On Monday June 19, dgc@xxxxxxx wrote: > > > I can see that shrink_dcache_sb could take a long time and should be > > fixed, which should be as simple as replacing it with > > shrink_dcache_parent; shrink_dcache_anon. > > But these are not guaranteed to reclaim all the dentries from a given > superblock. Yes, they move the dentries to the LRU, but other activity in the > system means that they may not get reclaimed during the subsequent calls > to prune_dcache() and hence they may live beyond the unmount.... > My proposed patch earlier in this thread (I can post it again if you like) addresses exactly this issue. Instead of moving dentries to the global LRU, it moves them to a private LRU, and the calls prune_dcache on that. So there is no room for other activity to get in the way of prune_dcache doing what needs to be done. I agree that using a single big LRU for everything doesn't work. I just don't think we need (or want) separate LRUs for each superblock. Rather we want separate temporary LRUs just for use when unmounting. > > But I'm still puzzled as to why a long dcache LRU slows down > > unmounting. > > > > Can you give more details? > > It's not the unmount that slows down - it's the fact that the dcache lock > is held for so long that rest of the system halts for time it takes > to run shrink_dcache_sb(). We've seen up to 50s to do a (touch fred; rm fred) > when the LRU has grown to several million dentries and shrink_dcache_sb() > is running. When this happens, it's not uncommon to see every CPU in the > machine spinning on the dcache_lock... Definitely a problem. Maybe it was hoped that the call to cond_resched_lock(&dcache_lock) would avoid this, but apparently not. I still maintain that we should replace shrink_dcache_sb with calls to shrink_dcache_anon and shrink_dcache_parent. That, together with my previous patch, should fix this problem quite cleanly. If I send you a combined patch against the latest -mm can you test? Thanks, NeilBrown - 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
