On Fri, 04 May 2007 11:39:22 +0400 Alex Tomas <alex@xxxxxxxxxxxxx> wrote: > Andrew Morton wrote: > > I'm still not understanding. The terms you're using are a bit ambiguous. > > > > What does "find some dirty unallocated blocks" mean? Find a page which is > > dirty and which does not have a disk mapping? > > > > Normally the above operation would be implemented via > > ext4_writeback_writepage(), and it runs under lock_page(). > > I'm mostly worried about delayed allocation case. My impression was that > holding number of pages locked isn't a good idea, even if they're locked > in index order. so, I was going to turn number of pages writeback, then > allocate blocks for all of them at once, then put proper blocknr's into > bh's (or PG_mappedtodisk?). ooh, that sounds hacky and quite worrisome. If someone comes in and does an fsync() we've lost our synchronisation point. Yes, all callers happen to do lock_page(); wait_on_page_writeback(); (I think) but we've never considered a bare PageWriteback() as something which protects page internals. We're OK wrt page reclaim and we're OK wrt truncate and invalidate. As long as the page is uptodate we _should_ be OK wrt readpage(). But still, it'd be better to use the standard locking rather than inventing new rules, if poss. I'd be 100% OK with locking multiple pages in ascending pgoff_t order. Locking the page is the standard way of doing this synchronisation and the only problem I can think of is that having a tremendous number of pages locked could cause the wake_up_page() waitqueue hashes to get overloaded and go slow. But it's also possible to lock many, many pages with readahead and nobody has reported problems in there. > > > > > >> going to commit > >> find inode I dirty > >> do NOT find these blocks because they're > >> allocated only, but pages/bhs aren't mapped > >> to them > >> start commit > > > > I think you're assuming here that commit would be using ->t_sync_datalist > > to locate dirty buffer_heads. > > nope, I mean sb->inode->page walk. > > > But under this proposal, t_sync_datalist just gets removed: the new > > ordered-data mode _only_ need to do the sb->inode->page walk. So if I'm > > understanding you, the way in which we'd handle any such race is to make > > kjournald's writeback of the dirty pages block in lock_page(). Once it > > gets the page lock it can look to see if some other thread has mapped the > > page to disk. > > if I'm right holding number of pages locked, then they won't be locked, but > writeback. of course kjournald can block on writeback as well, but how does > it find pages with *newly allocated* blocks only? I don't think we'd want kjournald to do that. Even if a page was dirtied by an overwrite, we'd want to write it back during commit, just from a quality-of-implementation point of view. If we were to leave these pages unwritten during commit then a post-recovery file could have a mix of up-to-five-second-old data and up-to-30-seconds-old data. > > It may turn out that kjournald needs a private way of getting at the > > I_DIRTY_PAGES inodes to do this properly, but I don't _think_ so. If we > > had the radix-tree-of-dirty-inodes thing then that's easy enough to do > > anyway, with a tagged search. But I expect that a single pass through the > > superblock's dirty inodes would suffice for ordered-data. Files which > > have chattr +j would screw things up, as usual. > > not dirty inodes only, but rather some fast way to find pages with newly > allocated pages. Newly allocated blocks, you mean? Just write out the overwritten blocks as well as the new ones, I reckon. It's what we do now. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
