From: Linus Torvalds <torvalds@transmeta.com>
To: "Stephen C. Tweedie" <sct@redhat.com>
Cc: masp0008@stud.uni-sb.de, "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [PATCH] Re: swapcache bug?
Date: Mon, 8 Feb 1999 09:32:24 -0800 (PST) [thread overview]
Message-ID: <Pine.LNX.3.95.990208092701.31153B-100000@penguin.transmeta.com> (raw)
In-Reply-To: <199902081639.QAA03290@dax.scot.redhat.com>
On Mon, 8 Feb 1999, Stephen C. Tweedie wrote:
>
> Good point, the line include/linux/pagemap.h:39,
>
> return s(i+o) & (PAGE_HASH_SIZE-1);
>
> should probably be
>
> return s(i+o+offset) & (PAGE_HASH_SIZE-1);
>
> to mix in the low order bits for swap entries. Well spotted. Anyone
> see anything wrong with this one-liner change?
Yes, the above will potentially result in different hash entries for the
same page, which means that we now have aliasing and basically just random
behaviour.
It _may_ be that the hash function is always called with a page-aligned
offset, but that was not how it was strictly meant to be: the way the
thing was envisioned you could just find the page at "offset" by doing
page_hash(inode,offset)
without page-aligning offset before you did this.
If anything, maybe the swap cache should just use the high bits in the
"offset" field (or at least prefer to do so: something like
page->offset = swap_entry_to_offset(entry);
and
entry = offset_to_swap_entry(page->offset);
that does a PAGE_MASK_BITS rotate on the bits..
Linus
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next prev parent reply other threads:[~1999-02-07 22:27 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-02-07 18:21 swapcache bug? Manfred Spraul
1999-02-07 21:30 ` Eric W. Biederman
1999-02-08 16:39 ` [PATCH] " Stephen C. Tweedie
1999-02-08 17:32 ` Linus Torvalds [this message]
1999-02-08 17:51 ` Stephen C. Tweedie
1999-02-08 18:48 ` Linus Torvalds
1999-02-08 21:13 ` Matti Aarnio
1999-02-09 7:15 ` Eric W. Biederman
1999-02-09 16:32 ` Linus Torvalds
1999-02-10 0:28 ` Eric W. Biederman
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