Re: 2.4.0-test8-pre1 is quite bad / how about integrating Rik's VM now?

From: Linus Torvalds (
Date: Sun Sep 03 2000 - 00:43:13 EST

On Sat, 2 Sep 2000, Alexander Viro wrote:
> +
> + /*
> + * So truncate in the middle of a hole not on a block boundary will
> + * allocate a block. BFD. Everything is still consistent, so trying
> + * to be smart is not worth the trouble.
> + */

You don't actually have to be smart.

There's a really simple way to avoid this: compare the thing you're going
to zero out against zero before you memset() it to zero. If it was already
zero, you just unlock the page and release.

Downside: you have to read the page in, in order to compare. However, this
turns out to be a non-issue: if there were no blocks on that page (like a
forward truncate) this ends up generating no IO anyway. If there _were_
blocks on the page, it's likely that one of them was the one we're going
to partially clear anyway, so the "prepare_write()" would have had to read
it in anyway.

Upside: you don't need to mark it dirty unnecessarily, assuming you just
extended the file. No extra allocation, and no extra write to disk.

Comments? Basically the "grab_cache_page()" would be a "read_cache_page()"
instead with all the wait-on-page etc stuff.


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