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

[Shaohua Li <shli@fb.com>]

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.

Thanks,
Shaohua

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