Re: [PATCH][RFC] splice support

[Jens Axboe]

On Thu, Mar 30 2006, Andrew Morton wrote:
> Jens Axboe <axboe@xxxxxxx> wrote:
> >
> > > find_get_pages() does "find me the next N pages above `index' which are
> >  > presently in pagecache'.  So it can return an array of page*'s which do not
> >  > represent contiguous pages in the file - there can be holes in there.
> >  > 
> >  > IOW: pages[n]->index !necessarily= pages[n+1]->index-1
> >  > 
> >  > Maybe the code handles that by making sure that all the pages in the range
> >  > are already in pagecache - I didn't check.  But that would take some heroic
> >  > locking.
> > 
> >  It doesn't, I'm assuming that find_get_pages() returns consequtive pages
> >  atm. Would seem like the sane interface :-)
> 
> Yeah, sorry.  It's a "gather what's presently there" thing.  For writeback.
> 
> Nick has some gang-lookup-slots code.  So instead of populating an array of
> page*'s you can populate an array of (effectively) page**'s.  Then one
> could walk that.   All while holding ->tree_lock.    This doesn't help ;)
> 
> Or you could walk the pages[] array until you hit an ->index which doesn't
> match and then toss the rest away.  That's a bit of extra work, but in the
> common case all the pages will be good.  Perhaps.
> 
> >  We continue doing find_or_create_page() on the remaining, but using 'i'
> >  as the 'index' addition. So if we had non-conseq pages, we'd be screwed.
> 
> Yup.
> 
> Probably the simplest for now is an open-coded find_get_page() loop.  Later
> on we should optimise that into a find_get_contig_pages() which only takes
> tree_lock a single time.
> 
> Doing it with a new radix_tree_gang_lookup_contig_name_me_longer() would be
> relatively straightforward too.  It would bale out as soon as it hit a
> not-present slot.

I'll go for the simple approach right now, going over the returned
find_get_pages() array and moving pages around and filling holes doesn't
sound too alluring. Thanks!

-- 
Jens Axboe

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