On Thursday after LSF, Hugh, Minchan, Mel, Johannes and I were sitting in the hallway talking about yet more VM things. During that discussion, we came up with a way to redesign the swap cache. During my flight home, I came with ideas on how to use that redesign, that may make the changes worthwhile. Currently, the page table entries that have swapped out pages associated with them contain a swap entry, pointing directly at the swap device and swap slot containing the data. Meanwhile, the swap count lives in a separate array. The redesign we are considering moving the swap entry to the page cache radix tree for the swapper_space and having the pte contain only the offset into the swapper_space. The swap count info can also fit inside the swapper_space page cache radix tree (at least on 64 bits - on 32 bits we may need to get creative or accept a smaller max amount of swap space). This extra layer of indirection allows us to do several things: 1) get rid of the virtual address scanning swapoff; instead we just swap the data in and mark the pages as present in the swapper_space radix tree 2) free swap entries as the are read in, without waiting for the process to fault it in - this may be useful for memory types that have a large erase block 3) together with the defragmentation from (2), we can always do writes in large aligned blocks - the extra indirection will make it relatively easy to have special backend code for different kinds of swap space, since all the state can now live in just one place 4) skip writeout of zero-filled pages - this can be a big help for KVM virtual machines running Windows, since Windows zeroes out free pages; simply discarding a zero-filled page is not at all simple in the current VM, where we would have to iterate over all the ptes to free the swap entry before being able to free the swap cache page (I am not sure how that locking would even work) with the extra layer of indirection, the locking for this scheme can be trivial - either the faulting process gets the old page, or it gets a new one, either way it'll be zero filled 5) skip writeout of pages the guest has marked as free - same as above, with the same easier locking Only one real question remaining - how do we handle the swap count in the new scheme? On 64 bit systems we have enough space in the radix tree, on 32 bit systems maybe we'll have to start overflowing into the "swap_count_continued" logic a little sooner than we are now and reduce the maximum swap size a little? -- All rights reversed -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>