Re: [pbs-devel] [PATCH proxmox-backup 5/6] datastore: verify: evict corrupt chunks from in-memory LRU cache

[Christian Ebner <c.ebner@proxmox.com>]

On 10/27/25 11:59 AM, Fabian Grünbichler wrote:
> On October 16, 2025 3:18 pm, Christian Ebner wrote:
>> Chunks detected as corrupt have been renamed on both, the S3 backend
>> and the local datastore cache, but not evicted from the in-memory
>> cache containing the LRU chunk digests. This can lead to the chunks
>> being considered as already present if their digest is still cached,
>> and therefore not being re-inserted in the local store cache and S3
>> backend on backup upload.
>>
>> Fix this by not only renaming the local datastore's chunk marker
>> file, but also removing it from the in-memory cache while holding the
>> chunk store mutex lock to exclude interference from concurrent chunk
>> inserts.
>>
>> Reported-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
>> Signed-off-by: Christian Ebner <c.ebner@proxmox.com>
>> ---
>>   pbs-datastore/src/datastore.rs | 10 +++++++++-
>>   1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/pbs-datastore/src/datastore.rs b/pbs-datastore/src/datastore.rs
>> index a7ea8fd96..c551679e3 100644
>> --- a/pbs-datastore/src/datastore.rs
>> +++ b/pbs-datastore/src/datastore.rs
>> @@ -2467,10 +2467,18 @@ impl DataStore {
>>   
>>           let _lock = self.inner.chunk_store.mutex().lock().unwrap();
>>   
>> -        match std::fs::rename(&path, &new_path) {
>> +        let result = match std::fs::rename(&path, &new_path) {
>>               Ok(_) => Ok(Some(format!("corrupted chunk renamed to {new_path:?}"))),
>>               Err(err) if err.kind() == std::io::ErrorKind::NotFound => Ok(None),
>>               Err(err) => bail!("could not rename corrupted chunk {path:?} - {err}"),
>> +        };
>> +
>> +        if let Some(cache) = self.cache() {
>> +            // Requiremets for calling the unsafe method are met, since snapshots referencing the
>> +            // corrupt chunk are to be considered corrupt. Ignore the error due to the missing file.
>> +            let _ = unsafe { cache.remove(digest) };
>>           }
> 
> not exactly related to this patch, but something I noticed while
> reviewing it:
> 
> how do we protect against concurrent actions on the same chunk between
> - verification detects chunk as corrupt
> - rename on S3 level
> - lock + rename and remove on chunk store/cache level
> 
> let's say to verification tasks and a backup writer race, all involving
> the same chunk:
> 
> - verification A and B detect chunk as corrupt
> - verification A renames it on the S3 level
> - verification A renames it on the chunk store level and removes it from
>    the cache
> - backup writer uploads it to S3
> - verification B renames it on the S3 level
> 
> and now either:
> - backup writer inserts it into the cache
> - verification B renames it on the chunk store level and removes it from
>    the cache
> 
> which means the backup writer creates a corrupt, but not detected as
> such backup
> 
> or
> - verification B attempts to rename it on the chunk store level
>    (ENOTFOUND) and removes it from the cache
> - backup writer inserts it into the cache
> 
> which means we just broke cache coherency - the chunk exists in the
> chunk store and cache, but not on S3 (anymore)
> 
> 
> or another racey variant:
> - verification A and B detect chunk as corrupt
> - verification A renames it on the S3 level
> - verification B renames it on the S3 level but fails because it doesn't
>    exist there??
> 
> I think we might need another synchronization mechanism for verification
> involving S3..

only option I see here is if the rename (copyObject) can only happen 
conditionally, e.g. via the x-amz-copy-source-if-unmodified-since header

Which might however not be available for all providers.


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