On Mon, May 02, 2011 at 06:59:31PM -0700, Darrick J. Wong wrote: > On Tue, Apr 26, 2011 at 07:33:18AM -0400, Chris Mason wrote: > > Excerpts from Darrick J. Wong's message of 2011-04-25 20:37:38 -0400: > > > On Fri, Apr 22, 2011 at 10:34:34PM +0200, Jan Kara wrote: > > > > On Fri 22-04-11 08:50:01, Chris Mason wrote: > > > > > Excerpts from Darrick J. Wong's message of 2011-04-21 20:02:26 -0400: > > > > > > Hi everyone, > > > > > > > > > > > > I've finally managed to get together a patch that seems to provide stable pages > > > > > > during writeback, or at least gets us to the point that after several days of > > > > > > running tests I don't see DIF checksum errors anymore. :) > > > > > > > > > > > > The last two pieces to go into this puzzle were (a) bio_integrity_prep needs to > > > > > > walk the process tree to find all userland ptes that map to a particular memory > > > > > > page and revoke write access, and > > > > > > > > > > Hmm, did you need the bio_integrity_prep change for all the filesystems? > > > > > This should be happening already as part of using page_mkwrite. > > > > Or more precisely page_mkclean() should do what you try to do in > > > > bio_integrity_prep()... It would certainly be interesting (bug) if you > > > > could write to the page after calling page_mkclean() without page_mkwrite() > > > > being called. > > > > > > Hm... in mpage_da_submit_io I see the following sequence of calls: > > > > > > 1. clear_page_dirty_for_io > > > 2. possibly one of: ext4_bio_write_page or block_write_full_page. > > > If ext4_bio_write_page, > > > 2a. kmem_cache_alloc > > > 2b. set_page_writeback > > > > > > Before and after #1, the page is locked but writeback is not set. > > > > > > Before #2, the page must be locked and writeback must not be set, because both > > > of those two functions want to set the writeback bit themselves. However, > > > ext4_bio_write_page tries to allocate memory with GFP_NOFS, which means it can > > > sleep (I think). > > > > Sleeping isn't the problem as long as you sleep with the page locked. > > The idea is that writepage will: > > > > 1) lock the page > > 2) clear_page_dirty_for_io (which calls page_mkclean) > > 3) set_page_writeback() > > 4) unlock the page > > 5) start the IO > > > > page_mkwrite will: > > > > 1) lock the page > > 2) wait on page writeback > > 3) do other stuff > > > > So if ext is calling set_page_writeback() on an unlocked page, that's a > > problem. Otherwise it should be working. > > You're right, at this point in time writepage and page_mkwrite in ext4 both > behave as you describe. I began backing out parts of my patches to > bio-integrity.c and discovered that with the current kernel (2.6.39-rc5) the > only part that seems useful is the set_memory_ro/rw pair from that old > debugging patch. Unfortunately, those two functions only seem to exist on x86; > I suppose I could port them to others. If that's even a sane idea. Never mind, I looked around for anything that would result in the kernel trying to map a page for the purpose of writing, and I think adding a wait_on_writeback to get_cache_page_for_write will solve that last hole without having to port set_memory_* to other arches. With that, a whole lot of code falls out of the patches, which I will post shortly. --D > > --D > > > > -chris > > -- > > 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 > -- > 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 > -- 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
