Re: [PATCH 2/9] cachefiles: drop direct usage of ->bmap method.

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On Tue, Aug 20, 2019 at 01:50:30PM +0100, David Howells wrote:
> Carlos Maiolino <cmaiolino@xxxxxxxxxx> wrote:
> 
> > > > +	block = page->index;
> > > > +	block <<= shift;
> > > 
> > > Can't this cause overflows?
> > 
> > Hmm, I honestly don't know. I did look at the code, and I couldn't really spot
> > anything concrete.
> 
> Maybe, though we'd have to support file sizes over 16 Exabytes for that to be
> a problem.

On 32-bit sysems page->index is a 32-bit value, so you'd overflow at
pretty normal sizes of a few TB.

> Note that bmap() is *only* used to find out if the page is present in the
> cache - and even that I'm not actually doing very well, since I really *ought*
> to check every block in the page.
> 
> I really want to replace the use of bmap entirely with iov_iter doing DIO.
> Cachefiles currently does double buffering because it works through the
> pagecache of the underlying to do actual read or write - and this appears to
> cause memory management problems.

Not related to this patch, but using iov_iter with dio is trivial, what
is the blocker therere?



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