Re: [PATCH 0/5] mm improvements

[Andrew Morton <akpm@osdl.org>]

Hugh Dickins <hugh@veritas.com> wrote:
>
> On Wed, 4 Feb 2004, Andrew Morton wrote:
> > Hugh Dickins <hugh@veritas.com> wrote:
> > >
> > >  Sorry, that BUG_ON is there for very good reason.  It's no disgrace
> > >  that your testing didn't notice the effect of passing a mapped page
> > >  down to shmem_writepage, but it is a serious breakage of tmpfs.
> > 
> > hm.  Can't I force writepage-of-a-mapped-page with msync()?
> 
> I hope not, __filemap_fdatawrite still starts off with:
> 
> 	if (mapping->backing_dev_info->memory_backed)
> 		return 0;

Sigh.  ->memory_backed is a crock.  It is excessively overloaded and needs
to be split up into several things which really mean something.

> Once upon a time you did have vmscan.c calling ->writepages, rather
> the effect that Nikita is trying for.  It was that writepages which
> led me to insert the BUG_ON and give tmpfs a dummy writepages.
> Later on you dropped the ->writepages from vmscan.c:
> do you remember why? would be useful info for Nikita.

I'd need to troll the changelogs to remember the exact reason.  I had the
standalone a_ops->vm_writeback thing in there, which was able to do
writearound against the targetted page.  iirc it was causing some difficulties and
as a big effort was underway to minimise the amount of writeout via vmscan
_anyway_, I decided to toss it all out, stick with page-at-a-time writepage.

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