7 deletions(-)
> 
> diff --git a/pbs-datastore/src/chunk_store.rs b/pbs-datastore/src/chunk_store.rs
> index 5e02909a1..a8c826353 100644
> --- a/pbs-datastore/src/chunk_store.rs
> +++ b/pbs-datastore/src/chunk_store.rs
> @@ -1,9 +1,11 @@
> +use std::os::unix::fs::MetadataExt;
>  use std::os::unix::io::AsRawFd;
>  use std::path::{Path, PathBuf};
>  use std::sync::{Arc, Mutex};
> +use std::time::{Duration, UNIX_EPOCH};
>  
> -use anyhow::{bail, format_err, Error};
> -use tracing::info;
> +use anyhow::{bail, format_err, Context, Error};
> +use tracing::{info, warn};
>  
>  use pbs_api_types::{DatastoreFSyncLevel, GarbageCollectionStatus};
>  use proxmox_io::ReadExt;
> @@ -13,6 +15,7 @@ use proxmox_sys::process_locker::{
>  };
>  use proxmox_worker_task::WorkerTaskContext;
>  
> +use crate::data_blob::DataChunkBuilder;
>  use crate::file_formats::{
>      COMPRESSED_BLOB_MAGIC_1_0, ENCRYPTED_BLOB_MAGIC_1_0, UNCOMPRESSED_BLOB_MAGIC_1_0,
>  };
> @@ -93,6 +96,7 @@ impl ChunkStore {
>          uid: nix::unistd::Uid,
>          gid: nix::unistd::Gid,
>          sync_level: DatastoreFSyncLevel,
> +        atime_safety_check: bool,

Not a huge fan of another flat parameter here, albeit it's a bit borderline.
I do not want to add scope creep, but if we really want this here, then moving
those options into a struct might be nicer, could be done afterward too though.

But stepping a bit back, why is this a parameter for the fn creating a new
chunkstore in the first place? Why cannot the call sites that are interested
in doing this call check_fs_atime_updates themselves on the chunkstore instance
after getting it instead of having to loop through a parameter?

We do not have many places that create datastores, so I do not think there's a
high likelihood that we forget to call it.

>      ) -> Result<Self, Error>
>      where
>          P: Into<PathBuf>,
> @@ -147,7 +151,17 @@ impl ChunkStore {
>              }
>          }
>  
> -        Self::open(name, base, sync_level)
> +        let chunk_store = Self::open(name, base, sync_level)?;
> +        if atime_safety_check {
> +            chunk_store
> +                .check_fs_atime_updates(true)
> +                .map_err(|err| format_err!("access time safety check failed - {err:#}"))?;
> +            info!("Access time safety check successful.");
> +        } else {
> +            info!("Access time safety check skipped.");
> +        }
> +
> +        Ok(chunk_store)
>      }
>  
>      fn lockfile_path<P: Into<PathBuf>>(base: P) -> PathBuf {
> @@ -442,6 +456,66 @@ impl ChunkStore {
>          Ok(())
>      }
>  
> +    /// Check if atime updates are honored by the filesystem backing the chunk store.
> +    ///
> +    /// Checks if the atime is always updated by utimensat taking into consideration the Linux
> +    /// kernel timestamp granularity.
> +    /// If `retry_on_file_changed` is set to true, the check is performed again on the changed file
> +    /// if a file change while testing is detected by differences in bith time or inode number.
> +    /// Uses a 4 MiB fixed size, compressed but unencrypted chunk to test. The chunk is inserted in
> +    /// the chunk store if not yet present.
> +    /// Returns with error if the check could not be performed.
> +    pub fn check_fs_atime_updates(&self, retry_on_file_changed: bool) -> Result<(), Error> {
> +        let (zero_chunk, digest) = DataChunkBuilder::build_zero_chunk(None, 4096 * 1024, true)?;
> +        let (pre_existing, _) = self.insert_chunk(&zero_chunk, &digest)?;
> +        let (path, _digest) = self.chunk_path(&digest);
> +
> +        // Take into account timestamp update granularity in the kernel
> +        std::thread::sleep(Duration::from_secs(1));

If there's the need for a v6 I'd maybe add to the comment that this normally
runs in a worker, so sleep blocking the whole thread is fine here.

Also, you write "sleep in-between stating and utimensat for 1 second", but
this is sleeping before the stat + touch + stat calls, or am I overlooking
something?

> +        let metadata_before =
> +            std::fs::metadata(&path).context(format!("failed to get metadata for {path:?}"))?;
> +
> +        // Second atime update if chunk pre-existed, insert_chunk already updates pre-existing ones
> +        self.cond_touch_path(&path, true)?;
> +
> +        let metadata_now =
> +            std::fs::metadata(&path).context(format!("failed to get metadata for {path:?}"))?;

Another tiny nit: maybe encode now/after in the context to be easier able
to distinguish which stat syscall failed – while it's very unlikely that
one succeeds and the other doesn't, it would not hurt to be able to tell
that case apart.

> +
> +        // Check for the unlikely case that the file changed in-between the
> +        // two metadata calls, try to check once again on changed file
> +        if metadata_before.ino() != metadata_now.ino() {
> +            if retry_on_file_changed {
> +                return self.check_fs_atime_updates(false);
> +            }
> +            bail!("chunk {path:?} changed twice during access time safety check, cannot proceed.");
> +        }
> +
> +        if metadata_before.accessed()? >= metadata_now.accessed()? {
> +            let chunk_info_str = if pre_existing {
> +                "pre-existing"
> +            } else {
> +                "newly inserted"
> +            };
> +            warn!("Chunk metadata was not correctly updated during access time safety check:");
> +            info!(
> +                "Timestamps before update: accessed {:?}, modified {:?}, created {:?}",
> +                metadata_before.accessed().unwrap_or(UNIX_EPOCH),
> +                metadata_before.modified().unwrap_or(UNIX_EPOCH),
> +                metadata_before.created().unwrap_or(UNIX_EPOCH),
> +            );
> +            info!(
> +                "Timestamps after update: accessed {:?}, modified {:?}, created {:?}",
> +                metadata_now.accessed().unwrap_or(UNIX_EPOCH),
> +                metadata_now.modified().unwrap_or(UNIX_EPOCH),
> +                metadata_now.created().unwrap_or(UNIX_EPOCH),
> +            );
> +            bail!("access time safety check using {chunk_info_str} chunk failed, aborting GC!");
> +        }
> +
> +        Ok(())
> +    }
> +
>      pub fn insert_chunk(&self, chunk: &DataBlob, digest: &[u8; 32]) -> Result<(bool, u64), Error> {
>          // unwrap: only `None` in unit tests
>          assert!(self.locker.is_some());


> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
> index 75c0c16ab..5558bb1ac 100644
> --- a/pbs-datastore/src/datastore.rs
> +++ b/pbs-datastore/src/datastore.rs
> @@ -1170,6 +1170,19 @@ 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(""))?,
> +            )?;
> +            if tuning.gc_atime_safety_check.unwrap_or(true) {
> +                self.inner
> +                    .chunk_store
> +                    .check_fs_atime_updates(true)
> +                    .map_err(|err| format_err!("atime safety check failed - {err:#}"))?;
> +                info!("Access time safety check successful, proceeding with GC.");
> +            } else {
> +                info!("Filesystem atime safety check disabled by datastore tuning options.");

Sounds IMO scarier than what this is, i.e. like the whole GC would not
be safe anymore, or would not use atime safety checks anymore.

I'd rather either drop that or reword it to something else, maybe just doing
s/safety/update/ would be enough.

> +            }
>  
>              info!("Start GC phase1 (mark used chunks)");
>  
> diff --git a/src/api2/config/datastore.rs b/src/api2/config/datastore.rs
> index fe3260f6d..35847fc45 100644
> --- a/src/api2/config/datastore.rs
> +++ b/src/api2/config/datastore.rs
> @@ -119,6 +119,7 @@ pub(crate) fn do_create_datastore(
>                  backup_user.uid,
>                  backup_user.gid,
>                  tuning.sync_level.unwrap_or_default(),
> +                tuning.gc_atime_safety_check.unwrap_or(true),
>              )
>              .map(|_| ())
>          } else {