On Wed, 7 Jul 2010, Shaohua Li wrote: > On Wed, Jul 07, 2010 at 09:32:54AM +0800, Andrew Morton wrote: > > > > The patch doesn't make shmem_getpage() any clearer :( :) > > > > shmem_inode_info.lock appears to be held too much. Surely > > lookup_swap_cache() didn't need it (for example). > > > > What data does shmem_inode_info.lock actually protect? > As far as my understanding, it protects shmem swp_entry, which is most used > to support swap. It also protects some accounting. If no swap, the lock almost > can be removed like tiny-shmem. That's right: shmem_info_info.lock protects what's in shmem_inode_info, plus what hangs off it (the shmem_swp blocks). We want that lock across the lookup_swap_cache() to be sure that what we find is still what we want (otherwise another thread might bring it out of swap and that swap be reused for something else) - the page lock is good once you have a page to lock, but until then.... I guess could be done by dropping the lock then retaking and rechecking after, but that would go right against the grain of this patch. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>