Hi Amir, Jan, et al, It's been a while since I worked with you on the patch series[1] that aimed to make __fsnotify_update_child_dentry_flags() a sleepable function. That work got to a point that it was close to ready, but there were some locking issues which Jan found, and the kernel test robot reported, and I didn't find myself able to tackle them in the amount of time I had. But looking back on that series, I think I threw out the baby with the bathwater. While I may not have resolved the locking issues associated with the larger change, there was one patch which Amir shared, that probably resolves more than 90% of the issues that people may see. I'm sending that here, since it still applies to the latest master branch, and I think it's a very good idea. To refresh you, the underlying issue I was trying to resolve was when directories have many dentries (frequently, a ton of negative dentries), the __fsnotify_update_child_dentry_flags() operation can take a while, and it happens under spinlock. Case #1 - if the directory has tens of millions of dentries, then you could get a soft lockup from a single call to this function. I have seen some cases where a single directory had this many dentries, but it's pretty rare. Case #2 - suppose you have a system with many CPUs and a busy directory. Suppose the directory watch is removed. The caller will begin executing __fsnotify_update_child_dentry_flags() to clear the PARENT_WATCHED flag, but in parallel, many other CPUs could wind up in __fsnotify_parent() and decide that they, too, must call __fsnotify_update_child_dentry_flags() to clear the flags. These CPUs will all spin waiting their turn, at which point they'll re-do the long (and likely, useless) call. Even if the original call only took a second or two, if you have a dozen or so CPUs that end up in that call, some CPUs will spin a long time. Amir's patch to clear PARENT_WATCHED flags lazily resolves that easily. In __fsnotify_parent(), if callers notice that the parent is no longer watching, they merely update the flags for the current dentry (not all the other children). The __fsnotify_recalc_mask() function further avoids excess calls by only updating children if the parent started watching. This easily handles case #2 above. Perhaps case #1 could still cause issues, for the cases of truly huge dentry counts, but we shouldn't let "perfect" get in the way of "good enough" :) Thanks, Stephen [1]: https://lore.kernel.org/all/20221013222719.277923-1-stephen.s.brennan@xxxxxxxxxx/ Amir Goldstein (1): fsnotify: clear PARENT_WATCHED flags lazily fs/notify/fsnotify.c | 26 ++++++++++++++++++++------ fs/notify/fsnotify.h | 3 ++- fs/notify/mark.c | 32 +++++++++++++++++++++++++++++--- include/linux/fsnotify_backend.h | 8 +++++--- 4 files changed, 56 insertions(+), 13 deletions(-) -- 2.43.0
