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Message-ID: <4BE877CB.30009@crca.org.au> Date: Tue, 11 May 2010 07:16:59 +1000 From: Nigel Cunningham <ncunningham@...a.org.au> To: Bill Davidsen <davidsen@....com> CC: pm list <linux-pm@...ts.linux-foundation.org>, LKML <linux-kernel@...r.kernel.org>, TuxOnIce-devel <tuxonice-devel@...onice.net> Subject: Re: Proposal for a new algorithm for reading & writing a hibernation image. Hi Bill. On 11/05/10 01:54, Bill Davidsen wrote: > Nigel Cunningham wrote: >> Hi all. >> >> Some discussions with Rafael a while ago (can't find the original >> message now, sorry) got me thinking about whether there might be a >> better way of writing a complete image of memory, particularly in the >> context of KMS breaking existing TuxOnIce algorithms. I finally got >> around to hammering out the algorithm last night, and thought I'd put >> it out there for others to comment on, particularly since I'm no >> expert on fault handling - it may be that what I'm thinking of is >> impossible on the hardware we support. >> >> The algorithm I'm thinking of trying to implement goes as follows: >> >> When saving the image >> ===================== >> >> 1. Modify driver suspend and resume routines so that the freeing of >> memory used for the storage of state is separated from restoring the >> resume methods. This will allow us to get the drivers to save their >> state prior to writing the image, without needing the memory allocated >> for this purpose to be atomically copied. >> 2. Prior to writing any of the image, also set up new 4k page tables >> such that an attempt to make a change to any of the pages we're about >> to write to disk will result in a page fault, giving us an opportunity >> to flag the page as needing an atomic copy later. Once this is done, >> write protection for the page can be disabled and the write that >> caused the fault allowed to proceed. >> 3. Write the entire contents of memory to disk. >> 4. Disable secondary CPUs (no need to do the driver suspend/resume >> again) and atomically copy pages that faulted while writing the image. >> 5. Write atomically copied data to disk, giving a complete image on >> disk of memory at the time of the atomic copy. >> >> When loading the image >> ====================== >> 1. Locate and allocate pages that can have data directly loaded (ie >> are free now and used in the saved image). These will be loaded >> without an 'atomic restore'. >> 2. For other pages: >> As each page is loaded: >> - Write protect existing data. >> - If contents are the same as what is being loaded >> Discard loaded version >> If contents change after being write protected, >> 1. make a copy of unmodified version to later atomically copy back. >> 2. remove write protection >> - If contents differ >> 1. set up atomic restore later >> 2. remove write protection >> 3. After loading memory and determining what needs to be atomically >> restored: >> - Do drivers suspend, atomic restore as is done at the moment >> >> The main difficulties I see with the above are - apart from not being >> sure that I can achieve the above with fault handling - are: >> >> 1. Memory requirements for the atomic copy wouldn't be known until the >> point where we get to the atomic copy. I guess, though, that with most >> things frozen, we'd expect the number to be reasonably consistent and >> small. >> 2. We also need extra memory for the driver suspend at resume time. >> That said, since it's not otherwise needed, it could be the same >> memory that's reserved for doing I/O and for atomically copied data >> when writing the image. >> >> Are there other issues people can see that I might have missed? >> > I doubt you "missed" considering compression, but you didn't mention it. Yeah. I was just focusing on the method of ensuring we get a consistent image. I'd be seeking in the first instance to modify the existing TuxOnIce code to work this way, so it would still have multithreaded I/O, compression and so on. What I really want to do is work on patches to improve swsusp, but I have to keep the existing TuxOnIce users happy too :) Regards, Nigel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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