On Fri, 04 May 2007 10:57:12 +0400 Alex Tomas <alex@xxxxxxxxxxxxx> wrote: > Andrew Morton wrote: > > On Fri, 04 May 2007 10:18:12 +0400 Alex Tomas <alex@xxxxxxxxxxxxx> wrote: > > > >> Andrew Morton wrote: > >>> Yes, there can be issues with needing to allocate journal space within the > >>> context of a commit. But > >> no-no, this isn't required. we only need to mark pages/blocks within > >> transaction, otherwise race is possible when we allocate blocks in transaction, > >> then transacton starts to commit, then we mark pages/blocks to be flushed > >> before commit. > > > > I don't understand. Can you please describe the race in more detail? > > if I understood your idea right, then in data=ordered mode, commit thread writes > all dirty mapped blocks before real commit. > > say, we have two threads: t1 is a thread doing flushing and t2 is a commit thread > > t1 t2 > find dirty inode I > find some dirty unallocated blocks > journal_start() > allocate blocks > attach them to I > journal_stop() 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(). > 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. 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. 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. I assume (hope) that your delayed allocation code implements ->writepages()? Doing the allocation one-page-at-a-time sounds painful... > > map pages/bhs to just allocate blocks > > > so, either we mark pages/bhs someway within journal_start()--journal_stop() or > commit thread should do lookup for all dirty pages. the latter doesn't sound nice, IMHO. > I don't think I'm understanding you fully yet. - 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
