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SubjectRe: [potential bug] generic_file_readahead()

Rik,

Ok, I understand what you're doing now. How is this going to affect
page cache based file systems.

Jeff

Rik van Riel wrote:
>
> On Wed, 25 Oct 2000, Alexander Viro wrote:
>
> > In generic_file_readahead():
> >
> > unsigned long end_index = inode->i_size >> PAGE_CACHE_SHIFT;
> > unsigned long index = page->index;
> > ...
> > max_ahead = 0;
> > ...
> > raend = index;
> > if (raend < end_index)
> > max_ahead = filp->f_ramax;
> > and later
> > ahead = 0;
> > while (ahead < max_ahead) {
> > ahead ++;
> > if ((raend + ahead) >= end_index)
> > break;
> > if (page_cache_read(filp, raend + ahead) < 0)
> > break;
> > }
> >
> > AFAICS it means that we have off-by-partial here - if the file size is not
> > a multiple of PAGE_CACHE_SIZE we are missing the last page. Proposed
> > fix:
> > make end_index = (inode->i_size + PAGE_CACHE_SIZE - 1)>>PAGE_CACHE_SHIFT;
> >
> > Objections?
>
> Indeed, this should fix another inefficiency in the
> readahead code. I'll prepare a patch for filemap.c
> soon (also containing ANOTHER small fix to the
> readahead code).
>
> > PS: folks, could those who wrote that function comment on the
> > readahead rules in general? This stuff looks really ugly
> > (presumably from the layers and layers of small modifications)
> > and comments on _intentions_ of that code (as opposed to "what
> > are we doing in the next two lines") would be very welcome.
>
> While I haven't written it, I'll try to come up with
> some documentation when I have the time (yeah right).
> Oh well, I'll try to at least comment on everything _I_
> try to change in the kernel ;)
>
> regards,
>
> Rik
> --
> "What you're running that piece of shit Gnome?!?!"
> -- Miguel de Icaza, UKUUG 2000
>
> http://www.conectiva.com/ http://www.surriel.com/
>
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