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SubjectRe: 2.4.8/2.4.9 VM problems
On August 20, 2001 06:13 pm, Benjamin Redelings I wrote:
> Daniel Phillips wrote:
> > Could you please try this patch against 2.4.9 (patch -p0):
> >
> > --- ../2.4.9.clean/mm/memory.c Mon Aug 13 19:16:41 2001
> > +++ ./mm/memory.c Sun Aug 19 21:35:26 2001
> > @@ -1119,6 +1119,7 @@
> > */
> > return pte_same(*page_table, orig_pte) ? -1 : 1;
> > }
> > + SetPageReferenced(page);
> > }
> >
> > /*
> >
> Well, I tried this, and.... WOW! Much better [:)]
> Was it really true, that swapped in pages didn't get marked as
> referenced before? It almost felt that bad, but that seems kind of
> crazy - I don't completely understand what this fix is doing...

With the use-once optimization, all pages start on the inactive queue
instead of the active ring. If the page doesn't get referenced before it
gets to the other end of the inactive queue then it will be evicted and
freed. This means that somebody has to "rescue" each page that is actually
used, before it gets to the end of the inactive queue. This is implemented
explicitly for generic_file_read and generic_file_write via the
check_used_once function (which implements the use-once logic) and implicitly
for buffers via the existing touch_buffer function.

There was no such rescue implemented for swap pages because when I originally
submitted patch as an [RFC] I was doing all my testing without using any swap
space, just file IO. At the time, also, there other things wrong with the
swap cache so that it was hard to see the bad effects of this omission.

Just setting the page referenced means that page_launder or reclaim_page will
see the referenced bit and move the page to the active list, so it can live
out its normal life cycle.

A similar thing has to be done in filemap_nopage (which will take care of
mmap pages) and also for any filesystems whose page accesses bypass
generic_read/write, for example, the new directory-in-pagecache code in ext2.
I'm thinking now about whether it's best to take an approach that plugs all
the holes in a generic way, or instead just hunt them down one by one. Once
you know such holes are there it's not particularly hard to find and fill
them. It's tempting to try and move this logic to a more central place - the
problem with that is, in the central place it's hard to filter out accesses
that aren't real uses, such as readahead.

A final note: though the swap cache is not able to take full advantage of the
use-once logic (because we don't have a good way of checking the state of the
hardware page referenced bit - yet) it still does get a small benefit from
the machinery: when we optimistically read ahead from swap, those pages that
are not actually used will not be faulted in, thus will not have their
referenced bit set, thus will be discarded quickly.

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