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SubjectRe: [BK PATCH 2.5] Introduce 64-bit versions of PAGE_{CACHE_,}{MASK,ALIGN}
William Lee Irwin III wrote:
>
> On Sun, Jul 28, 2002 at 05:10:48PM -0700, Andrew Morton wrote:
> > - Few pages use ->private for much. Hash for it. 4(ish) bytes
> > saved.
>
> Do you know an approximate reasonable constant of proportionality
> for how many pages have ->private attached?

Well, it depends on what the mapping is using ->private for.
In the case of ext2, ext3 and (soon) reiserfs mappings, ->private
is only used for pagecache pages which were written to with write(2).

But for other filesystems, basically all pagecache pages have
buffers at present, so I exaggerate. But as filesystems migrate
to using direct-to-BIO reads, the situation gets better.

It might be useful to buffer-strip written-to pages as well, if
a clean way of doing that presents itself. Maybe in refill_inactive
or something.

> On Sun, Jul 28, 2002 at 05:10:48PM -0700, Andrew Morton wrote:
> > - Remove the rmap chain (I just broke ptep_to_address() anyway). 4 bytes
> > saved. struct page is now 20 bytes.
>
> How did ptep_to_address() break? I browsed over your latest changes and
> missed the bit where that fell apart. I'll at least take a stab at fixing
> it up until the other bits materialize.

I broke it in my five-minute thought-coding exercise. By removing
page->index.

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