Hi, On Tue, Oct 21, 2008 at 04:32:21PM +0200, Miklos Szeredi wrote: > On Tue, 21 Oct 2008, steve@xxxxxxxxxxx > > Once thats done, the dlm/glock is dropped (again notification is via > > the dlm) and if Node A has outstanding requests queued up, it > > re-requests the glock. This is a slightly simplified explanation > > but, I hope it gives the general drift. > > Yes, thanks. > > > So to return to the original subject, in order to allow all > > this locking to occur with no lock ordering problems, we have > > to define a suitable ordering of page locks vs. glocks, and the > > ordering that we use is that glocks must come before page locks. The > > full ordering of locks in GFS2 is in Documentation/filesystems/gfs2-glocks.txt > > > > As a result of that, the VFS needs reads (and page_mkwrite) to > > retry when !PageUptodate() in case the returned page has been > > invalidated at any time when the page lock has been dropped. > > Since this commit PG_uptodate isn't cleared on invalidate: > > commit 84209e02de48d72289650cc5a7ae8dd18223620f > Author: Miklos Szeredi <mszeredi@xxxxxxx> > Date: Fri Aug 1 20:28:47 2008 +0200 > > mm: dont clear PG_uptodate on truncate/invalidate > > Testing for !page->mapping, however, is a reliable way to detect both > truncation and invalidation. > > So the page can have the following states: > > !PG_uptodate -> page has not been read > PG_uptodate && page->mapping -> page has been read and is valid > PG_uptodate && !page->mapping -> page has been read but no longer valid > > So PG_uptodate does not reflect the validity of the data, only whether > the data was ever made up-to-date. > > Does this make sense? Should it be documented somewhere? > Well I'm not sure why we'd need to distinguish between "page has not been read" and "page has been read but no longer valid". I guess I don't understand why those two cases are not the same from the vfs and filesystem points of view. I'm sure it should be documented :-) it certainly seems confusing and if we want to keep this scheme, can we change PG_uptodate to PG_wasread or PG_usedonce or something like that which more clearly reflects its purpose in that case, Steve. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
