Re: [PATCH] fadvise invalidating range fix

[WU Fengguang <wfg@mail.ustc.edu.cn>]

On Mon, Mar 08, 2004 at 12:03:22PM -0800, Andrew Morton wrote:
> WU Fengguang <wfg@mail.ustc.edu.cn> wrote:
> >
> > 
> >  - When 'offset' and/or 'offset+len' do no align to page boundary, we must
> >    decide whether to abandon the partial page at the beginning/end of the range. 
> >    My patch assumes that the application is scanning forward,
> >    which is the most common case.
> >    So 'end_index' is set to the page just before the ending partial page.
> 
> If you're going to preserve the partial page at `end' (which seems
> reasonable) then you should also preserve the partial page at `start', don't
> you agree?
> 
> -	start_index = offset >> PAGE_CACHE_SHIFT;
> +	start_index = (offset + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
> 
In fact, it depends on the access pattern.
I would expect the normal usage of fadvise to be:
	
	for() {
		read() and consume() data in [offset, offset+LEN);
		fadvise(fd, offset, LEN, POSIX_FADV_DONTNEED);
		offset += LEN; /*scanning forward*/
	}
	
In this case, the partial page at `start' should be freed.
Certainly there may be other patterns. 

I wonder the best solution in kernel is to code in favor of the normal case,
and the best(safe and portable) practice in usermode code is to always align
'offset' and 'offset+len' to page boundaries.
And some comment in the manual page of posix_fadvise is recommended.