Re: [PATCH 8/8] Revert "ext4: fix wrong gfp type under transaction"

[Michal Hocko]

On Thu 19-01-17 10:22:36, Jan Kara wrote:
> On Thu 19-01-17 09:39:56, Michal Hocko wrote:
> > On Tue 17-01-17 18:29:25, Jan Kara wrote:
> > > On Tue 17-01-17 17:16:19, Michal Hocko wrote:
> > > > > > 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.
> > > > 
> > > > But we do not enter the writeback path from the direct reclaim. Or do
> > > > you mean something other than pageout()'s mapping->a_ops->writepage?
> > > > There is only try_to_release_page where we get back to the filesystems
> > > > but I do not see any NOFS protection in ext4_releasepage.
> > > 
> > > Maybe to expand a bit: These days, direct reclaim can call ->releasepage()
> > > callback, ->evict_inode() callback (and only for inodes with i_nlink > 0),
> > > shrinkers. That's it. So the recursion possibilities are rather more limited
> > > than they used to be several years ago and we likely do not need as much
> > > GFP_NOFS protection as we used to.
> > 
> > Thanks for making my remark more clear Jack! I would just want to add
> > that I was playing with the patch below (it is basically
> > GFP_NOFS->GFP_KERNEL for all allocations which trigger warning from the
> > debugging patch which means they are called from within transaction) and
> > it didn't hit the lockdep when running xfstests both with or without the
> > enabled journal.
> > 
> > So am I still missing something or the nojournal mode is safe and the
> > current series is OK wrt. ext*?
> 
> I'm convinced the current series is OK, only real life will tell us whether
> we missed something or not ;)

I would like to extend the changelog of "jbd2: mark the transaction
context with the scope GFP_NOFS context".

"
Please note that setups without journal do not suffer from potential
recursion problems and so they do not need the scope protection because
neither ->releasepage nor ->evict_inode (which are the only fs entry
points from the direct reclaim) can reenter a locked context which is
doing the allocation currently.
"
 
> > The following patch in its current form is WIP and needs a proper review
> > before I post it.
> 
> So jbd2 changes look confusing (although technically correct) to me - we
> *always* should run in NOFS context in those place so having GFP_KERNEL
> there looks like it is unnecessarily hiding what is going on. So in those
> places I'd prefer to keep GFP_NOFS or somehow else make it very clear these
> allocations are expected to be GFP_NOFS (and assert that). Otherwise the
> changes look good to me.

I would really like to get rid most of NOFS direct usage and only
dictate it via the scope API otherwise I suspect we will just grow more
users and end up in the same situation as we are now currently over time.
In principle only the context which changes the reclaim reentrancy policy
should care about NOFS and everybody else should just pretend nothing
like that exists. There might be few exceptions of course, I am not yet
sure whether jbd2 is that case. But I am not proposing this change yet
(thanks for checking anyway)...
-- 
Michal Hocko
SUSE Labs