Re: vmalloc with GFP_NOFS

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Tue, Apr 24, 2018 at 10:27:12AM -0600, Michal Hocko wrote:
> fs/ext4/xattr.c
> 
> What to do about this? Well, there are two things. Firstly, it would be
> really great to double check whether the GFP_NOFS is really needed. I
> cannot judge that because I am not familiar with the code.

*Most* of the time it's not needed, but there are times when it is.
We could be more smart about sending down GFP_NOFS only when it is
needed.  If we are sending too many GFP_NOFS's allocations such that
it's causing heartburn, we could fix this.  (xattr commands are rare
enough that I dind't think it was worth it to modulate the GFP flags
for this particular case, but we could make it be smarter if it would
help.)

> If the use is really valid then we have a way to do the vmalloc
> allocation properly. We have memalloc_nofs_{save,restore} scope api. How
> does that work? You simply call memalloc_nofs_save when the reclaim
> recursion critical section starts (e.g. when you take a lock which is
> then used in the reclaim path - e.g. shrinker) and memalloc_nofs_restore
> when the critical section ends. _All_ allocations within that scope
> will get GFP_NOFS semantic automagically. If you are not sure about the
> scope itself then the easiest workaround is to wrap the vmalloc itself
> with a big fat comment that this should be revisited.

This is something we could do in ext4.  It hadn't been high priority,
because we've been rather overloaded.  As a suggestion, could you take
documentation about how to convert to the memalloc_nofs_{save,restore}
scope api (which I think you've written about e-mails at length
before), and put that into a file in Documentation/core-api?

The question I was trying to figure out which triggered the above
request is how/whether to gradually convert to that scope API.  Is it
safe to add the memalloc_nofs_{save,restore} to code and keep the
GFP_NOFS flags until we're sure we got it all right, for all of the
code paths, and then drop the GFP_NOFS?

Thanks,

						- Ted