On Tue 17-01-17 14:04:03, Andreas Dilger wrote: > On Jan 17, 2017, at 8:59 AM, Theodore Ts'o <tytso@xxxxxxx> wrote: > > > > On Tue, Jan 17, 2017 at 04:18:17PM +0100, Michal Hocko wrote: > >> > >> OK, so I've been staring into the code and AFAIU current->journal_info > >> can contain my stored information. I could either hijack part of the > >> word as the ref counting is only consuming low 12b. But that looks too > >> ugly to live. Or I can allocate some placeholder. > > > > Yeah, I was looking at something similar. Can you guarantee that the > > context will only take one or two bits? (Looks like it only needs one > > bit ATM, even though at the moment you're storing the whole GFP mask, > > correct?) > > > >> But before going to play with that I am really wondering whether we need > >> all this with no journal at all. AFAIU what Jack told me it is the > >> journal lock(s) which is the biggest problem from the reclaim recursion > >> point of view. What would cause a deadlock in no journal mode? > > > > We still have the original problem for why we need GFP_NOFS even in > > ext2. If we are in a writeback path, and we need to allocate memory, > > we don't want to recurse back into the file system's writeback path. > > Certainly not for the same inode, and while we could make it work if > > the mm was writing back another inode, or another superblock, there > > are also stack depth considerations that would make this be a bad > > idea. So we do need to be able to assert GFP_NOFS even in no journal > > mode, and for any file system including ext2, for that matter. > > > > Because of the fact that we're going to have to play games with > > current->journal_info, maybe this is something that I should take > > responsibility for, and to go through the the ext4 tree after the main > > patch series go through? Maybe you could use xfs and ext2 as sample > > (simple) implementations? > > > > My only ask is that the memalloc nofs context be a well defined N > > bits, where N < 16, and I'll find some place to put them (probably > > journal_info). > > I think Dave was suggesting that the NOFS context allow a pointer to > an arbitrary struct, so that it is possible to dereference this in > the filesystem itself to determine if the recursion is safe or not. Yes, but can we start with a simpler approach first? Even this approach takes quite some time to be used. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
