On 4/15/11 12:13 PM, Jan Kara wrote: > 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(). I've been kind of absent from this thread, sorry, but why would we wait in ext4_page_mkwrite(), rather than in mm/memory.c prior to any page_mkwrite call on any fs? no frozen fs should be able to dirty & write pages via mmap, right? -Eric > Honza -- 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
