Re: [PATCH 2/2] aio: make aio .mremap handle size changes

[Shaohua Li <shli@fb.com>]

On Tue, Feb 03, 2015 at 02:48:28PM -0500, Benjamin LaHaise wrote:
> On Tue, Feb 03, 2015 at 11:31:15AM -0800, Shaohua Li wrote:
> > On Tue, Feb 03, 2015 at 02:23:23PM -0500, Benjamin LaHaise wrote:
> > > On Tue, Feb 03, 2015 at 11:18:53AM -0800, Shaohua Li wrote:
> > > > mremap aio ring buffer to another smaller vma is legal. For example,
> > > > mremap the ring buffer from the begining, though after the mremap, some
> > > > ring buffer pages can't be accessed in userspace because vma size is
> > > > shrinked. The problem is ctx->mmap_size isn't changed if the new ring
> > > > buffer vma size is changed. Latter io_destroy will zap all vmas within
> > > > mmap_size, which might zap unrelated vmas.
> > > 
> > > Nak.  Shrinking the aio ring buffer is not a supported operation and will 
> > > cause the application to lose events.  Make the size changing mremap fail, 
> > > as this patch will not make the system do the right thing.
> > 
> > Yes, making the syscall fail (vma ops has .remap) is another option. If
> > the app uses io_getevents(), looks the app will not lose events, no? On
> > the other hand, I just want to make sure kernel does the right thing
> > (not zap unrelated vmas). If app does crazy things, it will break.
> 
> But reading events out of the ring buffer is a supported mode of operation.  
> Given that constraint, you should make an mremap changing the size of the 
> ring buffer fail.

But if app chooses not to read the ring buffer, the app isn't broken.
This makes me hesitate to make the syscall fail.
There is a grey area about what's the correct semantics for mremap. We
currently don't limit any vma shrink for mremap.

Thanks,
Shaohua

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