"The reference bit may be set to one by fetching data or instructions that are neither designated nor used by the program, and, under certain conditions, a reference may be made without the reference bit being set to one. Under certain unusual circumstances, a reference bit may be set to zero by other than explicit program action."
Jan.
From: Martin Schwidefsky <[EMAIL PROTECTED]> Reply-To: Linux on 390 Port <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Subject: Re: s390 storage key inconsistency? [was Re: msync() behaviour broken for MS_ASYNC, revert patch?] Date: Thu, 1 Apr 2004 20:13:40 +0200
Hi Stephen,
> I just happened to follow the function and noticed that on s390, > page_test_and_clear_dirty() has the comment: > > * Test and clear dirty bit in storage key. > * We can't clear the changed bit atomically. This is a potential > * race against modification of the referenced bit. This function > * should therefore only be called if it is not mapped in any > * address space. > > but in this case the page is clearly mapped in the caller's address > space, else we wouldn't have reached this. > > Is this a problem?
The clearing of the dirty bit while the page is still mapped somewhere races against the setting of the referenced bit. The worst that can happen is that the setting of a referenced bit gets lost. This can lead to a bad swapping decision because a page is considered to be old but in reality someone accessed it recently. This is not nice but it doesn't happen often. This isn't a problem as long as no dirty bit gets lost.
blue skies, Martin
Linux/390 Design & Development, IBM Deutschland Entwicklung GmbH Schvnaicherstr. 220, D-71032 Bvblingen, Telefon: 49 - (0)7031 - 16-2247 E-Mail: [EMAIL PROTECTED]
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