Re: vmalloc with GFP_NOFS

[Darrick J. Wong]

On Wed, May 09, 2018 at 03:42:22PM +0200, Michal Hocko wrote:
> On Tue 24-04-18 13:25:42, Michal Hocko wrote:
> [...]
> > > 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?
> > 
> > I can.
> 
> Does something like the below sound reasonable/helpful?
> ---
> =================================
> GFP masks used from FS/IO context
> =================================
> 
> :Date: Mapy, 2018
> :Author: Michal Hocko <mhocko@xxxxxxxxxx>
> 
> Introduction
> ============
> 
> FS resp. IO submitting code paths have to be careful when allocating

Not sure what 'FS resp. IO' means here -- 'FS and IO' ?

(Or is this one of those things where this looks like plain English text
but in reality it's some sort of markup that I'm not so familiar with?)

Confused because I've seen 'resp.' used as shorthand for
'responsible'...

> memory to prevent from potential recursion deadlocks caused by direct
> memory reclaim calling back into the FS/IO path and block on already
> held resources (e.g. locks). Traditional way to avoid this problem

'The traditional way to avoid this deadlock problem...'

> is to clear __GFP_FS resp. __GFP_IO (note the later implies clearing
> the first as well) in the gfp mask when calling an allocator. GFP_NOFS
> resp. GFP_NOIO can be used as shortcut.
> 
> This has been the traditional way to avoid deadlocks since ages. It

I think this sentence is a little redundant with the previous sentence,
you could chop it out and join this paragraph to the one before it.

> turned out though that above approach has led to abuses when the restricted
> gfp mask is used "just in case" without a deeper consideration which leads
> to problems because an excessive use of GFP_NOFS/GFP_NOIO can lead to
> memory over-reclaim or other memory reclaim issues.
> 
> New API
> =======
> 
> Since 4.12 we do have a generic scope API for both NOFS and NOIO context
> ``memalloc_nofs_save``, ``memalloc_nofs_restore`` resp. ``memalloc_noio_save``,
> ``memalloc_noio_restore`` which allow to mark a scope to be a critical
> section from the memory reclaim recursion into FS/IO POV. Any allocation
> from that scope will inherently drop __GFP_FS resp. __GFP_IO from the given
> mask so no memory allocation can recurse back in the FS/IO.
> 
> FS/IO code then simply calls the appropriate save function right at
> the layer where a lock taken from the reclaim context (e.g. shrinker)
> is taken and the corresponding restore function when the lock is
> released. All that ideally along with an explanation what is the reclaim
> context for easier maintenance.
> 
> What about __vmalloc(GFP_NOFS)
> ==============================
> 
> vmalloc doesn't support GFP_NOFS semantic because there are hardcoded
> GFP_KERNEL allocations deep inside the allocator which are quit non-trivial

...which are quite non-trivial...

> to fix up. That means that calling ``vmalloc`` with GFP_NOFS/GFP_NOIO is
> almost always a bug. The good news is that the NOFS/NOIO semantic can be
> achieved by the scope api.
> 
> In the ideal world, upper layers should already mark dangerous contexts
> and so no special care is required and vmalloc should be called without
> any problems. Sometimes if the context is not really clear or there are
> layering violations then the recommended way around that is to wrap ``vmalloc``
> by the scope API with a comment explaining the problem.

Otherwise looks ok to me based on my understanding of how all this is
supposed to work...

Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx>

--D

> -- 
> Michal Hocko
> SUSE Labs