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

[Benjamin LaHaise <bcrl@kvack.org>]

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.

		-ben

> Thanks,
> Shaohua

-- 
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