On Wed, 2011-11-23 at 10:01 +0800, Nai Xia wrote: > On Tue, Nov 22, 2011 at 2:56 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > > On Tue, 2011-11-22 at 02:36 +0800, Mel Gorman wrote: > >> This patch adds a lightweight sync migrate operation MIGRATE_SYNC_LIGHT > >> mode that avoids writing back pages to backing storage. Async > >> compaction maps to MIGRATE_ASYNC while sync compaction maps to > >> MIGRATE_SYNC_LIGHT. For other migrate_pages users such as memory > >> hotplug, MIGRATE_SYNC is used. > >> > >> This avoids sync compaction stalling for an excessive length of time, > >> particularly when copying files to a USB stick where there might be > >> a large number of dirty pages backed by a filesystem that does not > >> support ->writepages. > > Hi, > > from my understanding, with this, even writes > > to /proc/sys/vm/compact_memory doesn't wait for pageout, is this > > intended? > > on the other hand, MIGRATE_SYNC_LIGHT now waits for pagelock and buffer > > lock, so could wait on page read. page read and page out have the same > > latency, why takes them different? > > So for the problem you raised, I think my suggestion to Mel is to adopt the > following logic: > > if (!trylock_page(page) && !PageUptodate(page)) > we are quite likely to block on read, so we > depend on yet another MIGRATE_SYNC_MODE to decide > if we really want to lock_page() and wait for this IO. > > How do you think ? assume the PageUptodate() is at the check for 'goto out'. yes, looks reasonable to me. And we need similar check for buffer_head. Thanks, Shaohua -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>