Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Tue, Aug 15, 2017 at 3:57 PM, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >> >> Oh, and the page wait-queue really needs that key argument too, which >> is another thing that swait queue code got rid of in the name of >> simplicity. > > Actually, it gets worse. > > Because the page wait queues are hashed, it's not an all-or-nothing > thing even for the non-exclusive cases, and it's not a "wake up first > entry" for the exclusive case. Both have to be conditional on the wait > entry actually matching the page and bit in question. > > So no way to use swait, or any of the lockless queuing code in general > (so we can't do some clever private wait-list using llist.h either). > > End result: it looks like you fairly fundamentally do need to use a > lock over the whole list traversal (like the standard wait-queues), > and then add a cursor entry like Tim's patch if dropping the lock in > the middle. > > Anyway, looking at the old code, we *used* to limit the page wait hash > table to 4k entries, and we used to have one hash table per memory > zone. > > The per-zone thing didn't work at all for the generic bit-waitqueues, > because of how people used them on virtual addresses on the stack. > > But it *could* work for the page waitqueues, which are now a totally > separate entity, and is obviously always physically addressed (since > the indexing is by "struct page" pointer), and doesn't have that > issue. > > So I guess we could re-introduce the notion of per-zone page waitqueue > hash tables. It was disgusting to allocate and free though (and hooked > into the memory hotplug code). > > So I'd still hope that we can instead just have one larger hash table, > and that is sufficient for the problem. If increasing the hash table size fixes the problem I am wondering if rhash tables might be the proper solution to this problem. They start out small and then grow as needed. Eric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
