On Thu 19-01-17 10:44:05, Michal Hocko wrote:

On Thu 19-01-17 10:22:36, Jan Kara wrote:

quoted

On Thu 19-01-17 09:39:56, Michal Hocko wrote:

quoted

On Tue 17-01-17 18:29:25, Jan Kara wrote:

quoted

On Tue 17-01-17 17:16:19, Michal Hocko wrote:

quoted

quoted

quoted

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.
"

Could you comment on this Ted, please?
-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>