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;
>
I gave it a rethink today, and yes, we should preserve the starting
partial page as well. Doing so helps reducing unnecessary disk I/O for
any access pattern, and the kernel will behave more consistent and robust.

The only negative effect is, some useless pages are left to be freed at some
later time by the page replacement routines, and the cost of which is acceptable.

regards, Wu Fengguang