Hello, On Fri 15-04-11 22:39:07, Toshiyuki Okajima wrote: > > For ext3 or ext4 without delayed allocation we block inside writepage() > >function. But as I wrote to Dave Chinner, ->page_mkwrite() should probably > >get modified to block while minor-faulting the page on frozen fs because > >when blocks are already allocated we may skip starting a transaction and so > >we could possibly modify the filesystem. > OK. I think ->page_mkwrite() should also block writing the minor-faulting pages. > > (minor-pagefault) > -> do_wp_page() > -> page_mkwrite(= ext4_mkwrite()) > => BLOCK! > > (major-pagefault) > -> do_liner_fault() > -> page_mkwrite(= ext4_mkwrite()) > => BLOCK! > > > > >>>>Mizuma-san's reproducer also writes the data which maps to the file (mmap). > >>>>The original problem happens after the fsfreeze operation is done. > >>>>I understand the normal write operation (not mmap) can be blocked while > >>>>fsfreezing. So, I guess we don't always block all the write operation > >>>>while fsfreezing. > >>> Technically speaking, we block all the transaction starts which means we > >>>end up blocking all the writes from going to disk. But that does not mean > >>>we block all the writes from going to in-memory cache - as you properly > >>>note the mmap case is one of such exceptions. > >>Hm, I also think we can allow the writes to in-memory cache but we can't allow > >>the writes to disk while fsfreezing. I am considering that mmap path can > >>write to disk while fsfreezing because this deadlock problem happens after > >>fsfreeze operation is done... > > I'm sorry I don't understand now - are you speaking about the case above > >when writepage() does not wait for filesystem being frozen or something > >else? > Sorry, I didn't understand around the page fault path. > So, I had read the kernel source code around it, then I maybe understand... > > I worry whether we can update the file data in mmap case while fsfreezing. > Of course, I understand that we can write to in-memory cache, and it is not a > problem. However, if we can write to disk while fsfreezing, it is a problem. > So, I summarize the cases whether we can write to disk or not. > > -------------------------------------------------------------------------- > Cases (Whether we can write the data mmapped to the file on the disk > while fsfreezing) > > [1] One of the page which has been mmapped is not bound. And > the page is not allocated yet. (major fault?) > > (1) user dirtys a page > (2) a page fault occurs (do_page_fault) > (3) __do_falut is called. > (4) ext4_page_mkwrite is called > (5) ext4_write_begin is called > (6) ext4_journal_start_sb => We can STOP! > > [2] One of the page which has been mmapped is not bound. But > the page is already allocated, and the buffer_heads of the page > are not mapped (BH_Mapped). (minor fault?) > > (1) user dirtys a page > (2) a page fault occurs (do_page_fault) > (3) do_wp_page is called. > (4) ext4_page_mkwrite is called > (5) ext4_write_begin is called > (6) ext4_journal_start_sb => We can STOP! > > [3] One of the page which has been mmapped is not bound. But > the page is already allocated, and the buffer_heads of the page > are mapped (BH_Mapped). (minor fault?) > > (1) user dirtys a page > (2) a page fault occurs (do_page_fault) > (3) do_wp_page is called. > (4) ext4_page_mkwrite is called > * Cannot block the dirty page to be written because all bh is mapped. > (5) user munmaps the page (munmap) > (6) zap_pte_range dirtys the page (struct page) which is pte_dirtyed. > (7) writeback thread writes the page (struct page) to disk > => We cannot STOP! > > [4] One of the page which has been mmapped is bound. And > the page is already allocated. > > (1) user dirtys a page > ( ) no page fault occurs > (2) user munmaps the page (munmap) > (3) zap_pte_range dirtys the page (struct page) which is pte_dirtyed. > (4) writeback thread writes the page (struct page) to disk > => We cannot STOP! > -------------------------------------------------------------------------- > > So, we can block the cases [1], [2]. > But I think we cannot block the cases [3], [4] now. > If fixing the page_mkwrite, we can also block the case [3]. > But the case [4] is not blocked because no page fault occurs > when we dirty the mmapped page. > > Therefore, to repair this problem, we need to fix the cases [3], [4]. > I think we must modify the writeback thread to fix the case [4]. The trick here is that when we write a page to disk, we write-protect the page (you seem to call this that "the page is bound", I'm not sure why). So we are guaranteed to receive a minor fault (case [3]) if user tries to modify a page after we finish writeback while freezing the filesystem. So principially all we need to do is just wait in ext4_page_mkwrite(). Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- 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
