On Wed, 13 Aug 2008, Al Viro wrote: > > What _can_ a common helper do, anyway, when we are busy parsing an arseload of > possibly corrupt data in whatever weird format fs insists upon? Well, the parsing has to be done by the low-level filesystem code, yes. However, the whole thing with races with "f_pos" and all the locking - that's only because we see the filesystem "readdir" code as being the primary source of data. Quite frankly, if we had a "readdir page cache", the low-level filesystem would still have to parse the insane low-level data with corruption issues, but we could make it totally independent of f_pos (because we would never use in the _real_ file->f_pos - we would just populate the cache), and the locking issues would be only a cold-cache issue, with the hot-cache hopefully needing little locking at all. For an exmple of that: you did a good job with all the "seq_file" helpers, which meant that the low-level "filesystem" ops didn't need to know _anything_ about partial results etc, and it automatically did the right thing wrt f_pos updates and lseek etc. I'm not saying that readdir() would use the _same_ model, but I do suspect that a common format in between the disk format and the eventual readdir() output, that also could be cached, might mitigate a lot of the problems. As to the issues with lookup() - yes, a lookup would need to get the lock for writing, but only for the last entry, and only if O_CREAT is set. There's nothing wrogn with concurrent read-only lookups, I think (apart from having to protect the dentries from being duplicated, of course, but that would be a per-dentry lock flag, not a directory lock, methinks). I dunno. That said, I think you are right that we could also just improve on the current non-caching version with soem higher-level semantics. Including flags like "yes, we've seen the end", so that we don't need to always call into the low-level filesystem one extra time to see that final zero return. So yes, instead of separate "filldir_t" and "void *data" things, having a "struct filldir_t" with several fields in common might be worth it. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
