On Sun, 2009-04-26 at 17:13 +0200, Nick Piggin wrote: > On Sun, Apr 26, 2009 at 10:18:29AM -0400, Trond Myklebust wrote: > > On Sun, 2009-04-26 at 08:40 +0200, Nick Piggin wrote: > > > On Sat, Apr 25, 2009 at 10:57:08AM -0400, Trond Myklebust wrote: > > > > On Fri, 2009-04-24 at 05:26 -0400, Rince wrote: > > > > > Applied try 3 of Nick Piggin's patch to 2.6.30-rc3 (cleanly, no less!) > > > > > > > > > > Doesn't appear to have helped at all - I received my favorite BUG ON > > > > > write.c:252 just like always, within 24 hours of booting the kernel, > > > > > even. > > > > > > > > Can you apply the following incremental patch on top of Nick's. This > > > > appears to suffice to close the race on my setup. > > > > > > Thanks, yes that looks good. Note: I deliberately didn't try to > > > convert filesystems because it needs much better understanding > > > of each one. So any fs maintainers using page_mkwrite I hope have > > > looked at these patches and considered whether they need to > > > do anything differently (ditto for the page_mkwrite return value > > > fixup patch). > > > > Note that after applying this, I put a WARN_ON(!PageDirty()) in the NFS > > set_page_dirty() method, and ran some mmap stress tests. > > > > As far as I can tell, the WARN_ON is never triggering. I take this to > > mean that the remaining cases where the VM is calling set_page_dirty() > > are basically all cases where we've already seen a page fault and set > > the page dirty flag, but haven't yet written it out (i.e. we haven't yet > > called clear_page_dirty_for_io() and so the pte is still marked as > > dirty). > > That again implies that set_page_dirty() is now fully redundant for > > filesystems that define page_mkwrite(), provided that the filesystem > > takes charge of marking the page as dirty. > > > > This suggests an alternative fix for the stable kernels in the form of > > the following patch. > > Comments? > > This doesn't seem to fix the race, though... on kernels with the > race still there, it will just open a window where you can have > a dirty pte but the page not written out. > > I don't understand. I'm just pointing out that the NFS client already calls __set_page_dirty_nobuffers() while holding the page lock inside the nfs_vm_page_mkwrite() call, so having the VM do it too in the call to set_page_dirty_balance() is actually redundant. IOW: as far as the NFS code is concerned, we can get rid of the ->set_page_dirty() callback in that situation. I couldn't find any other places in the VM code where we can have a dirty pte without also having called page_mkwrite() (and hence __set_page_dirty_nobuffers). As I said, adding a WARN_ON(!PageDirty()) in ->set_page_dirty() didn't ever trigger any cases where the set_page_dirty() was actually setting the dirty bit (except in the case where we race with page writeout in do_wp_page() and __do_fault()). That's why I believe disabling ->set_page_dirty() is safe here, and will in fact suffice to fix the page writeout race. Cheers Trond -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
