On Thu, Aug 3, 2017 at 4:53 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 3 Aug 2017 16:25:46 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >> On Thu, Aug 3, 2017 at 4:11 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >> > >> > Where is this INIT_LIST_HEAD()? >> >> I think it's this one: >> >> list_del_init(&info->shrinklist); >> >> in shmem_unused_huge_shrink(). > > OK. > >> > I'm not sure I'm understanding this. AFAICT all the list operations to >> > which you refer are synchronized under spin_lock(&sbinfo->shrinklist_lock)? >> >> No, notice how shmem_unused_huge_shrink() does the >> >> list_move(&info->shrinklist, &to_remove); >> >> and >> >> list_move(&info->shrinklist, &list); >> >> to move to (two different) private lists under the shrinklist_lock, >> but once it is on that private "list/to_remove" list, it is then >> accessed outside the locked region. > > So the code is using sbinfo->shrinklist_lock to protect > sbinfo->shrinklist AND to protect all the per-inode info->shrinklist's. > Except it didn't get the coverage complete. Normally once we move list entries from a global list to a private one they are no longer visible to others, however in this case they could still be accessed via setattr() path. > > Presumably it's too expensive to extend sbinfo->shrinklist_lock > coverage in shmem_unused_huge_shrink() (or is it? - this is huge > pages). An alternative would be to add a new > shmem_inode_info.shrinklist_lock whose mandate is to protect > shmem_inode_info.shrinklist. Both find_lock_page() and iput() could sleep, I think this is why we have to defer these two calls after releasing spinlock. > >> Honestly, I don't love this situation, or the patch, but I think the >> patch is likely the right thing to do. > > Well, we could view the premature droppage of sbinfo->shrinklist_lock > in shmem_unused_huge_shrink() to be a performance optimization and put > some big fat comments in there explaining what's going on. But it's > tricky and it's not known that such an optimization is warranted. It is not for performance optimization, because we still traverse the list with the spinlock held. A typical optimization is to use a list_splice() with spinlock and traverse it without it, this is used by a few places in networking subsystem. Thanks. -- 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>
