Re: [seL4] Detecting dirty/accessed page tables

The accessed bit is a file system + OS feature. If it works in ways you expect life is good. OS meta data may live above the microkernel and needs to be checked. Asking the microkernel might be possible with an ioctl() to a driver you craft. The page table is both in memory and in the processor so knowing what to ask from user space is not obvious. There are security issues with all of these. Ity is also very dynamic. I do not think of the page table as one table. It is a complex multi layered structure managed by privileged code and extracting the portions any security model might allow seem a complex risk. Debugging tools might let you see what you want. Debugging tools need their own framework and support. Do not ignore cache architecture and MTTR and PAT... On Wed, Aug 12, 2015 at 8:37 PM, Raymond Jennings wrote:

On Wed, Aug 12, 2015 at 8:15 PM, Tom Mitchell wrote:

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You (or some other code block) needs to look at how the memory pages are marked in the TLB system. One way is to mark the page read only then on a write handle the exception validate the writer and mark the page writable for that context. Then mark a bit that the page needs to be pushed to disk.

Filesystem code also flows through code that can mark it as dirty and needing to be pushed to media. Since some file writes are well managed with exclusion flags at the open some things get easy.

Assumptions can be made as well on the last close and push any page in memory back to disk.

It is possible for user space file system code to do bookkeeping and tell the OS that pages have been modified. i.e. the content of the file belong to user space. Meta data and the block list for the file belong to the protected code in the kernel.

Without a TLB protection and exception there is no way to "know" and other code must do the book keeping.

Big cautions about multi processor, multi user, access control lists, files with holes (blocks of NULL) and as always meta data including extended meta data. Least I forget filesystem design is difficult.

On Wed, Aug 12, 2015 at 5:36 PM, Raymond Jennings wrote:

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Hey I'm just curious...how do you detect if a memory page mapped by a page table has been written to or accessed?

I imagine if you're writing a filesystem task, such information would help you know when to flush data back to disk or which pages to keep around if things get tight.

I hope this isn't a stupid question.

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So basically there's no way to actually peek at the page table or otherwise ask the microkernel if the page is accessed/dirty?

I was also asking about the accessed bit.

-- T o m M i t c h e l l