Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and _NONVOLATILE flags

[Dave Chinner <david@fromorbit.com>]

On Wed, Feb 15, 2012 at 12:37:50PM +1100, NeilBrown wrote:
> On Tue, 14 Feb 2012 16:29:10 -0800 John Stultz <john.stultz@linaro.org> wrote:
> 
> > But I'm open to other ideas and arguments.
> 
> I didn't notice the original patch, but found it at
> https://lwn.net/Articles/468837/
> and had a look.
> 
> My first comment is -ENODOC.  A bit background always helps, so let me try to
> construct that:
> 
>  The goal is to allow applications to interact with the kernel's cache
>  management infrastructure.  In particular an application can say "this
>  memory contains data that might be useful in the future, but can be
>  reconstructed if necessary, and it is cheaper to reconstruct it than to read
>  it back from disk, so don't bother writing it out".
> 
>  The proposed mechanism - at a high level - is for user-space to be able to
>  say "This memory is volatile" and then later "this memory is no longer
>  volatile".  If the content of the memory is still available the second
>  request succeeds.  If not, it fails.. Well, actually it succeeds but reports
>  that some content has been lost. (not sure what happens then - can the app do
>  a binary search to find which pages it still has or something).
> 
>  (technically we should probably include the cost to reconstruct the page,
>  which the kernel measures as 'seeks' but maybe that isn't necessary).
> 
>  This is implemented by using files in a 'tmpfs' filesystem.  These file
>  support three new flags to fadvise:
> 
>  POSIX_FADV_VOLATILE - this marks a range of pages as 'volatile'.  They may be
>         removed from the page cache as needed, even if they are not 'clean'.
>  POSIX_FADV_NONVOLATILE - this marks a range of pages as non-volatile.
>         If any pages in the range were previously volatile but have since been
>         removed, then a status is returned reporting this.
>  POSIX_FADV_ISVOLATILE - this does not actually give any advice to the kernel
>         but rather asks a question: Are any of these pages volatile?

What about for files that aren't on tmpfs? the fadvise() interface
is not tmpfs specific, and given that everyone is talking about
volatility of page cache pages, I fail to see what is tmpfs specific
about this proposal.

So what are the semantics that are supposed to apply to a file that
is on a filesystem with stable storage that is cached in the page
cache?

If this is tmpfs specific behaviour that is required, then IMO
fadvise is not the correct interface to use here because fadvise is
supposed to be a generic interface to controlling the page cache
behaviour on any given file....

> As a counter-point,  this is my first thought of an implementation approach
> (-ENOPATCH, sorry)
> 
> - new mount option for tmpfs e.g. 'volatile'.  Any file in a filesystem
>   mounted with that option and which is not currently open by any process can
>   have blocks removed at any time.  The file name must remain, and the file
>   size must not change.
> - lseek can be used to determine if anything has been purged with 'SEEK_DATA'
>   and 'SEEK_HOLE'.
> 
> So you can only mark volatility on a whole-file granularity (hence the
> question above).
>  'open' says "NONVOLATILE".
>  'close' says "VOLATILE".
>  'lseek' is used to check if anything was discarded.
> 
> This approach would allow multiple processes to share a cache (might this be
> valuable?) as it doesn't become truly volatile until all processes close
> their handles.

If this functionality is only useful for tmpfs, then I'd much prefer
a tmpfs specific approach like this....

Cheers,

Dave.


-- 
Dave Chinner
david@fromorbit.com