Re: [PATCH 2/3] xfs: add kmem_alloc_io()

[Vlastimil Babka]

On 8/22/19 12:14 PM, Dave Chinner wrote:
> On Thu, Aug 22, 2019 at 11:10:57AM +0200, Peter Zijlstra wrote:
>> 
>> Ah, current_gfp_context() already seems to transfer PF_MEMALLOC_NOFS
>> into the GFP flags.
>> 
>> So are we sure it is broken and needs mending?
> 
> Well, that's what we are trying to work out. The problem is that we
> have code that takes locks and does allocations that is called both
> above and below the reclaim "lock" context. Once it's been seen
> below the reclaim lock context, calling it with GFP_KERNEL context
> above the reclaim lock context throws a deadlock warning.
> 
> The only way around that was to mark these allocation sites as
> GFP_NOFS so lockdep is never allowed to see that recursion through
> reclaim occur. Even though it isn't a deadlock vector.
> 
> What we're looking at is whether PF_MEMALLOC_NOFS changes this - I
> don't think it does solve this problem. i.e. if we define the
> allocation as GFP_KERNEL and then use PF_MEMALLOC_NOFS where reclaim
> is not allowed, we still have GFP_KERNEL allocations in code above
> reclaim that has also been seen below relcaim. And so we'll get
> false positive warnings again.

If I understand both you and the code directly, the code sites won't call
__fs_reclaim_acquire when called with current->flags including PF_MEMALLOC_NOFS.
So that would mean they "won't be seen below the reclaim" and all would be fine,
right?

> What I think we are going to have to do here is manually audit
> each of the KM_NOFS call sites as we remove the NOFS from them and
> determine if ___GFP_NOLOCKDEP is needed to stop lockdep from trying
> to track these allocation sites. We've never used this tag because
> we'd already fixed most of these false positives with explicit
> GFP_NOFS tags long before ___GFP_NOLOCKDEP was created.
> 
> But until someone starts doing the work, I don't know if it will
> work or even whether conversion PF_MEMALLOC_NOFS is going to
> introduce a bunch of new ways to get false positives from lockdep...
> 
> Cheers,
> 
> Dave.
>