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 <shentino@gmail.com> wrote:

On Wed, Aug 12, 2015 at 8:15 PM, Tom Mitchell <mitch@niftyegg.com> wrote:
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 <shentino@gmail.com> wrote:
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.

_______________________________________________
Devel mailing list
Devel@sel4.systems
https://sel4.systems/lists/listinfo/devel

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

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