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To: Wolfgang Bumiller <w.bumiller@proxmox.com>
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Subject: Re: [pbs-devel] [PATCH v6 proxmox-backup 3/8] fix #5982: garbage
collection: check atime updates are honored
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On 3/20/25 09:36, Wolfgang Bumiller wrote:
> On Wed, Mar 19, 2025 at 06:24:27PM +0100, Christian Ebner wrote:
>> 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. To perform the check also when
>> reusing an existing datastore, open the chunks store also on reuse.
>>
>> 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 chunk insert
>> (including an atime update if pre-existing) and the subsequent
>> stating + utimensat for 1 second.
>>
>> Fixes: https://bugzilla.proxmox.com/show_bug.cgi?id=5982
>> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
>> ---
>> changes since version 5:
>> - Also perform check when reusing datastores, not just new one.
>> - Expand on comment for sleep, fix incorrect wording in commit message.
>> - Add additional error context to before/after stat calls.
>> - Soften wording of error messages if check is skipped because disabled.
>>
>> pbs-datastore/src/chunk_store.rs | 72 ++++++++++++++++++++++++++++++--
>> pbs-datastore/src/datastore.rs | 13 ++++++
>> src/api2/config/datastore.rs | 35 ++++++++++++----
>> 3 files changed, 109 insertions(+), 11 deletions(-)
>>
>> diff --git a/pbs-datastore/src/chunk_store.rs b/pbs-datastore/src/chunk_store.rs
>> index 5e02909a1..dcb499426 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,
>> };
>> @@ -177,7 +180,7 @@ impl ChunkStore {
>> /// Note that this must be used with care, as it's dangerous to create two instances on the
>> /// same base path, as closing the underlying ProcessLocker drops all locks from this process
>> /// on the lockfile (even if separate FDs)
>> - pub(crate) fn open<P: Into<PathBuf>>(
>> + pub fn open<P: Into<PathBuf>>(
>> name: &str,
>> base: P,
>> sync_level: DatastoreFSyncLevel,
>> @@ -442,6 +445,69 @@ 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
>> + // Blocking the thread is fine here since this runs in a worker.
>> + std::thread::sleep(Duration::from_secs(1));
>> +
>> + let metadata_before = std::fs::metadata(&path).context(format!(
>> + "failed to get metadata for {path:?} before atime update"
>> + ))?;
>> +
>> + // 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:?} after atime update"
>> + ))?;
>> +
>> + // 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),
>
> I wonder if rendering `UNIX_EPOCH` makes sense for failed values, we
> could just use `.ok()` and have it debug-render the `Option`?
Okay, will adapt that
>
>> + );
>> + 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 a6a91ca79..09b5808e0 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:#}"))?;
>
> ^ I see we have `{:#}` only exactly once in our code base from one of
> your patches. Do we really want to use this form?
>
> From what I can tell, this chains contexts with colons.
>
> I think we mostly use `.context`/`.with_context` nowadays instead?
> That way the choice of formatting is left to the caller and they can
> choose between `{}`, `{:#}` and `{:?}` and `{:#?}`...
But exactly that was the intention here? The check_fs_atime_updates()
returns some errors by adding context instead of reformatting the error
itself.
I didn't adapt this for the whole datastore logic though, therefore
limited it to this call side instead. I could of course bubble this up
further, but who should be the caller defining the error format then?
>
>> + info!("Access time update check successful, proceeding with GC.");
>> + } else {
>> + info!("Access time update check disabled by datastore tuning options.");
>> + }
>>
>> info!("Start GC phase1 (mark used chunks)");
>>
>> diff --git a/src/api2/config/datastore.rs b/src/api2/config/datastore.rs
>> index fe3260f6d..7e97a7de3 100644
>> --- a/src/api2/config/datastore.rs
>> +++ b/src/api2/config/datastore.rs
>> @@ -4,7 +4,7 @@ use ::serde::{Deserialize, Serialize};
>> use anyhow::{bail, format_err, Error};
>> use hex::FromHex;
>> use serde_json::Value;
>> -use tracing::warn;
>> +use tracing::{info, warn};
>>
>> use proxmox_router::{http_bail, Permission, Router, RpcEnvironment, RpcEnvironmentType};
>> use proxmox_schema::{api, param_bail, ApiType};
>> @@ -98,7 +98,15 @@ pub(crate) fn do_create_datastore(
>> )?;
>>
>> let res = if reuse_datastore {
>> - ChunkStore::verify_chunkstore(&path)
>> + ChunkStore::verify_chunkstore(&path).and_then(|_| {
>> + // Must be the only instance accessing and locking the chunk store,
>> + // dropping will close all other locks from this process on the lockfile as well.
>> + ChunkStore::open(
>> + &datastore.name,
>> + &path,
>> + tuning.sync_level.unwrap_or_default(),
>> + )
>> + })
>> } else {
>> let mut is_empty = true;
>> if let Ok(dir) = std::fs::read_dir(&path) {
>> @@ -120,19 +128,30 @@ pub(crate) fn do_create_datastore(
>> backup_user.gid,
>> tuning.sync_level.unwrap_or_default(),
>> )
>> - .map(|_| ())
>> } else {
>> Err(format_err!("datastore path not empty"))
>> }
>> };
>>
>> - if res.is_err() {
>> - if need_unmount {
>> - if let Err(e) = unmount_by_mountpoint(&path) {
>> - warn!("could not unmount device: {e}");
>> + match res {
>> + Ok(chunk_store) => {
>> + if tuning.gc_atime_safety_check.unwrap_or(true) {
>> + chunk_store
>> + .check_fs_atime_updates(true)
>> + .map_err(|err| format_err!("access time safety check failed - {err:#}"))?;
>
> ^ same here
>
>> + info!("Access time update check successful.");
>> + } else {
>> + info!("Access time update check skipped.");
>> + }
>> + }
>> + Err(err) => {
>> + if need_unmount {
>> + if let Err(e) = unmount_by_mountpoint(&path) {
>> + warn!("could not unmount device: {e}");
>> + }
>> }
>> + return Err(err);
>> }
>> - return res;
>> }
>>
>> config.set_data(&datastore.name, "datastore", &datastore)?;
>> --
>> 2.39.5
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