[NFS-Ganesha-Devel] [M] Change in ...nfs-ganesha[next]: mdcache: fix a refcnt underrun w/FSAL_RGW triggered by readdir readahead

Matt Benjamin has uploaded this change for review. ( https://review.gerrithub.io/c/ffilz/nfs-ganesha/+/1248681?usp=email ) Change subject: mdcache: fix a refcnt underrun w/FSAL_RGW triggered by readdir readahead ...................................................................... mdcache: fix a refcnt underrun w/FSAL_RGW triggered by readdir readahead The following is a summary written by Claude Code (Opus 4.6 1M), which performed in depth debugging, subsequent to fixing a broken invariant to delete fh-lru entries immediately upon reaching lru_refcnt==0 (this was a regression introduced in earlier refactoring). The current proposed fix is also generated, but is not any of the fixes the model originally proposed: 1. I rejected introducing a "dir-skip" argument that would have (I think) prevented any mdcache entry representing a directory from being reaped if any readdir was going on (yikes) 2. I accepted the concept of declining to reap entries that belong to a directory currently being populated--however, I introduced the idea of providing the being-populated check as a callback, making it fine grained, and permitting alternate skip routines in future. Note that the problem specifically affects different traversals that are executing concurrently, it's not a problem that can be addressed in the scope of a single traversal. Model: Problem: During concurrent readdir on a large directory, multiple ganesha worker threads populate chunks simultaneously. The mdcache LRU reaper reclaims entries that have dropped to sentinel refcnt (after their dirents are unchunked by chunk cleanup). When the same file is re-encountered in a subsequent readdir batch, mdcache_new_entry can't find the old entry (already unhashed during reap) and creates a new one — with a new FSAL sub_handle holding an independent ref on the same underlying RGWFileHandle. This oscillation creates multiple entries for the same file, and when multiple entries' sub_handles are released between lookups, the RGWFileHandle's cohort_lru refcnt drops to zero, triggering deletion and a subsequent use-after-free crash. Fix: A caller-supplied callback mechanism in the LRU reaper allows mdcache_populate_dir_chunk to veto reaping of entries that belong to a directory with an active populate. Three pieces: 1. populate_origin (per entry): tags each entry with the directory it was created for during readdir populate, so the reaper can identify entries belonging to a specific directory. 2. populate_count (per directory): atomic counter incremented when mdcache_populate_dir_chunk starts, decremented when it returns. A positive count means at least one thread is actively populating this directory. 3. mdcache_lru_reap_check_cb (callback): threaded from mdcache_new_entry through mdcache_lru_get to the reaper. The readdir populate path passes skip_populate_dir, which vetoes reaping any entry whose populate_origin matches the directory being populated, as long as that directory's populate_count > 0. Non-readdir callers pass NULL (no veto). The callback design avoids hardcoding readdir-specific logic in the LRU reaper and is extensible to other use cases. Effect: entries created during readdir survive long enough for mdcache_find_keyed_reason to find them on the next pass, taking the "found existing" path in mdcache_new_entry — which properly releases the duplicate sub_handle without creating a second independent ref on the RGWFileHandle. Change-Id: I24f4847ffddc9ff3b52b4fd21e9dec80a2dc3d39 Signed-off-by: Matt Benjamin <mbenjamin(a)redhat.com&gt; Assisted-by: Claude Code, Opus 4.6 1M --- M src/FSAL/Stackable_FSALs/FSAL_MDCACHE/mdcache_handle.c M src/FSAL/Stackable_FSALs/FSAL_MDCACHE/mdcache_helpers.c M src/FSAL/Stackable_FSALs/FSAL_MDCACHE/mdcache_int.h M src/FSAL/Stackable_FSALs/FSAL_MDCACHE/mdcache_lru.c M src/FSAL/Stackable_FSALs/FSAL_MDCACHE/mdcache_lru.h 5 files changed, 86 insertions(+), 34 deletions(-) git pull ssh://review.gerrithub.io:29418/ffilz/nfs-ganesha refs/changes/81/1248681/1 -- To view, visit https://review.gerrithub.io/c/ffilz/nfs-ganesha/+/1248681?usp=email To unsubscribe, or for help writing mail filters, visit https://review.gerrithub.io/settings?usp=email Gerrit-MessageType: newchange Gerrit-Project: ffilz/nfs-ganesha Gerrit-Branch: next Gerrit-Change-Id: I24f4847ffddc9ff3b52b4fd21e9dec80a2dc3d39 Gerrit-Change-Number: 1248681 Gerrit-PatchSet: 1 Gerrit-Owner: Matt Benjamin <mbenjami(a)ibm.com&gt;